Dædalus Coming up in Dædalus

Home , David Chilton Phillips, Firsov (crater), Hermann Staudinger, Hugh Huxley, John Kendrew, John Meurig Thomas

Cover_Fall 2014 9/8/2014 10:09 AM Page 1 Dædalus coming up in Dædalus:

What is the Brain Fred H. Gage, Thomas D. Albright, Emilio Bizzi & Robert Ajemian, Good For? Brendon O. Watson & György Buzsáki, A. J. Hudspeth, Joseph LeDoux, Dædalus Earl K. Miller & Timothy J. Buschman, Terrence J. Sejnowski, Larry R. Squire & John T. Wixted, and Robert H. Wurtz Journal of the American Academy of Arts & Sciences Fall 2014 On Water Christopher Field & Anna Michalak, Michael Witzel, Charles Vörösmarty, Michel Meybeck & Christopher L. Pastore, Terry L. Anderson, John Briscoe, Richard G. Luthy & David L. Sedlak, Stephen R. Fall 2014: From Atoms to the Stars Carpenter & Adena R. Rissman, Jerald Schnoor, and Katherine Jacobs From Atoms Jerrold Meinwald Introduction 5 to the Stars Christopher C. Cummins Phosphorus: From the Stars to Land & Sea 9 On an Aging Society John W. Rowe, Jay Olshansky, Julie Zissimopolous, Dana Goldman, John Meurig Thomas Foresight, Unpredictability & Chance Robert Hummer, Mark Hayworth, Lisa Berkman, Axel Boersch-Supan, in Chemistry & Cognate Subjects 21 Dawn Carr, Linda Fried, Frank Furstenberg, Caroline Hartnett, Martin Fred Wudl The Bright Future of Fabulous Materials Kohli, Toni Antonucci, David Bloom, and David Canning Based on Carbon 31 Chaitan Khosla The Convergence of Chemistry G. David Tilman, Walter C. Willett, Meir J. Stampfer & Jaquelyn L. Food, Health & & Human Biology 43 the Environment Jahn, Nathaniel D. Mueller & Seth Binder, Steven Gaines & Christopher K. N. Houk & Peng Liu Using Computational Chemistry to Understand Costello, Andrew Balmford, Rhys Green & Ben Phalan, G. Philip & Discover Chemical Reactions 49 Robertson, Brian G. Henning, and Steven Polasky Jeremiah P. Ostriker From the Atom to the Universe: Recent Astronomical Discoveries 67 plus The Internet; What’s New About the Old?; New Dilemmas in Ethics, Technology & War &c Anna Frebel Reconstructing the Cosmic Evolution of the Chemical Elements 71 Gáspár Áron Bakos Exoplanets, 2003–2013 81 Michael A. Strauss Mapping the Universe: Surveys of the Sky as Discovery Engines in Astronomy 93 Scott Tremaine The Odd Couple: Quasars & Black Holes 103 Pieter van Dokkum The Formation & Evolution of Galaxies 114 David N. Spergel Cosmology Today 125

U.S. $13; www.amacad.org Cherishing Knowledge · Shaping the Future Cover_Fall 2014 9/8/2014 10:09 AM Page 2 Book_Fall 2014_Shinner.qxd 9/8/2014 9:54 AM Page 1

Inside front cover: “Life seen as a miracle of organized atoms.” Artwork by Adam Hurwitz. © American Academy of Arts & Sciences. Book_Fall 2014_Shinner.qxd 9/8/2014 9:54 AM Page 2

Jerrold Meinwald and Jeremiah Ostriker, Guest Editors Phyllis S. Bendell, Managing Editor and Director of Publications D Peter Walton, Assistant Editor Emma Goldhammer, Senior Editorial Assistant J

Committee on Studies and Publications Jerrold Meinwald and John Mark Hansen, Cochairs; Jesse H. Choper, Denis Donoghue, Gerald Early, Carol Gluck, Sibyl Golden, Linda Greenhouse, John Hildebrand, Jerome Kagan, Philip Khoury, Steven Marcus, Eric Sundquist, Jonathan Fanton (ex of½cio), Don M. Randel (ex of½cio), Diane P. Wood (ex of½cio)

Dædalus is designed by Alvin Eisenman. Book_Fall 2014_Shinner.qxd 9/8/2014 9:54 AM Page 3

Dædalus Journal of the American Academy of Arts & Sciences

The pavement labyrinth once in the nave of Reims Cathedral (1240), in a drawing, with ½gures of the architects, by Jacques Cellier (c. 1550–1620)

Dædalus was founded in 1955 and established as a quarterly in 1958. The journal’s namesake was renowned in ancient Greece as an inventor, scientist, and unriddler of riddles. Its emblem, a maze seen from above, symbol- izes the aspiration of its founders to “lift each of us above his cell in the labyrinth of learning in order that he may see the entire structure as if from above, where each separate part loses its comfortable separateness.” The American Academy of Arts & Sciences, like its journal, brings together distinguished individuals from every ½eld of human endeavor. It was char- tered in 1780 as a forum “to cultivate every art and science which may tend to advance the interest, honour, dignity, and happiness of a free, independent, and virtuous people.” Now in its third century, the Academy, with its nearly ½ve thousand elected members, continues to provide intellectual leadership to meet the critical challenges facing our world. Book_Fall 2014_Shinner.qxd 9/10/2014 9:08 AM Page 4

Dædalus Fall 2014 Subscription rates: Electronic only for non- Issued as Volume 143, Number 4 member individuals–$47; institutions–$129. Canadians add 5% gst. Print and electronic for © 2014 by the American Academy nonmember individuals–$52; institutions– of Arts & Sciences $144. Canadians add 5% gst. Outside the United Phosphorus: From the Stars to Land & Sea States and Canada add $23 for postage and han- © 2014 by Christopher C. Cummins dling. Prices subject to change without notice. Published under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license Institutional subscriptions are on a volume- Reconstructing the Cosmic Evolution year basis. All other subscriptions begin with of the Chemical Elements the next available issue. © 2014 by Anna Frebel Single issues: $13 for individuals; $35 for insti- Editorial of½ces: Dædalus, American Academy of tutions. Outside the United States and Canada Arts & Sciences, 136 Irving Street, Cambridge ma add $6 per issue for postage and handling. 02138. Phone: 617 576 5085. Fax: 617 576 5088. Prices subject to change without notice. Email: daedalus@amacad.org. Claims for missing issues will be honored free Library of Congress Catalog No. 12-30299 of charge if made within three months of the publication date of the issue. Claims may be Dædalus publishes by invitation only and as- submitted to [email protected]. Members of sumes no responsibility for unsolicited manu- the American Academy please direct all ques- scripts. The views expressed are those of the tions and claims to [email protected]. author of each article, and not necessarily of the American Academy of Arts & Sciences. Advertising and mailing-list inquiries may be addressed to Marketing Department, mit Press Dædalus (issn 0011-5266; e-issn 1548-6192) Journals, One Rogers Street, Cambridge ma is published quarterly (winter, spring, summer, 02142-1209. Phone:617253 2866. Fax: 617 253 1709. fall) by The mit Press, One Rogers Street, Cam- Email: [email protected]. bridge ma 02142-1209, for the American Academy of Arts & Sciences. An electronic full-text version To request permission to photocopy or repro- of Dædalus is available from The mit Press. duce content from Dædalus, please complete the Subscription and address changes should be ad - online request form at http://www.mitpress dressed to mit Press Journals Customer Service, journals.org/page/permissionsForm.jsp, or con- One Rogers Street, Cambridge ma 02142-1209. tact the Permissions Manager at mit Press Jour- Phone: 617 253 2889; U.S./Canada 800 207 8354. nals, One Rogers Street, Cambridge ma 02142- Fax: 617 577 1545. Email: [email protected]. 1209. Fax: 617 253 1709. Email: journals-rights@ mit.edu. Printed in the United States of America by Cadmus Professional Communications, Science Corporations and academic institutions with Press Division, 300 West Chestnut Street, valid photocopying and/or digital licenses with Ephrata pa 17522. the Copyright Clearance Center (ccc) may reproduce content from Dædalus under the Newsstand distribution by Ingram Periodicals terms of their license. Please go to www Inc., 18 Ingram Blvd., La Vergne tn 37086. .copyright.com; ccc, 222 Rosewood Drive, Postmaster: Send address changes to Dædalus, Danvers ma 01923. One Rogers Street, Cambridge ma 02142-1209. The typeface is Cycles, designed by Sumner Periodicals postage paid at Boston ma and at Stone at the Stone Type Foundry of Guinda ca. additional mailing of½ces. Each size of Cycles has been sep arately designed in the tradition of metal types. Book_Fall 2014_Shinner.qxd 9/8/2014 9:59 AM Page 5

Introduction

Jerrold Meinwald

Why “From Atoms to the Stars”? In the Summer 2012 issue of Dædalus, entitled “Science in the 21st Century,” May Berenbaum and I sought to provide representative accounts of recent progress in the natural sciences. But it turned out that two areas of the physical sciences–astronomy and chemistry– cried out for more extensive attention than we were then able to provide. Consequently, Jeremiah Ostriker and I recruited a group of outstanding as - tronomers and chemists to write a set of essays to complement this earlier issue. Each of the new es - says in this volume discusses important scienti½c JERROLD MEINWALD, a Fellow of the American Academy since 1970, developments in astronomy and chemistry in spe - is the Goldwin Smith Professor of ci½c areas of study to which the authors themselves Chemistry Emeritus at Cornell Uni - have made major contributions. versity. His research has contrib - Philosophers, alchemists, and subsequently chem- uted to a wide range of chemical ists have examined the properties and transforma- and chemical biological subjects, tions of matter in all its diversity for over two mil- including organic photochemistry, lennia. The pace of progress of these studies (and, reaction mechanisms, the synthe- in fact, in all areas of science) picked up markedly sis of chiral inhalation anesthetics, natural product chemistry, and toward the end of the eighteenth century, and has chemical ecology. His publications been increasing rapidly ever since. It was not until include the edited volumes Chem - twenty years after the ½rst performance of Stravin- ical Ecology: The Chemistry of Biotic sky’s The Rite of Spring (and, I was shocked to realize, Interaction (with Thomas Eisner, during my own lifetime!) that it became clear, with 1995) and Science and the Educated James Chadwick’s discovery of the neutron in 1932, Amer ican: A Core Component of Lib- that all ordinary matter is made up simply of protons, eral Education (with John G. Hilde- brand, 2010). He is Secretary of the neutrons, and electrons. While protons and neutrons American Academy and Cochair of were at ½rst believed to be the fundamental parti- the Academy’s Committee on Stud- cles making up atomic nuclei, they have since the ies and Publications. 1960s been best understood as composite subatomic

Intro - particles, each made up of three inseparable istence of unknown “missing” elements duction quarks. Protons (which carry a single posi- that remained for future research to dis- tive charge) consist of two up quarks and cover. Our understanding of how, where, one down quark. Neutrons (electrically neu- and when all the elements with an atomic tral) comprise one up quark and two down number greater than two (quaintly re- quarks. Interestingly, this revolutionary ferred to as “metals” within the astronom- structural insight into the nature of mat- ical community) were produced is much ter has had no impact at all on our under- more recent and still somewhat incom- standing of chemistry. plete. Anna Frebel’s account in this volume The simplest atom, hydrogen (H), which of the origin of these elements following is not only the most abundant form of or - the Big Bang is a fascinating story that is dinary matter in the observable universe much less well known (even among chem- but also the most abundant atom in our ists) than it deserves to be. Her essay own bodies, consists of a single nuclear pro- (among the astronomy contributions) proton and a single planetary electron. The vides an ideal introduction to the very ex - addition of one or two neutrons to the pro- istence of chemistry and of life itself. ton yields the hydrogen isotopes deuterium and tritium, respectively. The combination Christopher Cummins responded to our of two nuclear protons and two neutrons, invitation to write an essay on inorganic along with two planetary electrons, pro- chemistry by examining the chemistry of a duces an atom of helium (He). With the ex- single element: phosphorus (P). In his exception of tiny amounts of lithium (Li), ploration of phosphorous, we learn that whose nucleus contains three protons, py rophoric (spontaneously combustible) these are the sole types of atom produced ele mental white phosphorus consists of P as a consequence of the Big Bang some 13.8 discrete 4 molecules in which each phos- billion years ago. From a chemist’s view- phorus atom occupies the vertex of a tet- point, there are some extremely important rahedron, the simplest of the ½ve Platonic differences between hydrogen and helium solids. (By a striking coincidence, Plato atoms. Hydrogen atoms are able to bond tells us that Timaeus considered the “ele- to many other types of atoms to form stable ment” ½re to be composed of tetrahedral molecules such as elemental hydrogen particles!) H H O NH ( 2), water ( 2 ), ammonia ( 3), meth- Cummins’s research on how the synthe- CH ane ( 4), and literally millions of other sis of phosphorus-containing compounds “organic” compounds (all of which also might be greatly simpli½ed exempli½es contain carbon). Helium atoms, in con- the oretical and experimental chemical trast, prefer to remain alone. thinking at its best. From a consideration It was not until the mid-nineteenth cen- of the complex genesis and low abundance tury (1869) that Dmitri Mendeleev taught of phosphorus in the universe (where it is us that all the known elements, when listed relatively rare) and in living organisms according to increasing atomic number (where it occurs in concentrations much (the number of protons in the nucleus), higher than it does in our solar system), we could be arranged into a “periodic table.” move on to accounts of its vital importance His table revealed that the fewer than one in agriculture and industry. hundred naturally occurring elements fall Tracing the passage of phosphorus from into periodically recurring groups (such as the soil into plants, then into animals, and noble gases and halogens), a ½nding that ½nally into the sea illuminates some seri- enabled him to predict (correctly) the ex- ously underappreciated ecological con-

6 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 9:59 AM Page 7

cerns. The sad tale of the rise and fall of nucleic acids, and the myriad “small mole- Jerrold the chemically innovative Canadian re- cules” that serve as molecular messengers Meinwald search project that aimed at improving throughout nature, dramatic and even un - agricultural phosphorus use by creating a imaginable improvements in the practices breed of pig, the Enviropig, able to digest of medicine (including psychiatry) and ag- plant-derived phosphorus compounds in riculture are certain to play a prominent its diet better than any previously existing role in the twenty-½rst century. In another breed of pig, brings this essay to a close. It direction, with the successful synthesis ½rst may come as a surprise that the study of a of the simplest organic molecules (urea, single element reaches into such a wide ethyl alcohol, vanillin) and then of many of range of human concerns, spanning agri- the much more complex structures (cho- culture, industry, the health sciences, ecol- lesterol, vitamin B-12, insulin) far behind ogy, and even the social sciences. Readers of us, can the construction of synthetic vi - Cummins’s essay will be rewarded not on- ruses and even living cells be far off? ly with insights into some beautiful sci ence, Another major opportunity for research but also with extensive material for chem - that is occupying the attention of many istry-based cocktail party conversation. contemporary chemists is the development of materials science, an area discussed The borders between the classical scien- in Fred Wudl’s essay. As a result of many ti½c disciplines are rapidly disappearing, advances in physics, we know vastly more but continuing in the more or less tradi- than we did only a few decades ago about tional ½elds of inorganic and physical chem- the way atomic and molecular interactions istry, John Meurig Thomas has given us influence the macroscopic properties of an intriguing essay on chemical catalysis, all sorts of materials. We know why some embedded in a wide-ranging examination materials are brittle, flexible, good elec- of the importance of unpredictability and trical conductors, good electrical insula- chance within and beyond chemistry. His tors, magnets, or light emitters or ab- vision of the “chemist” leans in the direc- sorbers. As the physics of all these phe- tion of what used to be termed “natural nomena is better understood, it becomes philosopher.” He provides a refreshing possible for chemists to design and pro- view of the world of chemical research, duce new materials from which everything with an emphasis on the importance of from “improved” fabrics to computers, entirely unanticipated discoveries and un- airplanes, and televisions can be made. foreseen practical applications. His case Quite remarkably, the element carbon studies serve to remind us of the remark- plays a central role in the design of many able value of curiosity-driven research. of the novel materials with desirable prop- There is an important message here for erties, such as “self-healing” plastic (vitri- society at large with respect to shaping the mers) or solar photovoltaic cells. Wudl’s most productive science policy. essay outlines how this area of chemistry The discipline of chemistry has quietly evolved and what we may expect from it undergone an absolutely remarkable trans - in the decades to come. formation (or perhaps expansion would be Chaitan Khosla has focused his essay on a better term) over the last half-century. the ½eld that has become known as chem- This development has manifested itself in ical biology. Perhaps influenced by the an - part through the examination of biology as cient Greek aphorism “know thyself,” he a molecular science. With our increasing places particular emphasis on the roles understanding of the chemistry of proteins, that chemistry plays in understanding (as

143 (4) Fall 2014 7 Book_Fall 2014_Shinner.qxd 9/8/2014 9:59 AM Page 8

Intro - well as improving) the lives of Homo sapi- The world of computational chemistry duction ens. Chemistry lies at the heart of much is a far cry from the chemistry labs of our medical research, from the development youth, with their litmus paper, Bunsen of noninvasive imaging techniques such burners, and distilling flasks. The pungent as mri and pet scans to the discovery of odors of bromine or nitrobenzene, the new molecular targets that may serve as beau ty of deep-purple potassium perman- the basis for the design of much-needed, ganate crystals, the brilliance of burning novel anti-infective agents to help battle magnesium, the eerie blue glow of luminol malaria, Lyme disease, sars, and many treated with hydrogen peroxide (experi- other threats to human health. In some ences that have attracted generations of areas, the chemistry and biology relevant young students to chemistry in the past) to health is already fairly well understood, are absent from this new world. They are and “translation” from theory to applica- replaced by the less sensual but neverthe- tion can be expected to proceed smoothly. less deeply satisfying insights that only the In others, such as the chemical/biological computer can give! There can be no doubt understanding of brain functioning or the that much of the sort of chemical research control of development, basic research re- that is now being carried out in the con- mains an essential precursor to human ventional laboratory with actual chemicals ap plications. Khosla’s essay illuminates a will be done within the next few decades ½eld the chemical basis of which is not faster, cheaper, and more safely by compu- yet widely enough appreciated. tational chemists sitting in their of½ces.

Chemistry is an experimental science. In summation, what we have here is a ½ve- Some of its complexity derives from the course chemical tasting menu. It would fact that it deals typically with huge num- have been possible to choose ½ve entirely bers of molecules at a time; after all, an different aspects of chemistry that would 24 H O ounce of water contains about 10 2 have given an equally appropriate account molecules, roughly equal to the estimated of the rapid advance of this lively disci- number of stars in the observable universe. pline. In many menus, some of the courses Nevertheless, the enormous power of con - or wine-pairing descriptions use unfamil- temporary computers, combined with fun- iar terminology. Understandably, we are in - damental physical insights provided by the clined to avoid incomprehensible or un- development of quantum mechanics, has pronounceable items. Describing chem- resulted in the birth of computational istry presents an analogous challenge; one chemistry, which provides both explana- of the great problems in writing about sci- tions and predictions of chemical proper- ence is how to eliminate jargon. But this ties and behavior. K. N. Houk and Peng Liu dif ½culty can be readily taken care of these describe examples of the power of com- days simply by googling the ob scure terms putational chemistry in predicting the (Wikipedia also offers highly informative products of chemical reactions, in under- accounts of everything from the Platonic standing the course of newly discovered solids, neutrons, the periodic table, and catalytic reactions, and even in designing phosphogypsum to the Enviropig and synthetic enzymes capable of catalyzing pet scans). Whether chemistry always in - reactions for which no natural enzymes trigued you, or whether it was your worst exist. Although the power of computation- subject in high school, I hope you will ½nd al chemistry is now apparent, the science yourself enjoying what our dedicated au- is still in its infancy. thors have to say. Bon appétit!

8 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 9

Phosphorus: From the Stars to Land & Sea

Christopher C. Cummins

Abstract: The chemistry of the element phosphorus offers a window into the diverse ½eld of inorganic chemistry. Fundamental investigations into some simple molecules containing phosphorus reveal much about the rami½cations of this element’s position in the periodic table and that of its neighbors. Addition - ally, there are many phosphorus compounds of commercial importance, and the industry surrounding this element resides at a crucial nexus of natural resource stewardship, technology, and modern agriculture. Questions about our sources of phosphorus and the applications for which we deploy it raise the provocative issue of the human role in the ongoing depletion of phosphorus deposits, as well as the transfer of phosphorus from the land into the seas.

Inorganic chemistry can be de½ned as “the chemistry of all the elements of the periodic table,”1 but as such, the ½eld is impossibly broad, encompassing everything from organic chemistry to materials science and enzymology. One way to gain insight into and appreciate the rapidly moving and diverse ½eld of inorganic chemistry is to view the science from the perspective of the elements themselves, since they are the basic ingredients for assembling molecules or materials–and indeed, all matter, living or inanimate. Although phosphorus may be less cele- brated than carbon or hydrogen, it joins those elements (along with nitrogen, oxygen, and sulfur) to constitute the six “biogenic elements” (those needed CHRISTOPHER C. CUMMINS,a Fellow of the American Academy in large quantities to make living organisms; see Fig - 2 since 2008, is Professor of Chemis - ure 1). Let us take a look at some of the issues that try at the Massachusetts Institute of arise in inorganic chemistry from the perspective of Technology. His research focuses on phosphorus, illustrating in the process the notion innovating new methods of in or- that each element has its own story to tell. gan ic synthesis, as well as the syn - the sis of new simple substances. His work has recently appeared in Inor - Many phosphorus-containing chemical com- ganic Chemistry, Science, Journal of the pounds are commercially valuable and have interest- 3 American Chemical Society, and Chem - ing or important applications. Lithium hexafluoro - ical Science, among other journals. phosphate, for example, is the electrolyte in common

9 Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 10

Phosphorus: Figure 1 From the Periodic Table with Nonmetals, Including Biogenic Elements, Above the Stair-Step Line Stars to Land & Sea

Biogenic elements are H, C, N, O, P, S in the “nonmetals” region of the periodic table, indicated by the heavy line. Source: Adapted from a graphic found on http://www.openclipart.org.

lithium-ion batteries, which are used in phorus if chlorine is not even present in consumer electronics (such as laptops) the products, such as lithium hexafluoro- and automotive applications.4 So how is phosphate, that are the target of synthesis? it made? The synthesis route begins with These industry standard processes suggest the white form of elemental phosphorus, there is room for improvement: if manu- a simple molecular form of the element facturers eliminated the use of chlorine P consisting of tetrahe dral 4 molecules in the synthesis of important phosphorus (Figure 2).5 White phosphorus is com- compounds in which chlorine is absent, bined with elemental chlorine in order to both hazards and waste would be signi½- bring the phosphorus to the correct oxi- cantly reduced. dation state (+5), and then, in a second Because our research has shown that it is step, chloride is replaced by fluoride. indeed possible to derive organo-phospho- This process is also frequently used to rus compounds directly from white phos- synthesize many organo-phosphorus com- phorus, this is an opportunity for inor- pounds that are important components of ganic chemistry to improve the safety and catalysts used in the chemical industry.6 ef½ciency of the manufacturing process. In these applications, again, white phos- In one advance, we showed that phospho- phorus is ½rst oxidized using chlorine, and rus-carbon bonds can be generated by then the chloride provides the basis for the using white phosphorus together with a formation of carbon-phosphorus bonds.7 source of organic radicals.9 Each of the six But notably, elemental chlorine is hazard- phosphorus-phosphorus bonds present in ous to use and ship, and environmental a molecule of white phosphorus absorbs groups have called for an outright ban on two organic radicals in the process of being 8 it. So why use chlorine to oxidize phos- broken; each P4 tetrahedron is broken

10 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 11

Figure 2 Christopher P C. Cummins Tetrahedral Arrangement of Atoms in a 4 (White Phosphorus) Molecule

Source: Generated by the author using the platon program. See A. L. Spek, “Single-Crystal Structure Valida- tion with the Program platon,” Journal of Applied Crystallography 36 (2003): 7–13.

completely apart, and each phosphorus in a wide variety of structural arrange- atom becomes incorporated into a freshly ments, all of which are networks exclusive- formed organo-phosphorus compound. ly based upon phosphorus-phosphorus Our method for developing this new sin gle bonds, three for every phosphorus pro cess was derived from basic inquiries node. The variant known as red phospho- into phos phorus’s relationship to the ele- rus, for example, has cages of phosphorus ments neighboring it on the periodic ta ble. atoms connected into linear tubes (see Fig- Phosphorus is immediately beneath nitro- ure 3),12 which in turn are cross-linked to - gen on the periodic table, suggesting that geth er to form a polymeric net work. these el e ments should have some sim ilar i - Knowing this, we were inspired to ask: ties in their chemical properties. Then why, can we design and synthesize a molecule we won dered, was it the case that, while that would be prone to a fragmentation re- Earth’s atmosphere consists mainly of ac tion wherein one of the fragments pro- N P triply-bonded 2 molecules, a similar di - duced would be the diatomic molecule 2? atom ic molecular form of phosphorus is If we could, we would have the opportu- neither prevalent nor even particularly sta- nity to study the properties and chemical b l e ? 10 Part of the answer is that nitrogen characteristics of an all-phosphorus mol- is unusual because the stability of its mul- ecule structurally analogous to the main tiple bond far exceeds that of the sum of constituent of Earth’s atmosphere. In our an equivalent number (three) of its single ½rst attempt to produce it, the selected de- bonds. So the only stable form of elemen- sign incorporated a feature patterned after tal nitrogen is the diatomic molecular form the reaction used to inflate an automobile floating innocuously about in the atmos- airbag in the event of a collision, a process phere we breathe; in contrast, phosphorus that rapidly generates nitrogen gas from a (like its diagonal relative, carbon)11 exists solid precursor. Our target molecule em -

143 (4) Fall 2014 11 Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 12

Phosphorus: Figure 3 From the Arrangement of Atoms in One of the Representative Structural Forms of Red Phosphorus Stars to Land & Sea

The box encloses one crystallographic unit cell. Source: Generated by the author using crystallographic coordinates from M. Ruck et al., “Fibrous Red Phosphorus,” An gewandte Chemie International Edition 44 (2005), doi:10.1002/ anie.200503017.

bed ded a diphosphorus moiety into the generation should not matter. Could there P sta bilizing environment of a niobium be a way to access the 2 molecule by start- complex (niobium is a transition metal; ing from a stable form of the element, rath- it forms complexes by arranging sets of er than from an exotic niobium complex? molecules or ions–called ligands–around We found the suggestion in a lightly cited itself ), from which it could be released by 1937 paper that the photochemical conver- a stimulus of mild heating.13 Carrying out sion of white phosphorus into the red form the fragmentation reaction in the presence of the element may occur with P2 as the of other molecules permitted the mapping key intermediary, which is initially gener - of the reactivity patterns of diatomic phos - ated and subsequently polymerizes.14 phorus. One important result was the dis - We found by experiment (see Figure 5) that P covery that 2 easily undergoes addition to the addition of methyl isoprene to a solu- unsaturated organic molecules, such as 1,3- tion of white phosphorus during irradia- cyclohexadiene (see Figure 4). tion both inhibits the production of red If diatomic molecular phosphorus is in - phos phorus and yields molecules in the deed capable of direct combination with same class of organo-phosphorus com- organic molecules, then the means of its pounds that we studied earlier in connec-

12 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/18/2014 2:44 PM Page 13

Figure 4 Christopher P C. Cummins A Niobium Complex that Can Act as an “Eliminator” of 2 under Thermal Fragmentation neat neat

65 °C

P In the depicted sequence, transient 2 (not observed) combines with two molecules of 1,3-cyclohexadiene resulting in four new P-C single bonds in the stable ½nal product (shown both as a line drawing and in a thermal ellipsoid representation from a single-crystal X-ray diffraction analysis). Abbreviations: tBu is tert-butyl, Ar is aryl (spe - M C H M ci½cally 3,5- e2 6 3), and es* is supermesityl. Source: Adapted from material published in N. A. Piro, J. S. Figueroa, J. T. McKellar, and C. C. Cummins, “Triple-Bond Reactivity of Diphos phorus Molecules,” Science 313 (2006), doi: 10.1126/science.1129630.

A tion with niobium-mediated access to di - phase s4 mole cules, one heats grey arse - phosphorus molecules.15 Hence, in effect nic (which has a layered sheet structure and in principle, we have shown that in rem iniscent of graph ite or black phospho - certain cases the hazardous and wasteful rus) to about 550 degrees C while flowing a use of chlorine in the synthesis of organo- carrier gas over it. The As4 molecules, en- phosphorus com pounds can be replaced trained in the carrier gas, can be led into a with a process rely ing on ultraviolet radia- solvent and used for reaction chemistry be- tion. fore re-polymer ization to grey arsenic can take place. If con densed to a solid on a A After viewing the beautiful tetrahedral cold surface, the s4 condensate is “yellow molecular form of elemental phosphorus arsenic,” but it cannot be kept. Warming in Figure 2, one might wonder whether this to room temper ature or ex posure to light arrangement of phosphorus is unique to brings about a facile return to the grey this particular element. Arsenic (As) lies form.16 just below phosphorus on the periodic ta- Phosphorus and arsenic lie on either side ble, separated from it by the stair-step line of the divide (marked on Figure 1) separat - dividing the metals from the nonmetals ing the metals from the non-metals. White (see Figure 1). Once again, the periodicity phosphorus is stable enough that it can be of chemical properties suggests that mol - stored as a pure liquid above its melting ecular arsenic might adopt a similar tet - point of 44 degrees Cand pumped into tank rahedral struc ture to that of phosphorus. cars for shipping; while, conversely, sam- Indeed, it does, but only in the gas phase ples of yellow arsen ic are eva nes cent. We where the molecules are well isolated from therefore wondered: would it be possible one another, or in solution at low temper- to synthesize a stable substance whose tet- ature and in the dark. To generate gas- rahedral mol ecules would be composed of

143 (4) Fall 2014 13 Book_Fall 2014_Shinner.qxd 9/18/2014 2:46 PM Page 14

Phosphorus: Figure 5 From the Combination of Phosphorus with Methyl Isoprene under the Action of Ultraviolet Light Stars to Land & Sea

P Methyl isoprene is the precursor to the original synthetic rubber. The reaction mechanism may involve 2 as a reactive photo-generated intermediate.17 Source: Generated by the author using data from Daniel Tofan and Chris - topher C. Cummins, “Photochemical Incorporation of Diphosphorus Units into Organic Molecules,” Angewandte Chemie International Edition 49 (2010), doi:10.1002/anie.201004385.

Figure 6 P A From the Commodity-Chemical 4 Molecule to the Unstable s4 Molecule

commodity chemical ? ? ? unstable

What are the properties of substances composed of tetrahedral molecules of mixed composition? Source: Adapted A P from Brandi M. Cossairt and Christopher Cummins, “Properties and Reactivity Patterns of s 3: An Experimental and Computational Study of Group 15 Elemental Molecules,” Journal of the American Chemical Society 131 (42) (2009), doi:10.1021/ja906294m.

a mixture of phosphorus and arsenic (see provides a benchmark for theorists work- Figure 6)? To test this idea, we made a ni - ing on the a priori predic tion of properties; obium complex carrying a P33− unit, and heavy elements pose the greatest challenge A P combined this with a source of arsenic in this regard. The elements in the s 3 (3+), effectively knitting together the neu - molecule are packaged together in a 1:3 A P 18 tral s 3 mole cule in a se lective fashion. ratio at the molecular level; and now this The new sub stance turned out to have a substance is readily avail able as a start ing waxy appearance much like that of white material. Substitution of a single nitro gen phosphorus, and it could be puri½ed by atom into the P4 tetrahedron has scarcely sublimation, wherein the pure material is been considered; one possibility involves condensed on to a cold probe. Because of stabiliza tion inside a recently dis covered A P B the volatile nature of s 3, we and our col - spherical 80 molecule that is analogous to C 20 laborators determined its prop erties by a Buckmin sterful lerene ( 60). variety of techniques, including electron diffraction, microwave spectros copy, and To ask why diatomic phosphorus is nei- photoelectron spectro scopy.19 Obtaining ther stable nor prevalent is really to ask a gas-phase property data on a sim ple mol- larger question: why is elemental phospho- ecule containing a heavy element (arsenic) rus not found on Earth as a pure substance,

14 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 15

uncombined with other chemical ele- ed great quantities of human urine, which Christopher ments? It is because elemental phosphorus he concentrated to a paste and subjected to C. Cummins is especially prone to oxidation, a process reductive distillation.) encouraged by Earth’s atmosphere at this Phosphate rock is not only the basis for point in history. Elements that form very the ½ne chemicals industry of phosphorus; stable oxides (such as aluminum, phos pho - it is also the starting point for the (much rus, and silicon) are not found in uncom- larger) phosphorus side of the fertilizer in- bined form on our planet unless they can dustry. The “wet process” of puri½cation be formed by biological or geological pro- uses sulfuric acid to generate phosphoric cesses taking place under anaer obic condi - acid from phosphate rock, after which it tions (as in the case of volcanic sulfur, or can be made into critical fertilizers such carbon in the form of coal and diamond). If as monoammonium phosphate, or map. we cannot obtain phosphorus in pure form Around 1940, the human population of our directly by digging it out of the ground, planet began to rise more rapidly than it where do we get it? had previously (since my birth in 1966 the Phosphate rock (also known by its min- population has doubled).25 This critical eral name apatite) is essentially the bones rise in population growth coincided with and teeth of ancient marine organisms two important developments in the fertiliz - formed into concentrated deposits where er industry: the worldwide commercial de- long-evaporated seas once stood.21 It is ex- ployment of the Haber-Bosch ammonia tracted through strip mining and forms synthesis (whereby ammonia for agricul- the basis for the phosphorus ½ne chemicals tural applications is obtained by direct industry. One of the principal methods for combination of the elements hydrogen and white phosphorus22 production is the nitrogen); and the large-scale mining of “thermal process,” which involves use of phosphate rock deposits, mainly for fertil - an electric arc furnace, carbon in the form iz er applications. Prior to the mid-twenti- of coke as a reducing agent, and silica to ab - eth century, humankind had been largely sorb the oxide ions liberated in the heating limited to locally available nutrients for process.23 The elemental phosphorus is crop production. Now, ammonia can be thus extracted from the rock in what is es - had in essentially limitless supply by com- sentially an expensive puri½cation process. bining the atmosphere’s inexhaustible sup- Note that most phosphorus-containing ply of nitrogen with hydrogen (which is commercial chemicals contain phospho- currently derived from natural gas by steam rus in the +5 oxidation state: the same as reforming). Can phosphorus keep up? is found in phosphate rock when it is dug out of the ground. The typical puri½cation Stars are the element factories.26 They process reduces phosphorus’s oxidation con sist mainly of our universe’s lightest state from +5 to zero; however, when it is and most abundant elements: hydrogen converted to other chemicals, chlorine is and helium. Red giants are more evolved often used to return the phosphorus to its stars with an onion-like layered structure; highest oxidation state (zero back to +5). the most abundant metallic elements, iron (This method of making white phospho- and nickel, make up their core, and layers rus is, in fact, reminiscent of the one used of progressively lighter elements surround by the alchemist Hennig Brand, who made them, moving outward to the surface. El- phosphorus the thirteenth element to be ements heavier than iron and nickel are obtained in pure form.24 In search of the formed by neutron capture when a massive philosopher’s stone, the alchemist collect- star explodes in a supernova, and these (in-

143 (4) Fall 2014 15 Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 16

Phosphorus: cluding the precious gold sought by the exhausted, leaving behind a legacy of ra - From the alchemist) are of minimal cosmic abun- dioactive phosphogypsum stacks and col - Stars to 30 Land & Sea dance. It is one of the peculiarities of nu - lapsing sinkholes. The term “peak phos- clear physics that nuclei of odd atomic phorus” is now used with reference to the num ber (odd Z) are generally less stable point in time when phosphate rock pro- and less abundant than those of even Z. duction (mining) will inevitably begin to The only stable isotope of phosphorus is taper off.31 Current estimates place peak 31P(Z = 15), and the 31Pnucleus is the prod - phosphorus some time later in the twenty- uct of an extremely improbable sequence ½rst century. of nuclear reactions (the ½nal reaction in Off the coast of Brittany, France, there the sequence converts 31Si into 31P by pro - are sometimes blooms of marine algae vast ton capture), only taking place during an enough to be visible from space.32 Brit- explosive neon burning phase in the core tany is a livestock-producing region where of massive, hot stars.27 Accordingly, the large amounts of phosphate from feed is cosmic abundance of phosphorus is lower transferred to the ground water and ulti- –by orders of magnitude–than that of the mately to the ocean. This perfectly illus- other ½ve biogenic elements. Indeed, to trates two consequences of the large-scale quote astrobiologist Douglas Whittet: mining of phosphate rock and industrial- “The only biogenic element present in the ized agricultural activity: ½rst, we are de- human body (and in biological tissue gen- pleting the concentrated reservoirs of this erally) at a concentration substantially key nutrient; second, its dispersal into the above its solar abundance is P. If one were world’s oceans can have negative effects on to attempt to place an upper limit on the marine ecosystems, chiefly by causing eu- total biomass present in the Universe at trophication through overgrowth of cer- large, on the basis of cosmic abundances, tain species of phytoplankton. then the critical element would be phos- What can we do to mitigate the move- phorus.”28 This is in keeping with the ob - ment of phosphorus from land to sea? Ef - servation that, in many of the ecosystems forts are being directed at optimizing the on Earth, phosphorus is life-limiting. This separation and recovery of phosphorus means that the addition of phosphorus from waste water, which is an important (usually in the form of phosphate) will direction.33 In some countries (such as In- bring about an abrupt bloom of life, since dia and Sweden), the use of toilets that sep- the absence of phosphorus was all that was a rate liquid from solid waste is being adopt- holding it back. ed; phosphate can then be recovered from Our land reserves of phosphorus are ½ - urine as the crystalline mineral struvite, nite. And given the ongoing depletion of while solid waste is composted.34 Pigs can- phosphate rock reserves, it is natural to not digest plant-derived phosphate because ask what is left, where it is, and how long it of the phytic acid form in which plants will last. The U.S. Geological survey indi- store it (see Figure 7), so researchers at the cates that roughly three-quarters of the University of Guelph in Canada developed available reserves are concentrated in Mo - the Enviropig.35 This genetically engi- rocco and Western Sahara.29 Mining loca- neered pig secretes the enzyme phytase in tions in Florida and Idaho contain the most its saliva, enabling the pig to digest the signi½cant amount of phosphate rock in plant phosphate, whereupon its excreta are the United States, but these constitute a phosphate-poor, leading to an improve- small percentage of global reserves. And ment in waste water quality. While the Central Florida’s mines have been largely meat of the Enviropig is the same as that of

16 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 17

Figure 7 Christopher Molecular Structure of Phytic Acid C. Cummins

Source: Generated by the author using the program MarvinSketch. Marvin is used for drawing, displaying, and characterizing chemical structures, substructures, and reactions. See ChemAxon, Marvin 6.1.3 (2014), http://www.chemaxon.com.

an unmodi½ed pig, concerns about this identify ways of using this limited resource cre ative kind of genetic engineering have that minimize waste, but we acknowledge effectively blocked its adoption thus far. our limited ability to grapple with the con- The chemistry of an element is a fasci- sequences of enormous demand for phos - nating thing, and we have explored several phorus–a markedly limited resource– of the issues that flow naturally from ask- stemming from a rapidly rising human ing questions about where an element population. Phosphorus, therefore, is in - comes from, what we use it for, and how teresting not only for its chemistry but we might gain an improved understanding also in light of the rich texture of its larg- of it. Motivated to study phosphorus by cu- er story, only one of the many stories that riosity and a desire to expand on funda- emerge when we view inorganic chem- mental science, we have come to appreci- istry from the perspective of a single ele- ate the vital role played by this relatively ment. precious element that forms the inorganic backbone of dna, the energy currency of atp, and the main component of bones and teeth. We have demonstrated the ability to

143 (4) Fall 2014 17 Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 18

Phosphorus: endnotes From the Stars to Author’s Note: I am grateful for stimulating conversations with Professor Brandi M. Cossairt, Land & Sea Pro fessor Anna Frebel, Professor Jerrold Meinwald, and Dr. Willem Schipper. This material is based upon work supported by the National Science Foundation under che-1362118. 1 Norman Neill Greenwood and Alan Earnshaw, Chemistry of the Elements (Oxford: Butterworth- Heinemann, 1997). 2 D. C. B. Whittet and J. E. Chiar, “Cosmic Evolution of the Biogenic Elements and Compounds,” The Astronomy and Astrophysics Review 5 (1993), doi:10.1007/BF00872922. 3 B. Elvers and U. Fritz, Phosphorus Compounds, Inorganic to Plastics, Additives (Weinheim, Ger- many: Wiley-vch, 2011). 4 Kazunori Ozawa, ed., Lithium Ion Rechargeable Batteries: Materials, Technology, and New Appli- cations (Weinheim, Germany: Wiley-vch, 2009). 5 Greenwood and Earnshaw, Chemistry of the Elements. 6 Paul C. J. Kamer and Piet W. N. M. van Leeuwen, Phosphorus (III) Ligands in Homogeneous Catal - ysis: Design and Synthesis (West Sussex; Chichester, U.K.: John Wiley & Sons, 2012). 7 Ibid. 8 R. Ayers, “The Life-Cycle of Chlorine, Part I: Chlorine Production and the Chlorine-Mercury Connection,” Journal of Industrial Ecology 1 (1997), doi:10.1162/jiec.1997.1.1.81. 9 Brandi M. Cossairt and Christopher C. Cummins, “Radical Synthesis of Trialkyl, Triaryl, Trisi- lyl and Tristannyl Phosphines from P4,” New Journal of Chemistry 34 (2010), doi:10.1039/c0nj 00124d. 10 W. E. Dasent, Nonexistent Compounds: Compounds of Low Stability (New York: M. Dekker, 1965). 11 Keith B. Dillon, François Mathey, and John F. Nixon, Phosphorus: The Carbon Copy: From Organo- phosphorus to Phospha-organic Chemistry (New York: Wiley, 1998). 12 M. Ruck, D. Hoppe, B. Wahl, P. Simon, Y. Wang, and G. Seifert, “Fibrous Red Phosphorus,” An- gewandte Chemie International Edition 44 (2005), doi:10.1002/anie.200503017. 13 N. A. Piro, J. S. Figueroa, J. T. McKellar, and C. C. Cummins, “Triple-Bond Reactivity of Diphos - phorus Molecules,” Science 313 (2006), doi:10.1126/science.1129630. 14 G. Rathenau, “Optische und photochemische versuche mit phosphor,” Physica 4 (1937): 503– 514, http://www.sciencedirect.com/science/article/B6X42-4F0H2V0-6T/2/62b38c13a477cb0 dc6b01d06d9a76727. 15 Daniel Tofan and Christopher C. Cummins, “Photochemical Incorporation of Diphosphorus Units into Organic Molecules,” Angewandte Chemie International Edition 49 (2010), doi:10.1002/ anie.201004385. 16 A. Rodionov, R. Kalendarev, J. Eiduss, and Yu. Zhukovskii, “Polymerization of Molecular (Yel- low) Arsenic,” Journal of Molecular Structure 380 (1996), doi:10.1016/0022-2860(95)09195-5. 17 Lee-Ping Wang, Daniel Tofan, Jiahao Chen, Troy Van Voorhis, and Christopher C. Cummins, “A Pathway to Diphosphorus from the Dissocation of Photoexcited Tetraphosphorus,” Royal Society of Chemistry Advances 3 (2013), doi:10.1039/c3ra43940b. 18 Brandi M. Cossairt, Mariam-Céline Diawara, and Christopher C. Cummins, “Facile Synthesis of AsP3,” Science 323 (2009), doi:10.1126/science.1168260. 19 Brandi M. Cossairt, Christopher C. Cummins, Ashley R. Head, Dennis L. Lichtenberger, Raphael J. F. Berger, Stuart A. Hayes, Norbert W. Mitzel, and Gang Wu, “On the Molecular and Electronic Structures of AsP3 and P4,” Journal of the American Chemical Society 132 (2010), doi:10.1021/ja102580d; and Adam M. Daly, Brandi M. Cossairt, Gavin Southwood, Spencer J. Carey, Christopher C. Cummins, and Stephen G. Kukolich, “Microwave Spectrum of Arsenic

18 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 19

Tri phosphide,” Journal of Molecular Spectroscopy 278 (2012): 68–71, http://www.sciencedirect Christopher .com/science/article/pii/S0022285212000410. C. Cummins 20 Jules Tshishimbi Muya, Erwin Lijnen, Minh Tho Nguyen, and Arnout Ceulemans, “Encapsula- tion of Small Base Molecules and Tetrahedral/Cubane-Like Clusters of Group V Atoms in the Boron Buckyball: A Density Functional Theory Study,” The Journal of Physical Chemistry A 115 (2011), doi:10.1021/jp107630q. 21 Eric Oelkers and Eugenia Valsami-Jones, “Phosphate Mineral Reactivity and Global Sustain - ability,” Elements 4 (2008): 83–87, http://elements.geoscienceworld.org/cgi/content/abstract/4/ 2/83. 22 White phosphorus is a commodity chemical that is also in demand for military applications, in which setting it is valued for its incendiary effects (including the production of smoke screens) and is known as “Willy Pete.” Small quantities of white phosphorus for research pur - poses were previously available from many chemical suppliers. After 2001, white phosphorus ceased to be available from catalog suppliers in the United States, but since red phosphorus was still available and can be converted to the white form by simple thermal depolymerization, this was not a major impediment to researchers. Now, both red and white phosphorus are Drug En - forcement Agency (dea) List 1 controlled chemicals, which has led to dif½culty in purchas- ing either form of the element. Why is phosphorus on this list? While there is no phosphorus whatsoever in the chemical composition of the much-abused drug methamphetamine, a pop - ular street method for synthesizing the drug involves a combination of phosphorus and hydro - gen iodide as a reducing agent for ephedrine. For the same reason, elemental iodine is also a dea List 1 chemical. My research group was fortunate to receive a gift of white phosphorus from Thermphos (see endnote 23); after arriving from the Netherlands, however, the shipment (which was around half a kilogram) was held up in customs in New Jersey until, with the assistance of mit’s general council and a customs broker properly licensed to receive shipments of List 1 chemicals, we were able to free the shipment. For more on the conversion of red to white phosphorus, see J. Brodkin, “Preparation of White Phosphorus from Red Phosphorus,” Journal of Chemical Education 37 (2) (1960), doi:10.1021/ ed037pA93.1. For more on the use of phosphorus in methamphetamine synthesis, see Harry F. Skinner, “Methamphetamine Synthesis via Hydriodic Acid/Red Phosphorus Reduction of Ephedrine,” Forensic Science International 48 (1990): 123–134, http://www.sciencedirect .com/science/article/pii/0379073890901047. 23 A company called Thermphos (recently gone out of business) in the Netherlands was situated adjacent to a nuclear power plant for the cheap electricity, and took its phosphate rock shipments from Florida, now mostly mined out. Thermphos was taking a leadership role toward the exciting goal of making phosphorus from waste and thereby realizing a vision of a sustainable phosphorus industry. White phosphorus is made in the United States as the ½rst step in the syn - thesis of glyphosate. See A. D. E. Grossbard, The Herbicide Glyphosate (London; Boston: Butter - worths, 1985). 24 John Emsley, The 13th Element: The Sordid Tale of Murder, Fire, and Phosphorus (Malden, Mass.: John Wiley & Sons, Inc., 2000). 25 Jan Willem Erisman, Mark A. Sutton, James Galloway, Zbigniew Klimont, and Wilfried Win - iwarter, “How a Century of Ammonia Synthesis Changed the World,” Nature Geoscience 1 (2008), doi:10.1038/ngeo325. 26 Anna Frebel, “Reconstructing the Cosmic Evolution of the Chemical Elements,” Dædalus 143 (4) (2014): 71–80; and Sean G. Ryan and Andrew J. Norton, Stellar Evolution and Nucleosyn thesis (Cambridge: Cambridge University Press, 2010). 27 Enrique Maciá, “The Role of Phosphorus in Chemical Evolution,” Chemical Society Reviews 34 (2005), doi:10.1039/B416855K. 28 Whittet and Chiar, “Cosmic Evolution of the Biogenic Elements and Compounds.”

143 (4) Fall 2014 19 Book_Fall 2014_Shinner.qxd 9/8/2014 10:00 AM Page 20

Phosphorus: 29 Tina-Simone S. Neset and Dana Cordell, “Global Phosphorus Scarcity: Identifying Synergies From the for a Sustainable Future,” Journal of the Science of Food and Agriculture 92 (2012), doi:10.1002/ Stars to jsfa.4650. Land & Sea 30 P. Rutherford, M. Dudas, and R. Samek, “Environmental Impacts of Phosphogypsum,” Science of the Total Environment 149 (1994), doi:10.1016/0048-9697(94)90002-7; and C. Hull and W. Bur nett, “Radiochemistry of Florida Phosphogypsum,” Journal of Environmental Radioactivity 32 (3) (1996), doi:10.1016/0265-931X(95)00061-E. 31 Dana Cordell and Stuart White, “Peak Phosphorus: Clarifying the Key Issues of a Vigorous De - bate about Long-term Phosphorus Security,” Sustainability 3 (2011): 2027–2049, http://www .mdpi.com/2071-1050/3/10/2027. 32 Gabriel M. Filippelli, “The Global Phosphorus Cycle: Past, Present, and Future,” Elements 4 (2008): 89–95, http://elements.geoscienceworld.org/cgi/content/abstract/4/2/89. 33 S. A. Parsons and J. A. Smith, “Phosphorus Removal and Recovery from Municipal Waste- waters,” Elements 4 (2008): 109–112, http://elements.geoscienceworld.org/cgi/content/ abstract/4/2/109; and Kersti Linderholm, Anne-Marie Tillman, and Jan Erik Mattsson, “Life Cycle Assessment of Phosphorus Alternatives for Swedish Agriculture,” Resources, Conservation and Recycling 66 (2012): 27–39, http://www.sciencedirect.com/science/article/pii/S092134 4912001048. 34 Dana Cordell, Jan-Olof Drangert, and Stuart White, “The Story of Phosphorus: Global Food Security and Food for Thought,” Global Environmental Change 19 (2009): 292–305, http:// www.sciencedirect.com/science/article/pii/S095937800800099X. 35 S. P. Golovan, R. G. Meidinger, A. Ajakaiye, M. Cottrill, M. Z. Wiederkehr, D. J. Barney, C. Plante, J. W. Pollard, M. Z. Fan, M. A. Hayes, J. Laursen, J. P. Hjorth, R. R. Hacker, J. P. Phillips, and C. W. Forsberg, “Pigs Expressing Salivary Phytase Produce Low-Phosphorus Manure,” Na ture Biotechnology 19 (2001), doi:10.1038/90788.

20 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 21

Foresight, Unpredictability & Chance in Chemistry & Cognate Subjects

John Meurig Thomas

Abstract: In numerous branches of natural philosophy, the ways in which major, transformative ad - vances are achieved are often cloaked in mystery, or arrived at through a fortunate concatenation of circumstances. This theme is pursued here with the aid of some examples from my own work on catalysis (the speeding up of the attainment of chemical equilibria), as well as from the work of others. The emergence of the maser (forerunner of the laser), the development of positron emission tomography, and the crea - tion of blood-glucose sensors for use by those suffering from type 2 diabetes are among the innovations adumbrated here. In addition to describing the unpredictable nature of much scienti½c discovery, I also describe areas in which new chemical technology will be especially bene½cial to society. I foresee that open- structure solid catalysts are likely to transform many of the ways in which chemicals, now manufactured in an environmentally harmful manner, will be produced in the future. Also outlined is the vital need to un - derstand and exploit photocatalysts so as to harness solar energy. Finally, I touch upon the absolute value of chemistry in the quest for beauty and truth.

Nearly ½fty years ago, while watching Nobel lau- bbc JOHN MEURIG THOMAS, a For- reate John Kendrew present a program on the eign Honorary Member of the molecules of life, I was surprised to hear him remark American Academy since 1990, is that even expert scientists cannot usually predict Honorary Professor in the Depart- what will happen in their ½elds more than three years ment of Materials Science at the in advance.1 How could it be, I wondered, that sci- University of Cambridge and Emer - enti½c giants like him could hold such a view? Do itus Professor at the Davy-Faraday road maps used by scientists become invalid after a Laboratory, London. Formerly, he mere three years? As my knowledge of advances in was Director of the Royal Institu- tion of Great Britain, London, and chemistry and adjacent ½elds grew, however, I began Head of the Department of Physi- to feel that Kendrew’s view is close to the truth. Be- cal Chemistry and Master (Head) fore venturing into the past fortunes and future pos - of Peterhouse, Cambridge. His pub - sibilities of chemistry, I will ½rst demonstrate the lications include Principles and Prac- veracity of Kendrew’s statement with the aid of three tice of Heterogeneous Catalysis (with historical examples that underline the unpredictable W. J. Thomas, 2014) and Michael nature of advances in science and technology. Faraday and the Royal Institution: The Genius of Man and Place (1991). He In 1937, President Roosevelt asked a group of expert was knight ed in 1991 for services to scientists, engineers, and businessmen for advice on chem istry and the popularization what developments in science and technology could of science. likely be expected in the future, in part so that he

Foresight, could better serve his fellow Americans and the dirigible for long-distance and trans- Unpredict - foster the common good. The report cor- oceanic travel, and the airplane for over- ability & 4 Chance in rectly foresaw that agricultural science land routes. It offered the hypothesis that Chemistry would play an ever-increasing role in the “experimental work which is being car- & Cognate Subjects economy of the United States. It also pre- ried out in producing steam drive for air- dicted that gasoline and other useful prod- plane service has given such promising re - ucts could be readily generated from coal.2 sults that it is quite possible a combined But in retrospect, it is not what the experts steam boiler and steam turbine will be in identi½ed as likely advances that make the extensive use within the next few years.” report so interesting: it is the advances that This never came to pass for a variety of were missed.3 reasons that require no elaboration here.5 Roosevelt’s carefully selected experts My third example involves the discov- can be forgiven for not mentioning phe - ery of the maser, the forerunner of the la- nom ena or effects that had not yet been ser. In the 1950s at Columbia University, dis cov ered: nuclear ½ssion, nuclear fusion, phys icist Charles Townes became in - the transistor, the laser, space satellites, and trigued by the possibility that the popula- jet aircraft; or, looking further ahead, ge - tion of energy levels in simple molecules ne tic engineering, genetic ½ngerprinting, could be inverted, and by what the optical the struc ture of dna, and immunosup - consequences of such an inversion might pres sive drugs (which make organ trans - be. When he proposed an experiment in- plantation possible). However, this collec- volving the inversion process, his colleague tion of experts surprisingly omitted to iden- Is idor Rabi, a Nobel Prize–winning physi - ti fy antibi otics, fuel cells, fax machines, or cist, told him he was wasting his time. Oth- syn chro tron radiation, all of which were er eminent scientists, including phys icist al ready known well before 1937. Perspica- Niels Bohr and mathematician John von cious phys icist members of that panel Neumann, doubted the worthiness of the might have even identi½ed tomography, experiment. But Townes stubbornly per- the mathematical foundations of which se vered and so discovered the maser. Its had been described by mathematician Jo- logical successor, the laser, has changed hann Radon in Leipzig in 1917. Also in 1917, our world comprehensively. (In both ma - Einstein published his famous paper in sers and lasers, mi cro wave and visible light, which Einstein coef½cients (which pertain respectively, are emitted in a polarized to elec tronic energy levels in atoms and state, with high, ad justable intensity and ma terials) were ½rst described. From this a sharply de½ned wavelength.) In addition, account, an “expert” might even have pre- Townes’s work led to the discovery that dicted the existence of masers and la sers! nearby galaxies emit maser light, which My second example comes from the falls upon Earth. j o u rn a l Scienti½c American (a publication to It is against a background of such errant which my education owes a great deal). In and incomplete predictions on the part of 1920, as part of a special anniversary unim peachable experts that I embark on issue, it ran a series of articles authored the present essay on foresight and chance by eminent experts on the theme of “The in chemistry. As we shall see, some prac- Fu ture as Suggested by Developments of tical advances emerge through foresight the Past Seventy-Five Years.” Notably, an and as a result of rectilinear progress in a article by the selected expert on aviation well-trod den ½eld; others, however, arise claimed that, in the future, aerial travel from unexpected sources and observa- vessels would be divided into two types: tions.

22 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 23

My principal areas of research as a come poisoned by accumulated build-up John chemist are heterogeneous catalysis and of carbonaceous material at their exterior Meurig Thomas chemical electron microscopy. In the for- surface. By carefully burning the “poison” mer, a solid catalyst speeds up the rate of in air, the catalyst can be regenerated. attainment of chemical equilibrium when Although my own studies of catalysts are one, two, or more potential reactant mol- primarily of an academic nature, catalysis ecules of different kinds of gases or liquids is a vitally important means of sustaining are brought together in its presence. The modern civilized life. One example of this kind of question I seek to answer is the fol- is the enormous array of products derived lowing. How is it that molecules impinging from oil, almost always with the assistance upon certain catalytic surfaces at velocities of catalysts (see Figure 1). of typically 1600 km/h–1 can be converted Another striking reminder of how vital at that surface, with high ef½ciency and catalysis is to modern life is illustrated in often with spectacular selectivity, into a Figure 2, which shows how the girth of a desired product; whereas the same sub- tree that grew in a Norwegian forest un - stance colliding with other (inert) surfaces derwent a remarkable increase in its rate of merely rebounds with more or less reten- growth once ammonia fertilizer was ad- tion of translational, vibrational, and rota- min istered to the forest after the tree’s ½rst tional energy? A good catalyst has three twenty-½ve years of life. main characteristics: it must have high ac- The catalyst that has long been used for N tiv ity, meaning that it must facilitate fruit - the conversion of nitrogen ( 2) and hydro- H NH ful reaction as rapidly as possible; it should gen ( 2) to yield ammonia ( 3) was dis - be selective, meaning that it must favor just covered by the German chemist Fritz Ha - one of the various options that are thermo- ber in Karlsruhe in 1909. It is composed dynamically allowed as reaction pathways, principally of metallic iron, but tinctures A O thereby yielding a desired product; and it of potassium and alumina ( l2 3) are must have longevity, meaning that it added to it to optimize its effectiveness and should continue to function actively and longevity. Many companies worldwide are selectively for as long as possible before its able to prepare long-lived, durable, and catalytic performance degrades. I am pre- high ly active ammonia-synthesis catalysts occupied with ways of designing and syn- that produce millions of metric tons of thesizing new catalysts that meet these ammonia each year. Some of their iron- stand ards. based catalysts last for over a decade in High-school textbooks often state, erro- continuous use without signi½cant loss of neously, that a catalyst is an agent that does performance. not itself undergo change. This is not quite In order to devise new and better solid true. All catalysts interact with the species catalysts, it is necessary ½rst to understand that they transform, and in so doing, they precisely how existing ones work. Often may ultimately change their nature or be- this is an intellectually and experimentally come contaminated with molecular frag- dif½cult task, especially when some cata- ments that gradually diminish or eliminate lysts (like the iron one for ammonia syn- their ef½cacy. It is sometimes possible to thesis) operate under elevated tempera- reactivate a “dead” catalyst by judicious tures and pressures. There are very few ex- chemical manipulation. For example, the perimental techniques available for in situ solid catalysts used extensively to convert studies of catalysts that operate (within petroleum to useful products, such as gas - either a thick ceramic or other refractory oline or polymer precursors, gradually be- chamber) at nearly 400 degrees C and

143 (4) Fall 2014 23 Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 24

Foresight, Figure 1 Unpredict - A Selection of the Products Made from Petroleum ability & Chance in Chemistry Gasoline Toothpaste Perfumes Cassettes Dresses & Cognate Heating oil Heart valves TV cabinets Dishwasher parts Tires Subjects Tents Candles Shag rugs Toolboxes Golf bags Crayons Trash bags Electrician’s tape Transparent tape Percolators Parachutes House paint Tool racks Shoe polish Life jackets Telephones Water pipes Car battery cases Helmets Rubbing alcohol Enamel Hand lotion Epoxy Caulking Tennis rackets Pillows Roller skates Paint Petroleum jelly Rubber cement Dishes Surfboards Mops CD players Fishing boots Cameras Shampoo Faucet washers Vaporizers Anesthetics Wheels Antiseptics Balloons Arti½cial turf Paint rollers Clothesline Sunglasses Arti½cial limbs Shower curtains Curtains Solvents Bandages Guitar strings Basketballs Diesel fuel Dentures Luggage Soap Motor oil Model cars Aspirin Vitamin capsules Bearing grease Folding doors Safety glasses Antihistamines Ink Hair curlers Antifreeze Purses Floor wax Cold cream Awnings Fishing rods Food preservatives Ballpoint pens Movie ½lm Eyeglasses Lipstick Shoes Football cleats Soft contact lenses Clothes Denture adhesive Dashboards Upholstery Drinking cups Toothbrushes Linoleum Cortisone Sweaters Fan belts Ice chests Ice cube trays Deodorant Boats Car enamel Footballs Synthetic rubber Footballs Insecticides Shaving cream Combs Speakers Putty Bicycle tires Ammonia CDs and DVDs Plastic wood Dyes Sports car bodies Refrigerators Paint brushes Electric blankets Pantyhose Nail polish Golf balls Detergents Glycerin Refrigerant Fishing lures

Source: Prepared by Jay Keasling of the Lawrence Berkeley National Laboratory

around 200 atmospheres of pressure. ed energetics of these atoms at the iron Hence, it is necessary to use indirect model surface and hence explain fully how, from studies. To illustrate how dif½cult it is to transitory diatomic and triatomic frag- NH NH NH elucidate the iron-ammonia-synthesis cat - ments like and 2, gaseous 3 is alysts’ modes of operation, we consider the ½nally (catalytically) formed. For this dis- work of Gerhard Ertl of the Fritz Haber cov ery, Ertl earned the Nobel Prize in 2007. Institute of the Max Planck Gesellschaft in Berlin. In the early 1980s, he was able after For the last three decades, my own work much effort to demonstrate be yond doubt on the synthesis, characterization, and de- the sequence of individual chemical steps ployment of new solid catalysts has fo - that occur at the surface of the iron cata- cused on open-structure solids. I ½rst mod- lyst when it produces ammonia. Briefly, i½ed or synthesized clay minerals (not un- N H the process begins when 2 and 2 disso- like mica) that are composed of negatively ciate to yield relatively loosely bound charged sheets of atoms consisting mainly nitrogen and hydrogen atoms at the iron of aluminum, silicon, and oxygen. I could surfaces. By an ingenious sequence of ex - then convert such solids into powerful solid periments, Ertl was able to work out the acids–as strong as sulfuric acid–and capi - frequency of collisions and the associat- talize on their consequential catalytic ac-

24 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 25

Figure 2 John Growth Increase of a Tree Following Administration of Ammonia Fertilizer Meurig Thomas

A dendrochronological illustration of how effective fertilizers are. Helicopters sprinkled ammonia on a Norwe- gian forest regularly after the trees had been growing for twenty-½ve years. Thereafter, rapid growth occurred. Source: Image provided to the author by the late Professor Joseph Chatt, University of Sussex.

tiv ity. This entails populating the spaces be - Because clean chemical processes will like- tween the sheets of the synthetic clay with ly be demanded by legislators and the pub- positively charged hydrogen atoms (pro- lic for the inde½nite future, one can safely n tons), which are very acidic. In this way, predict that there will be a great need for m my colleagues at the University of Wales catalysts that make such processes feasible. and I were able to produce a highly active For far too long, the chemical industry, as and selective catalytic method of gener - well as researchers in our colleges and uni- ating the well-known sweet fragrance ethyl versities, have blithely used numerous acetate.6 In fact, this advance is now uti- chemical reagents that are noxious, toxic, lized industrially on a massive scale in the potentially explosive, or otherwise haz- k United Kingdom to produce three hun- ardous. In the future, the use of powerful dred thousand metric tons of ethyl acetate oxidants (such as nitric acid or potassium in a one-step, solvent-free process, the at - permanganate) or powerful acids–like tributes of which are of great importance oleum or hydrofluoric acid (which nowa- in this age of clean technology and green days are widely used in chemical technolo- y chemistry. gy)–will almost certainly be banned.7 y This is just one of many examples that Fortunately, there are now solid catalysts d illustrate how catalysts can render chemi- that can be tailored to effect oxidations and d cal processes both clean (environmentally other transformations using air or oxygen sustainable) and ef½cient (requiring less or hydrogen peroxide as the key reagents. energy to generate the desired product). I have myself been involved in fashioning

143 (4) Fall 2014 25 Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 26

Foresight, such catalysts. One way of designing pow - the energy needs of humans by around Unpredict - erful new catalysts that effect environmen- 2050 could not possibly be met by nuclear ability & Chance in tally responsible and economically impor- reactors. At present, human activity on Chemistry tant chemical transformations is to learn Earth requires thirteen terawatts (or thir- & Cognate Subjects how to prepare open-structure solids, like teen trillion watts) per year to sustain itself. the one depicted in Figure 3. Such solids A minimum of ten thousand nuclear ½s - possess enormous internal surface areas; sion reactors would have to be built by typically a gram of such a mesoporous sol - 2050 to meet the predicted energy needs id has an area in excess of that of a football of the human population, as pointed out ½eld. And by adroit chemical manipula- by Nathan Lewis.11 This would require us tion, one may “place” designed catalyti- to build a new nuclear power plant some- cally active centers spaced suf½ciently far where in the world every other day for the apart so as to ensure that each active site next half-century. At present, the only re - works independently, like those in an en - alistic way in which humankind’s energy zyme. A wide and versatile range of chem - demands can be met is through harness- ical synthesis and other processes may now ing solar radiation, and that will almost be effected using such open-structure sol - certainly necessitate the discovery of new ids.8 Given that hundreds of thousands of photocatalysts or variants of the nano- such solids should, in principle, exist, and technologic cells involving semiconduct- that only a few hundred or so have been ing liquid junctions of the kind now under prepared to date, it is safe to predict that development in various research labora- many powerful new catalysts will in the tories.12 future be tailored to effect reactions that One may safely predict that, for survival yield no by-products and do not generate reasons alone, chemists of the future will carbon dioxide.9 certainly focus on this pressing problem.13 While it will be perfectly possible to ½nd But by its very nature, scienti½c research catalysts that can convert renewable feed- will see many unexpected discoveries that stocks (like plants and the oils that they could well transform future life, so the so- create, such as soyabean, corn, and jatro- lutions may not come in the forms we pha) or microalgae (which are much bet- expect (one recalls here the prognostica- ter feedstocks than most living systems) tions of the editors of Scienti½c American in into useful products in a sustainable man- 1920). Here, it is prudent to focus on the ner, there are many other pressing scien- kind of developments that have come our ti½c problems that must be borne in mind. way in the chemical sciences: examples not The most vital of these is the creation of of failed predictions but of unexpected energy to satisfy the rising living standards successes. of a growing world population. The combustion of biomass or the harnessing of My ½rst example of an unexpected, un- wind, hydro, or nuclear forces cannot (even predictable advance was the discovery of in concert) meet the pressing need for green fluorescent protein (which earned more energy to power our world. The only Roger Tsien, Martin Chal½e, and Osamu possible source that is essentially limit- Shimomura the Nobel Prize in Chemistry less is solar energy; and here new photo- in 2008). This major advance in biochem- catalysts need to be designed and built. istry, which only a few generations ago This topic has been eloquently discussed was inconceivable, owes an enormous debt by Daniel Nocera in a previous issue of to Shimomura’s lifelong pertinacity and Dædalus.10 He showed, for example, that devotion in collecting and studying, out of

26 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 27

Figure 3 John An Open-Structure Solid Meurig Thomas

Various electron tomographical images of mesoporous silica. The nanopores in this solid catalyst show up as dark circles. Their diameter is close to 10 nm (ten billionths of a meter). Source: These micrographs were recorded by my colleagues on samples provided by V. Alfredsson of the University of Lund, using the techniques described in R. K. Leary, Paul A. Midgley, and John Meurig Thomas, “Recent Advances in the Application of Electron To - mography to Materials,” Accounts of Chemical Research 45 (10) (2012): 1782.

sheer curiosity, 850,000 specimens of jel - porate relativistic features into the Schrö - ly½sh. dinger equation. Dirac’s mathematical for- My second example of an unexpected mulations led him to propose in 1927 the advance has its origins in the borderland existence of the positron: the ½rst-ever sug - between theoretical physics and quantum gestion that antimatter was a reality. It took chemistry. In the 1920s, the young Paul another four years before the positron’s Dirac undertook to study quantum mech - existence was incontrovertibly established anics, stimulated by the work of physicists through experimental proof by Carl An - Werner Heisenberg and Max Born in Ger - dersen at the California Institute of Tech- many, and motivated by a desire to incor- nology. For many decades thereafter, the

143 (4) Fall 2014 27 Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 28

Foresight, positron was regarded as a novelty with When Hinshelwood was president of the Unpredict - lit tle prospect of ever being harnessed for London Chemical Society more than sixty ability & Chance in practical purposes. Now, however, al most years ago, he elaborated on the merit of his Chemistry every major hospital in the developed subject in ways that still resonate today. & Cognate Subjects world uses positrons in the noninvasive Not only is chemistry a mental discipline, medical technique of positron emission it is an adventure and an aesthetic experi- tomography. Its many uses include chart- ence. Its followers seek to know the hidden ing cerebral activity and identifying stages causes that underlie the transformations of in the growth of tumors. our changing world; to learn the essence of My third example of an unexpected sci- the rose’s color, the lilac’s fragrance, and enti½c leap forward relates to research ac- the oak’s tenacity; and to understand the tivities conducted by Allen Hill and his secret paths by which the sunlight and air group at the University of Oxford’s Inor - create these wonders. To this knowledge ganic Chemistry Laboratory in the mid- they attach an absolute value: that of truth 1970s. At that time, Hill began to wonder and beauty. In its pursuit, numerous fas- whether a metalloenzyme could readily ex- cinating, unexpected, and often extraor- change electrons with an electrode. In 1982, dinary discoveries are made. following up his curiosity-driven question, As I have ampli½ed elsewhere,15 scien- he invented a simple sensor for glucose in ti½c researchers know that discoveries can- blood, a breakthrough that has subsequent - not be planned: they pop up, like Puck, in ly led to the worldwide use of billions of unexpected corners. sensor strips by patients suffering from type 2 diabetes (a disease afflicting over 290 million people as of 2010).14 The measure - ment of blood glucose levels is re quired of all patients to whom insulin is prescribed for this disease.

Irrespective of its practical applicability, chemistry as a discipline has a validity of its own. I ½rst realized this fact at the age of eighteen, when I read the ½rst few pages of Sir Cyril Hinshelwood’s textbook, The Kinetics of Chemical Change, as part of my undergraduate course. His opening para- graph grips me now as it did then: That everything changes is an inescapable fact which from time immemorial has moved poets, exercised metaphysicians, and excited the curiosity of natural philo sophers. Slow chemical transformations, pursuing their hidden ways, are responsible for corrosion and decay, for development, growth and life. And their inner mechanisms are mysteries into which it is fascinating to in quire.

28 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 29

endnotes John Meurig Author’s Note: I am grateful for stimulating conversations with J. D. Dunitz, P. P. Edwards, Thomas H. A. O. Hill, R. Ritz, P. A. Midgley, and R. K. Leary. 1 John Kendrew repeated this statement in his book Thread of Life: An Introduction to Molecular Biology (London: Bell and Hyman, 1966). Kendrew’s own success owed much to a concatenation of fortunate circumstances and de - velopment of new instruments. First, he declined Sir Lawrence Bragg’s offer to join him when Bragg became Director of the Royal Institution (ri) in London in 1953. He preferred to stay in Cambridge, but he agreed to visit the ri regularly as Honorary Reader there. This brought him in touch with David Chilton Phillips and Ulrich Wolfgang Arndt, who together had just produced the ½rst-ever so-called linear automatic X-ray diffractometer, a major advance in instrumentation that enormously sped up the collection of X-ray data. But there were two other fortunate occurrences. The ½rst took place in 1951 when Kendrew’s gifted research student, Hugh Huxley, was carrying out a series of laborious X-ray–related computations by hand in his Cambridge lodgings, which he shared with an Australian research student named J. M. Bennett. The latter quickly pointed out to Huxley and Kendrew that the unique Electronic De - lay Storage Automatic Calculator (edsac), designed a few years earlier by Maurice Wilkes in Cambridge, could cope very easily with such calculations. Kendrew and Bennett soon wrote a de½nitive article (“The Computation of Fourier Syntheses with a Digital Electronic Calculating Machine”) that constituted a turning point in the processing of X-ray crystallographic data. The second fortunate circumstance was that Kendrew was part-time Deputy Chief Scienti½c Advisor to the Ministry of Defence in London–a position that enabled him to become acquainted with the most powerful computer in Britain, which he used to accelerate the interpretation of his raw data. With the ultra-rapid data collection of the linear X-ray diffractometer and an equally rapid means of processing data, Kendrew and colleagues were able to describe the ½rst-ever three-dimensional model of a protein, myoglobin (the primary oxygen- carrying pigment of muscle tissue), in a 1958 issue of Nature. By now, the technique pioneered by Kendrew and Max Perutz (who solved the structure of hemoglobin a few years later) is used extensively worldwide. Some one hundred thousand three-dimensional structures are in the Protein Data Bank, which was created in the 1970s in the Brookhaven National Laboratory and then transferred to Rutgers University. It now contains information about the structures of tens of thousands of proteins and nucleic acids, and it is updated weekly. 2 There was nothing prophetic in this prediction. The German workers Fischer and Tropsch had already shown in the early 1920s that hydrocarbons for fuels could be prepared, by use of appropriate catalysts, from the products of partially oxidized coal. 3 In 1984–1985, I served as a member of the U.K. Government’s Cabinet Of½ce on a panel set up to investigate promising areas of scienti½c research. Of the many predictions we made, only two became a reality: magnetic resonance imaging (mri) and the extensive use, in bio - logy and medicine, of confocal light microscopy. 4 A patent for a jet engine was taken out in the early 1930s by the young British flying of½cer Frank Whittle. 5 The Hindenberg disaster in 1927 put an end to the popularity of long-distance travel by diri - gible. 6 J. A. Ballantine, J. H. Purnell, and J. M. Thomas, “Sheet Silicates: Broad Spectrum Catalysts for Organic Synthesis,” Journal of Molecular Catalysis 27 (1) (1984): 157. 7 See John Meurig Thomas, “Solid Acid Catalysts,” Scienti½c American 266 (4) (1992): 112. 8 John Meurig Thomas, Design and Applications of Single-Site Heterogeneous Catalysts: Contribu- tions to Green Chemistry, Clean Technology and Sustainability (London: Imperial College Press, 2012).

143 (4) Fall 2014 29 Book_Fall 2014_Shinner.qxd 9/8/2014 10:01 AM Page 30

Foresight, 9 John Meurig Thomas and Jacek Klinowski, “Systematic Enumeration of Microporous Solids: Unpredict - Towards Designer Catalysts,” Angewandte Chemie International Edition 46 (38) (2007): 7160. ability & Chance in 10 Daniel G. Nocera, “On the Future of Global Energy,” Dædalus 135 (4) (2006): 112. See also Daniel Chemistry G. Nocera, “Can We Progress from Solipsistic Science to Frugal Innovation?” Dædalus 141 & Cognate (3) (2012): 45. Subjects 11 Nathan S. Lewis, “Powering the Planet,” MRS Bulletin 32 (2007): 808. 12 Ibid. 13 Predictions in general have exercised the thoughts of numerous historians of science as well as those of business-school economists. See, for example, Stephen G. Brush, “Prediction and Theory Evaluation: The Case of Light Bending,” Science 246 (4934) (1989): 1124; Clayton M. Christensen, The Innovators’ Dilemma: When New Technologies Cause Great Firms to Fail (Boston: Harvard Business School Press, 1997); and Norman R. Augustine, “They Never Saw It Com- ing,” Science 339 (6118) (2013): 373. 14 Anthony E. G. Cass, Graham Davis, Graeme D. Francis, H. Allen O. Hill, William J. Aston, I. John Higgins, Elliot V. Plotkin, Lesley D. L. Scott, and Anthony P. F. Turner, “Ferrocene- Mediated Enzyme Electrode for Amperometric Determination of Glucose,” Analytical Chem- istry 56 (4) (1984): 667; and Jane E. Frew and H. Allen O. Hill, “Electrochemical Biosensors,” Analytical Chemistry 59 (15) (1987): 933A. 15 John Meurig Thomas, “Intellectual Freedom in Academic Scienti½c Research Under Threat,” Angewandte Chemie International Edition 52 (22) (2013): 5654.

30 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 31

The Bright Future of Fabulous Materials Based on Carbon

Fred Wudl

Abstract: Our current civilization belongs to the organic materials age. Organic materials science pervades nearly all aspects of our daily life. This essay sketches the evolution of materials science up to the pres ent day. Plastics as textiles and structural materials dominate human civilization. The element carbon is at the core of this development because of its diverse interconnections with itself and other elements of the pe r iodic table. While silicon will not be supplanted from its role in electronics, carbon will provide the most versatile electronics applications, through inexpensive, flexible electronic devices.

Mr. McGuire: Ben, just one word. Ben: Yes, sir? Mr. McGuire: Are you listening? Ben: Yes, I am. Mr. McGuire: Plastics! Ben: How do you mean? Mr. McGuire: There is a great future in plastics, think about it. –The Graduate (1967)

Mr. McGuire was right in his assessment. Plastics, or better yet, organic materials, contribute signi½ - cant ly to making our everyday lives exciting and pro ductive. It is hard to imagine a world without plas - tics. Over the last several decades, organic materials have giv en us polyesters, polystyrene, formica, light- FRED WUDL, a Fellow of the Amer- weight containers, ½berglass boats, airplanes that i can Academy since 2001, is Re - consume less fuel (thanks to lighter structural ma- search Professor of Chemistry and terials), ½bers that are stronger, nylon, and much Materials at the University of Cal- more recently, flat screen displays with brilliant, vi - ifornia, Santa Barbara. His current brant colors. And organic materials are today leading research interests include the opti- us into a much less energy-intensive future, thanks to cal and electro-optical properties of processable conjugated polymers, organic solar cells (“plastic solar cells”) and organic the organic chemistry of fullerenes, transistors. and the design and preparation of How have we come so far so quickly: from un - self-mending polymers. imaginable to reality in as little as sixty years? Let’s

The Bright backtrack to examine this history a bit is its ability to form stable double bonds (π Future of more closely. bonds), particularly with itself. The π bonds Fabulous Materials Materials science might well be labeled consisting of two pairs of shared electrons Based on the “central science,” a moniker given to are the lowest common denominator in or- Carbon the discipline of chemistry in the recent ganic electronic materials. Another funda- past.1 Materials predate chemistry by mil - mental difference between organic solids lennia, and human civilization is inextrica- and inorganic solids (again at the atomic bly connected with materials. In fact, the level) is that the former are molecular solids stages of early human civilization have while the latter are extended solids. Extended been characterized by the materials that solids are materials in which the entire hu mans used as civilization evolved. Thus bulk has all atoms bonded to each other: the progress in prehistory follows the se - for example, a chunk of silicon, a diamond, quence stone age (Neolithic), followed by a gold nugget, iron, silicon oxide (quartz, bronze age, and then iron age. Thereafter glass), or iron oxide (magnetite). On the (c. 5000 bce), civilization became rather other hand, molecular solids are composed complex; but if we were to jump forward of discrete molecules that are not bonded to the present day, we would ½nd that every to each other. The molecules themselves aspect of current civilization is touched by elec- can consist of a diverse number of atoms, tronics, and speci½cally, electronics based on rang ing from two (iodine) to millions (ul - highly processed silicon. This brings us to tra high molecular weight poly[ethylene], electronic materials that have been domi- vectra, dna). The molecules pack into a nated by semiconductor elements, in par- solid in which intermolecular forces hold ticular silicon, germanium, and compound the bulk material together. Intermolecular semiconductors such as gallium arsenide, attractive forces are many orders of mag- indium gallium arsenide, and so on. In par - nitude weaker than the interatomic bonds allel with the evolution of solid-state elec- of solids. Thus, molecular solids in general tronics, organic materials (particularly are soluble in most solvents and have rel- plastics) have fully permeated civilization, atively low melting points, whereas all ex- hence a proper name for the current civi- tended solids are insoluble in all solvents, lization, in terms of materials, would be the and with the exceptions of mercury, cesi - silicon age, or the organic materials age. The um, gallium, rubidium, and potassium, as much more modern aspect of materials well as their alloys, extended solids have science, namely, organic electronic materials, rather high melting points. is in its nascency; its origin can be traced to The obvious advantage of organic mate- the second half of the twentieth century, rials is that they are much easier to process speci½cally to 19732 and 1987.3 (that is, convert to useful items) than are tradi- Inherent in the de½nition of organic ma- tional inorganic materials. Thus, organic materials is the classic de½nition of organic terials are considerably less energy inten- chemistry as the chemistry of carbon com - sive in the procedures employed to convert pounds. The foundation of organic mate - them to useful objects. Organic materials rials is based on the unique way in which have had profound effects on society via car bon atoms interact with one another their roles as textiles, structural materials, and and with other elements of the periodic very recently, electronic materials. The latter table. The key difference, at the atomic lev- will be the main subject of this essay. el, between carbon and all the elements below it in its column of the periodic table Organic materials in textiles had their (that is, silicon, germanium, tin, and lead) start with natural flax ½bers going back to

32 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 33

the Neolithic age, roughly thirty thousand known about these materials at either the Fred years ago;4 this was followed by cotton5 at atomic or molecular level. Many chemists Wudl least seven thousand years ago. It was not were convinced that plastics consisted of until the beginning of the twentieth centu- another form of matter, one that did not in- ry that synthetic, organic polymer–based volve traditional chemical bonds. It was (“plastics”) ½bers became prevalent. They not until the fundamental contributions of had originated in the mid-nineteenth cen- Herman Staudinger at the turn of the twen- tury with celluloid, the ½rst ½lm and bulk tieth century8 and Paul Flory9 in the mid- plastic-parts-forming material. Celluloid twentieth century that we were able to ex - had its origin with nitrocellulose, the explain all the properties of plastics as result- plosive guncotton. In 1855, in Birmingham, ing from the properties of giant molecules England, Alexander Parkes was the ½rst to (macromolecules or polymers). Both Stau- convert nitrocellulose into a plastic by mix- dinger and Flory received Nobel Prizes (in ing a nitrocellulose solution, known as col- 1953 and 1974, respectively) for their con- lodion, with camphor and then evaporat- tributions to polymer chemistry. Nylon ing the solvent. He named his invention and all its successors (for example, poly- Parkesine, and the material is thought to ester, acrylic, saran, and spandex) were in- mark the start of the plastics industry.6 vented based on our understanding of the By the 1870s, it became clear that certain chemistry of macromolecules at a funda- objects made from elephant tusk ivory, mental level. such as billiard balls and piano keys, were The terms polymer and macromolecule have becoming very expensive. Enter John Wes- become synonymous, while plastic usually ley Hyatt, an entrepreneur and inventor refers to a material made of synthetic poly - who expanded on Parkes’s invention and mers. As the name implies, a polymer con- labeled his material celluloid. A myriad of sists of many (Greek, poly) iden tical units articles were made of celluloid between the or parts (Greek, mer) joined together. The second half of the nineteenth century and simplest plastics are those in which a long the early twentieth century, including cin- molecular chain is formed of repeat units ematographic ½lm. Not surprisingly, the (chain links) that are all identical (homo- ½lm was very unstable and almost “spon- polymers), as is the case with polyethylene, taneously” combustible, resulting in tragic polypropylene, and polystyrene. One can movie theater ½res. These semi-synthetic imagine that because carbon is so versatile plastics had a limited lifetime, either suf- in its bonding ability, the poten tial for fering discoloration or breaking down. In different homopolymers is almost in½nite. 1907, Leo Baekeland invented a synthetic Now imagine combining two different resin made out of inexpensive synthetic mono mers to form a molecular chain. They ma terials (phenol and formaldehyde) and could be joined in at least three different called it Bakelite. Bakelite was stronger types of arrangements: they could alter- than celluloid, was very stable, and was not nate, occur at random, or occur in blocks. nearly as combustible. Soon a wide range Each of these arrangements can be of everyday objects were made of Bakelite: achieved, and each results in designable fountain pens, telephone housings, knife prop erties. Chemists and materials scien - handles, art deco objects, and phonograph tists, thanks to carbon’s unique chemistry, records, among many others. have at their disposal an almost in½nite In 1924, another semi-synthetic materi- number of organic plastic materials that al was obtained from wood cellulose pro- lend themselves to the production of ev- cessed into rayon ½bers,7 yet very little was erything from rubber bands to airplanes.

143 (4) Fall 2014 33 Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 34

The Bright Although the natural polymers making Engineering polymers–developed for the Future of up linen, cotton, and wool are still major pur pose of replacing metals–made up the Fabulous Materials textile ½bers, the highest strength ½bers, last major class of materials to spring from Based on as well as those with special applications, the Golden Age of Plastics. Carbon are all synthetic. For the same weight, a –Alexander H. Tullo, Chemical & Engineer ing Kev lar ½ber is stronger than steel. How is News, September 9, 2013 it possible to have a material that, in ½ber form, is stronger than steel and yet is a molecular solid? There are two reasons: The mechanically stronger and, in gener- 1) the macromolecules are very long, result- al, more brittle plastics such as Bakelite or ing in their forming regions of entangle- Formica have their long molecules con - ments or “mechanical bonds”; and 2) the nect ed to each other, with interchain bonds extensive regions that are not entangled or crosslinks. In essence they are a form of are highly ordered and have enhanced in- extended solids. Hence, they are insoluble termolecular attractive forces. These inter - and infusible. The only way to process molecular attractions are hydrogen bond- them is by forming the part from its mono - ing, dipole-dipole, π-π stacking, and Van mer components in a mold through a der Waals attractive forces. They were method called thermosetting. Because poly- known to chemists for decades but were meric solids are so easily manipulated, by not exploited until the second half of the further chemical transformation, by disso- twentieth century, at which point organic lution, or by melting, they can also be con- chemists were able to design molecules verted to very strong and tough materials, exploiting these weaker attractive forces. particularly by forming composites with These “designer molecules” would then strengthening ingredients such as glass ½- or der themselves into a predetermined bers, carbon ½bers, or steel mesh. Our most structure by “self assembly.” advanced airplanes (the F-117 stealth ½ght- Kevlar was designed to maximize all er, the B-2 bomber, the Boeing 787 Dream - these intermolecular forces, starting with liner) now feature extensive use of plastic- the monomer, the method to link the carbon ½ber composites. monomers to each other (polymerization), The giant molecules used in structural and ½nally the processing into ½bers. The materials, while often containing π bonds, chain entanglements are the main reason do not take advantage of the π bonds’ ex - why polymeric materials are plastic (flex- quisite electronic properties. Thanks to the ible, malleable, and ductile), and hence fundamental research of organic chemists, con vertible into ½bers, ½lms, devices, and mostly in the twentieth century, π bonds machine parts. The latter are in the realm can be tailored to have a useful electronic of structural or engineering materials. property and this ability is the essence of organic electronic materials. An isolated dou ble bond between two carbon atoms is relatively uninteresting from an organic Our everyday objects are lighter, more du- electronics perspective because the energy rable, cheaper to manufacture, sleeker and required to manipulate these π electrons more sanitary than their predecessors. The is too high for electronic devices. Figure 1 world before plastics was that of butcher pa- shows that as π bonds are connected, the per, tin soldiers, wooden crates, broken glass energy required to excite their electrons and rusting metal–less convenient and less becomes smaller. Thus the wavelength of abundant in consumer goods than today. . . . light required to excite electrons from the

34 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 35

relatively high-energy ultraviolet region of unusual properties of graphene.11 Graph- Fred the spectrum for ethylene (one π bond) to ene can in fact be considered the concep- Wudl the lower energy visible region in carotene tual germ of two other new forms of car- (eleven π bonds). By increasing the num- bon, buckminsterfullerene12 and carbon ber of alternating π bonds in a molecule nanotubes, illustrated in Figure 3.13 and including other elements that interact For the discovery of fullerenes, R. F. Curl, with the elec trons of π bonds (oxygen, sul- H. W. Kroto, and R. E. Smalley received the fur, nitrogen), one can extend the absorp- Nobel Prize in Chemistry in 1996. Fuller - tion of light to wavelengths of light corre- enes, especially carbon nanotubes, can be sponding to relatively low energies (to the envisioned as arising from the curving of near infrared region of the spectrum). a graphene sheet onto itself. The electronic Ethylene and all other π-bonded carbon properties of buckminsterfullerene and atoms are planar: that is, all four hydrogen carbon nanotubes are directly related to atoms of ethylene are in the same plane; the strain of their bent π bonds. In buck- deforming the plane results in a highly minsterfullerene, the strain is manifested “strain ed” bond. The most extremely in - by an increase in electronegativity, mean- terconnected π-bonded carbon structure ing that each π bond becomes mildly known is graphene, a small section of electron-attracting, making buckminster - which is shown in Figure 2. fullerene an electron acceptor (ea) mole- In nature, carbon occurs in two major cule. With the advent of buckminsterful - “allotropes”: diamond and graphite. The lerene, chemists were presented for the former, as mentioned above, is an extend - ½rst time with a set of π bonds arranged on ed three-dimensional solid. The structure a spherical surface. Buckminsterfullerene of a very tiny piece of diamond is shown is also the ½rst carbon allotrope in the form below. of a molecular solid that is soluble in several solvents and sublimable under vacuum at readily accessible temperatures. Buck- minsterfullerene molecules tend to aggre - gate, a property that is auspicious for the development of organic solar cells, as discussed below. Buckminsterfullerene and carbon nano - tubes are at the heart of organic nanoscience In this structure, each carbon atom is and nanotechnology, a relatively new branch joined to four other carbon atoms by sin- of chemistry, physics, and materials sci- gle bonds. On the other hand, graphite is ence.14 Nanotechnology and nanoscience composed of a very large number of planar deal with properties of matter in the realm graphene sheets stacked on top of each of 1–100 nm. In this scale, properties may oth er, with perfect registry from graphene be dominated by quantum mechanical ef- layer to layer, held together by weak Van fects. Two typical examples (of many)15 of der Waals forces. Each sheet contains many current nanotechnology applications in bonds. Because these inter-graphene forces materials science are bandages impregnat- are so weak, in writing with a graphite pen - ed with silver nanoparticles, to take advan- cil, one scrapes off layers of graphene onto tage of their remarkable antibiotic proper- the paper. In 2010, the Nobel Prize in Phys- ties, and nano-sized titanium dioxide as ics was awarded to A. K. Geim and K. S. well as zinc oxide for particularly effective Novoselov 10 for their discovery of the very sun screen ointments.16 A truly up-to-date

143 (4) Fall 2014 35 Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 36

The Bright Figure 1 Future of Carotene, with Its Eleven Linked (Conjugated) π Bonds Fabulous Materials H Based on H Carbon H H EthyleneEthylene is “1 Ππ bond”bond”

H3C H3C CH3 CH3 CH3

CH3 CH CH H3C CH3 3 3

Figure 2 The π Bonds in Graphene (left); an Atomic Force Micrograph of Graphene (right)

One nanometer (nm) is approximately one ten-billionth of an inch. Source: E. Stolyarova, K. T. Rim, S. Ryu, J. Maultzsch, P. Kim, L. E. Brus, T. E. Heinz, M. S. Hybertsen, and G. W. Flynn, “High-Resolution Scanning Tun- neling Microscopy Imaging of Mesoscopic Graphene Sheets on an Insulating Surface,” Proceedings of the National Academy of Sciences 104 (2007): 9209–9212.

application of carbon nanotubes is in com- provide the design and fabrication of de - puting, where nanotubes are beginning to vices for the consumer. take the place of silicon in a computer’s in- The close collaboration of chemists and tegrated circuit.17 physicists helped establish that the most effective way to generate and delocalize To make signi½cant advances in organic electrons in organic solids is to use two electronics, scientists need to be truly types of π bond–containing molecules: inter disciplinary. Equal participation of electron acceptors (ea) and electron do- chemists, physicists, and engineers is re - nors (ed).18 We already saw that buckmin- quired. The chemists design and synthesize sterfullerene is an electron acceptor. Elec- the organic materials, the physicists pro- tron donor molecules have π electrons that vide the theory and experimental proce- are relatively loose and easily given up, dures for transporting electrons through leav ing behind a positive charge or “hole.” organic molecular solids, and the engineers The solid resulting from transferring an

36 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 37

Figure 3 Fred C Wudl Structure of Buckminsterfullerene with Sixty Carbon Atoms ( 60) Containing Thirty π Bonds (left); Artist’s rendering of a Single and Multiwall (Double) Carbon Nanotube (right)

μm

0.2-5

0.36 nm

1-2 nm 2-25 nm

Source: http://en.wikipedia.org/wiki/File:Buckminsterfullerene-2D-skeletal.png; A. Hirsch, “Funktionalisierung von einwandigen Kohlenstoffnanoröhrern,” Angewandte Chemie 114 (2002): 1933–1939; R. M. Reilly, “Carbon Nano - tubes: Potential Bene½ts and Risks of Nanotechnology in Nuclear Medicine,” Journal of Nuclear Medicine 48 (7) (2007): 1039–1042; and S. Iijima, “Carbon Nanotubes: Past, Present, and Future,” Physica B (323) (2002): 1–5.

electron from an ed to the ea is known as research on organic metals ultimately led “a charge transfer complex.” It was further to the discovery of organic superconduc- established that both ed and ea molecules tors. These are materials that exhibit in½- had to form in½nite pancake-like stacks in nite conductivity (zero resis tance) below the solid state. Electrons and holes travel a particular transition temperature. 20 along these stacks much more readily than This was a truly amazing devel opment be - they do between stacks. With this funda- cause up to that point it was believed by mental knowledge, scientists were able to all experts in the ½eld that in order to ob- invent organic solids that conduct electric- serve superconductivity, one needed an ex- ity just as well as many metals do (but still tended solid. not as well as copper). Another property From a fundamental point of view, that these organic solids shared with met- achieving metallic and superconducting als was that their conductivity increased properties with organic materials was clear- with decreasing temperature and as a re - ly a remarkable accomplishment. But from sult, these solids were known as organic a materials engineering viewpoint this was metals . not the case, because organic metals were Up to the time of this momentous discov - tiny, brittle crystals that could not be pro - ery, organic materials were useful in elec- cessed into useful devices. What was re- trical engineering and electronics be cause quired was a polymer or plastic that would they were excellent insulators, not con duc - exhibit high conductivity when converted tors. Prior to this breakthrough point in the to a charge transfer complex. Not until 1977 history of organic electronics,19 the mea- did this momentous discovery take place, surement of conductivity as a function of when collaborating scientists in Japan and temperature of organic solids afforded only the United States identi½ed an electrically very low conductivity that decreased with conducting polymer.21 This ½nding led to de creasing temperature, a property that the 2000 Nobel Prize in Chemistry being de½nes semiconductors in general. Further awarded to A. J. Heeger, A. MacDiarmid,

143 (4) Fall 2014 37 Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 38

The Bright and H. Shirakawa. From an organic chem - important step in the development of or- Future of ist’s perspective, this was a deceptively sim - gan ic electronics, the ½rst organic electron - Fabulous 23 Materials ple polymer, a polyacetylene. It was simply ics device that was reported came in 1987; Based on a solid made up of molecules that consisted it was also a light-emitting device, but was Carbon of long chains of conjugated π bonds not based on polymers. Rather, it was a (where n is a dif½cult-to-determine large small-molecule organic light-emitting di- number in the hundreds to thousands). ode (oled). Both kinds of devices are very bright and colorful, but so far, pleds are still in the development stage while oleds have advanced into the com mercial sphere, seen everywhere from flat screen display n devices to smartphones. Because electrons and holes carry op- A sample of polyacetylene looks like a posite charges, they attract each other and piece of aluminum foil, but it tarnishes can actually combine, resulting in loss of quickly when exposed to the atmosphere. charge carriers. The origin of the light emit- This material was chiefly of academic in- ted by oleds and pleds is based on the fact terest, but it provided a trove of fundamen- that when electrons recombine with holes, tal information, much like fruit flies do for they raise molecules to a high-energy excit- the ½eld of genetics. In quick succession, ed state. When the excited molecules return the organic electronics community devel - to a ground state they emit light or heat. oped much more stable polymers that Again, π bonds have a greater tendency could be useful as materials. The paradigm to emit light than heat. In oleds and of the day was to achieve ever higher con- pleds, the positive and negative electrodes ductivities, even conductivities that might of the diode create the holes and electrons, be achievable without having to re sort to respectively. The excited state of an elec- charge transfer complex formation. As a tron-hole pair is called an exciton. An exci- result, the chemistry and physics commu - ton can also be created by absorption of nity concentrated on achieving metal-like light of the wavelength corresponding to properties until Richard Friend’s group the energy required to produce an electron- (in 1990) discovered that one of the most hole pair. So, if one were able to separate stable polymers could be processed into a the holes from the electrons before they light-emitting diode.22 This was a semi- had a chance to recombine, then one conductor property, not a metal property. would create an electric ½eld or voltage by A light-emitting diode is a particular the absorption of light, and one would have semi conductor device that emits light a “photovoltaic device” or “solar cell” if when a voltage is applied across it. the light absorbed were that corresponding The discovery caused an immediate par- to the solar spectrum. Many attempts to a digm shift by the research community, produce organic photovoltaic cells were from the search for metal properties to the made throughout the period from the 1960s search for semiconductor properties. In to 1980s by combining eds and eas and fact, judging from the number of publica- irradiating them. Unfortunately, though tions, research on organic metals appears the devices produced electricity, they had to have ceased in the United States, al - extremely low ef½ciencies, mostly because though it is still pursued in Japan and Eu - the electron-hole recombination rates rope. While the discovery of the polymeric were too high. Another way to view this light emitting diode (pled) was clearly an observation is that the acceptors were “too

38 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 39

willing” to give the electron back to the purse for the powering of a cell phone. Fred C Wudl holes. None of these devices use fullerene 60 as As it turned out, the negatively charged the acceptor, but rather a derivative called fullerenes, resulting from accepting an pcbm (shown below), an acronym for C electron, were less strained than the slight- [6,6]-phenyl- 61-butyric acid methyl ester. ly smaller-diameter neutral fullerenes.

This property resulted in a one thousand OCH3

to ten thousand times slower recombina- O tion24 with the hole of an exciton generated in an ed polymer than was observed previously with any other ea. Even though the ½rst organic solar cells–“plastic solar cells”–based on fullerene, discovered in While fullerene is relatively insoluble, 1992, had much higher ef½ciencies (0.04 its electronic structure makes it slightly per cent)25 in the conversion of solar en- too strong of an electron acceptor, where- ergy to electrical energy (power conversion as pcbm is more soluble and is a slightly ef½ciency [pce]) than any previous or - weaker electron acceptor, better matching ganic device, it was still far too low to be the electron-donating properties of most of more than academic interest. Another polymeric eds. The development of pcbm important aspect of fullerenes for this ap - provides yet another example of how many plication was that the very tight aggregates times science progresses by serendipity. fullerenes tend to form allowed the nega- This fullerene derivative was originally pre - tive charge to be carried quickly toward pared as part of a program to make a wa - the negative electrode of the cell. With ter-soluble agent to inhibit the active site of con centrated research efforts in the United hiv protease, an enzyme used by the aids States, Europe, Japan, and China, the pce virus in its replication process. The active increased rapidly to the current record of site of the protease is a cavity of ap prox i - 11 percent,26 a 275-fold increase from the mately 1 nm in diameter, correspond ing to original ef½ciency. While this number may the diameter of fullerene. To be able to ex- seem small, keep in mind that solar cells amine the biological properties of any mol- based on amorphous silicon ex hibit a pce ecule, the molecule needs to have some wa- of 10 to 11.9 percent. The most ef½cient so- ter solubility. Fullerene is insoluble in wa- lar cell reported to date is 38.8 percent ter, and pcbm was prepared as a water- ef½cient, and commercial cells are only 11 soluble fullerene derivative. Another rea- to 19 percent ef½cient.27 The most ef½cient son that fullerene pcbm was prepared is non-organic experimental cell is reported simply because the chemistry of fullerene to be 44.4 percent ef½cient, but this involves was being explored in my research group the use of a solar concentrator, a device, as well as in several other groups around the like a magnifying lens, that concentrates world; and one of the better known meth- light; without a concentrator, the maxi- ods to transform fullerene was by the re- mum pce is 32.6 percent.28 There are now action that led to pcbm. At the same time, several small companies manufacturing the “materials” properties of fuller ene, par - plastic solar cells that are adequately ef½- ticularly the optoelectronic properties for cient for relatively small applications, such organic electronics, were being examined as on the roof of a bus shelter for the min- by Dr. Saiciftci in Alan Heeger’s group at imal electricity needs there (lights, for ex- the University of California, Santa Barbara. ample), or on the outer flap of a woman’s He wanted a soluble fullerene derivative

143 (4) Fall 2014 39 Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 40

The Bright because he had just determined that fuller - tors are solid-state microscopic switches Future of ene would accept an electron from a photo- and ampli½ers that are the main way to Fabulous Materials excited ed polymer. Changing from ful - con trol electron flow in electronic devices, Based on ler ene as an electron acceptor to pcbm particularly in computing and displaying. Carbon dramat ically enhanced the pce to 2.9 per- Organic electronics is influencing the cent.29 From 1995 to 2013, the pce im - electronics industry with organic transis- proved to 11 percent. This signi½cant en - tors. The easiest way to convert an organic hancement was a consequence of a change semiconductor to a transistor is to use the in the ed polymer architecture from ho - tendency of organic materials to readily mo polymers to alternating co-polymers. form thin ½lms. Organic transistors are This structural modi½cation allowed for thin ½lm transistors (tft). A particularly more versatile design of electronic charac - simple tft is the ½eld effect transistor ter of the π backbone of the polymer. (fet); in this case an organic fet would In order to fabricate a conventional sil- be an ofet.30 Pentacene, a 22-π-bonded icon-based solar cell one must ½rst process carbon molecule, its derivatives, and some the silicon. Because the melting point of polymeric materials that are much better silicon is 1414 degrees C (2577 degrees F), ½lm-formers easily outperform amor- and because in order to fabricate silicon- phous silicon and have already yielded based devices one must crystallize silicon practical devices, such as drivers for flex- from the melt, the manufacture of solar ible oled and liquid crystalline displays. cells is a very energy-intensive process. On the other hand, plastic solar cells are based In conclusion, by exploiting the π bond- on molecular solids. They can be processed ing capability of carbon, organic materials from solution, called “inks,” with very have been designed to exhibit unmatched simple devices such as a dot matrix printer advantages: they are lightweight, flexible, or a roll-to-roll printer. Thanks to their and low-cost; and they have low energy light weight, flexibility, ease of manufac- fab rication demands. Devices based on ture, and low cost, plastic solar cells can be these materials can be expected to prolif- expected to have a profound effect on the erate rapidly. Cheaper and lighter large- world’s non-fossil fuel, non-nuclear elec- scale structures (dwellings and bridges, for tricity generating capacity. A similar bright instance); countless electronic devices; future for the reduction of energy-con- cheaper, lighter, and recyclable containers; sumption can be foreseen for oled display more ef½cient modes of transportation; devices, since these devices do not need and improved techniques for the harvest- strong backlighting. To observe a non-lu - ing of solar energy: all will be part of this minescent device, such as a liquid crystal branch of organic chemistry’s contribution display (lcd), one needs a source of illu- to sustainable life on Earth. mi nation; for flat screens this is best ac - complished from the back, hence “back - lighting.” As stated above, when π bonds are not in charge transfer complexes, they behave as semiconductors, the backbone of modern electronics. Diodes and transistors are the simplest units of semiconductor electronic devices. We already saw an organic diode application in the form of oleds. Transis-

40 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 41

endnotes Fred Wudl Author’s Note: This essay has bene½ted from the feedback of Linda Wudl and Jerrold Meinwald, to whom I am indebted for their kind help. 1 Attributed to Theodore E. Brown, who wrote a freshman chemistry text titled Chemistry, the Central Science. 2 J. Ferraris, D. O. Cowan, V. V. Walatka, Jr., and J. H. Perlstein, “Electron Transfer in a New Highly Conducting Donor-Acceptor Complex,” Journal of the American Chemical Society 95 (1973): 948–949; and F. Wudl, D. Wobschall, and E. J. Hufnagel, “Electrical Conductivity by the Bis- 1,3-dithiole-Bis-1,3-dithiolium System,” Journal of the American Chemical Society 94 (1972): 670–672. 3 C. W. Tang and S. A. Van Slyke, “Organic Electroluminescent Diodes,” Applied Physics Letters 51 (1987): 913–915. 4 Eliso Kvavadze, Ofer Bar-Yosef, Anna Belfer-Cohen, Elisabetta Boaretto, Nino Jakeli, Zinovi Matskevich, and Tengiz Meshveliani, “30,000-Year-Old Wild Flax Fibers,” Science 325 (5946) (2009): 1359; and Richard Harris, “These Vintage Threads Are 30,000 Years Old,” All Things Considered, September 10, 2001, npr. 5 Christophe Moulherat, Margareta Tengberg, Jerome-F. Haquet, and Benoit Mille, “First Evidence of Cotton at Neolithic Mehrgarh, Pakistan: Analysis of Mineralized Fibers from a Copper Bead,” Journal of Archaeological Science 29 (12) (2002): 1393–1401. 6 P. C. Painter and M. M. Coleman, “Essentials of Polymer Science and Engineering,” destech Publications, Pennsylvania, 2009. 7 Ibid. 8 Hermann Staudinger, “Über Polymerisation,” Berichte der Deutschen Chemischen Gesellschaft 53 (6) (1920): 1073–1085. 9 Paul Flory, Principles of Polymer Chemistry (Ithaca, N.Y.: Cornell University Press, 1953). 10 http://www.nobelprize.org/nobel_prizes/physics/laureates/2010/. 11 K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, “Electric Field Effect in Atomically Thin Carbon Films,” Science 306 (5696) (2004): 666–669. 12 C H. W. Kroto, J. R. Heath, S. C. O’Brien, R. F. Curl, and R. E. Smalley, “ 60: Buckminsterfuller - ene,” Nature 318 (6042) (1985): 162–163, Bibcode:1985Natur.318..162K, doi:10.1038/318162a0. 13 Sumio Iijima “Helical Microtubules of Graphitic Carbon,” Nature 354 (6348) (1991): 56–58, Bibcode:1991Natur.354...56I, doi:10.1038/354056a0. 14 Ed Regis, Nano: The Emerging Science of Nanotechnology (Boston: Little, Brown and Company, 1995). 15 Ibid. 16 M. E. Kurtoglu, T. Longenbach, P. Reddington, and Y. Gogotsi, “Effect of Calcination Tem- perature and Environment on Photocatalytic and Mechanical Properties of Ultrathin Sol– Gel Titanium Dioxide Films,” Journal of the American Ceramic Society 94 (4) (2011): 1101–1108, doi:10.1111/j.1551-2916.2010.04218.x. 17 Max M. Shulaker, Gage Hills, Nishant Patil, Hai Wei, Hong-Yu Chen, H.-S. Philip Wong, and Subhasish Mitra, “Carbon Nanotube Computer,” Nature 501 (26 September 2013): 526–530, doi:10.1038/nature12502. 18 Regis, Nano: The Emerging Science of Nanotechnology. 19 Ferraris et al., “Electron Transfer in a New Highly Conducting Donor-Acceptor Complex,” 948–949; and Wudl, “Electrical Conductivity by the Bis-1,3-dithiole-Bis-1,3-dithiolium Sys- tem,” 670–672.

143 (4) Fall 2014 41 Book_Fall 2014_Shinner.qxd 9/8/2014 10:02 AM Page 42

The Bright 20 J. M. Williams, J. R. Ferraro, R. J. Thorn, K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini, and Future of M.-H. Whangbo, Organic Superconductors (Upper Saddle River, N.J.: Prentice Hall, 1992). Fabulous Materials 21 H. Shirakawa, E. J. Lewis, A. G. MacDiarmid, C. K. Chiang, and A. J. Heeger, “Synthesis of Elec- CH Based on trically Conducting Organic Polymers: Halogen Derivatives of Polyacetylene, ( )x,” Journal Carbon of the Chemical Society, Chemical Communications (16) (1977): 578. 22 J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, “Light-Emitting Diodes Based on Conjugated Polymers,” Nature 347 (1990): 539. 23 Tang, “Organic Electroluminescent Diodes.” 24 N. S. Sariciftci, L. Smilowitz, A. J. Heeger, and F. Wudl, “Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene,” Science 258 (1992): 1474. 25 N. S. Sariciftci, D. Braun, C. Zhang, V. I. Srdanov, A. J. Heeger, G. Stucky, and F. Wudl, “Semiconducting Polymer-Buckminsterfullerene Heterojunctions: Diodes, Photodiodes, and Photovoltaic Cells,” Applied Physics Letters 62 (1993): 585. 26 M. C. Scharber and N. S. Sariciftci, “Ef½ciency of Bulk-Heterojunction Organic Solar Cells,” Progress in Polymer Science (2013), doi:10.1016/j.progpolymsci.2013.05.001. 27 http://www.nrel.gov/ncpv/images/ef½ciency_chart.jpg. 28 Ibid. 29 G. Yu, J. Gao, J. C. Hummelen, F. Wudl, and A. J. Heeger, “Polymer Photovoltaic Cells: En- hanced Ef½ciencies via a Network of Internal Donor-Acceptor Heterojunctions,” Science 270 (1995): 1789. 30 Ioannis Kymissis, Organic Field Effect Transistors: Theory, Fabrication and Characterization (New York: Springer, 2009).

42 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:15 AM Page 43

The Convergence of Chemistry & Human Biology

Chaitan Khosla

Abstract: Over the past two decades, “chemical biology” has emerged as the term of choice to describe the interface between chemistry and biology. As its name suggests, the ½eld draws upon chemical insights and tools to understand or engineer living things. This essay focuses on the scienti½c, societal, and pedagogical potential of an emerging frontier for chemical biologists: namely, the study of Homo sapiens. My goal is to highlight the opportunities and challenges presented to chemistry by human biology at a time when it costs less to sequence an individual’s genome than it does to buy a car. But how does chemical biology differ from other similar-sounding ½elds? By ½rst reaching a clear understanding of the scope of chemical biology, we may address more pertinent questions such as: What is the promise of the emerging interface between chemistry and human biology? Why is it important to nurture the relationship between these ½elds? And what are the attributes of individuals and environments that are well poised to contribute signi½cantly to this interface?

To someone only vaguely familiar with disciplines such as biochemistry, structural biology, molecular biology, and medicinal chemistry, the introduction of yet another related name may seem unnecessarily CHAITAN KHOSLA, a Fellow of the confusing. How does chemical biology differentiate American Academy since 2007, is itself from these established ½elds? The simple an - Professor of Chemistry, Chemical swer is that it does not. Each of the aforementioned Engineering, and (by courtesy) Bio - disciplines arose when a few talented and farsighted chemistry at Stanford University. chemists pivoted from contemporary problems in He also directs Stanford ChEM-H, an institute that brings together chemistry (which, as we are taught in high school, at- chem ists, engineers, biologists, and tempts to explain the properties of matter by under- clinicians to understand life at a standing its structure and reactivity at an atomic lev- chemical level and apply that knowl- el) to emerging challenges in biology. edge to improving human health. Biochemistry seeks to reconstitute the essence of a His work on antibiotic biosynthesis biological phenomenon by placing a well-de½ned and the molecular basis for celiac set of molecules in a highly controlled environment disease has been published in Sci- ence, Journal of the American Chemical such as a test tube. Not only does biochemistry play a Society, and Proceedings of the Nation- critical role in elucidating cell function, it also facili - al Academy of Sciences, among many tates deeper insight into the chemistry of life. Struc - others. tural biology elucidates the structures of spectacular ly

The complex biological molecules and as sem - The interface between chemistry and hu - Conver - blies at an atomic level. To do so, it em ploys man biology holds considerable promise gence of Chemistry exper i men tal and computational tools de- for science, medicine, and society. At a fun- & Human velop ed by chemists in the context of damental level, chemistry can strengthen Biology studying simp ler forms of matter. The pi- the foundations of human biology in a oneers of mo lecular biology harnessed their manner that is entirely analogous to its en- in sights in to dna structure and reac tiv - abling role in many of biology’s most nota - ity to transform biology from an obser va - ble advances in the twentieth century. Hu- tional science into an intervention al one. man beings are different from other living Finally, medi ci n al chemistry emerged as a dis- creatures (and indeed, even from each oth- cipline when chemists were task ed with er) in many interesting ways: our brains, the goal of engineering potent drugs that our diets, and our immune systems are mod u lated hu man physiology in a tar geted just a few examples. Explaining these dif- manner. ferences in the language of chemistry is a In its broadest de½nition, chemical bi - scienti½c frontier that has proven to have ology exploits a chemist’s knowledge of profound consequences for human health. mo lecular structure and reactivity, togeth er Imagine a future where it is possible to with his or her skills in molecular design, meaningfully discuss the chemical basis of synthesis, and analysis, to understand or speci½c thoughts and emotions; or a time engineer living organisms. In essence, this when our understanding of the immune represents a return to the pioneering spirit system is deep enough to interpret its con- of biochemists, structural biologists, mo - stantly changing responses to the effects lecular biologists, and medicinal chemists of aging, diet, and infection on each of us. from an earlier generation. The difference The age-old debate of nature versus nur- today is that chemistry itself has become a ture has taken on a new meaning with the far more powerful science than it was half a discovery of epigenetic phenomena that century ago. Our knowledge of the chemi- can be passed on from one generation to cal properties of large swaths of the period- the next. Unraveling this epigenetic code ic table has grown enormously. This in turn represents an exciting opportunity for has paved the road to cost-effective syn the - chem ical biologists to penetrate the mys- ses of incredibly complex molecules, some teries of complex illnesses. of which have become life-saving drugs. The chemistry–human biology inter- Similarly, back when antibiotics were ½rst face also holds a special place in the future isolated from soil microbes, their biosyn- of drug discovery, development, and eval - thetic origins were incomprehensi ble. To- ua tion. As human beings struggle to recon- day, we can not only decode the chemical cile their dreams for healthy aging with the logic of antibiotic biosynthe sis, but also need for cost-effective healthcare, inno- engi n eer antibiotics ourselves. Mean while, vative medicines are expected to be the chemistry’s analytical methods have be- panacea. A surprisingly large fraction of ef- come so sophisticated that single mole- forts to translate groundbreaking biolog- cules can be visualized with micro scopes ical discoveries into patient care are bottle- and useful information can be extracted necked by the lack of suitably engineered from even the tiniest sample, such as a bi - molecules or molecular assemblies. By fo- opsy from a patient with an unde½ned ill- cusing on problems where innovative mo- ness. In this way, chemical biologists har- lecular design, synthesis, or analysis is cru- ness the evolving science of chemistry to cial, chemistry can accelerate the transla- interrogate or modify biology. tion of advances in human biology into

44 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:15 AM Page 45

clin ical practice. Take, for example, the ½eld to envision a resurgence of interest in Chaitan of infectious diseases. At a time when our chemistry in mainstream medical educa- Khosla armamentarium of effective antibiotics has tion, dual strengths in chemistry and med- reached alarmingly low levels, our knowl - icine could foster a new breed of physician- edge of nature’s repertoire of antibiotic scientists who are also physical scientists. biosynthetic strategies is exploding. Chem- Their talent for molecular de sign and anal - ical biology is poised to exploit these in - ysis, coupled with their passion for human sights to engineer pathogen-speci½c ther - biology, would allow them to play leading apies. Consider also the ½eld of radiology. roles in reshaping the healthcare industry. New chemical probes and measurement methods such as mri (magnetic resonance Several compelling examples point to- imaging), pet (positron emission tomog - ward the opportunity that lies ahead at the raphy), and ultrasound have the potential chemistry–human biology interface. A re- to noninvasively visualize human anat - cent report on innovation in drug discov- omy and physiology effortlessly and at un- ery, development, and evaluation by the precedented resolution. Equipped with an President’s Council of Advisors on Science unimaginably sophisticated set of acces- and Technology highlighted both the con - sories and apps, the iPhone of the future tinuing need for innovative medicines as may be just as important a communication well as widespread concerns about their tool between healthcare consumers and development pace and cost.1 The report providers as its present-day version is be - pro posed doubling the current annual out - tween two teenagers. Chemical biologists put of innovative new medicines as an am- are also opening new doors in preventive bitious goal. While most industry watch- medicine. Until recently, vaccines were ers will concur that seventy to eighty in - prin cipally used to protect against deadly novative new drugs per year would indeed or debilitating infectious diseases. Today, represent a dramatic increase in research syn thetic vaccines are being developed and development productivity, two other against cancer and allergy. Our bodies also ½gures put the importance of this goal in - play host to innumerable bacterial cells to clearer perspective. First, of the roughly (col lec tively referred to as the “microbi - 23,000 proteins encoded by the human ome”) whose myriad health bene½ts re- genome (the functions of as many as half main to be understood and perhaps even of which remain unknown), fewer than 3 engi neer ed. Last but not least, regulatory percent are targeted by fda-approved sci ence re pre sents an attractive but over - drugs. Targeting proteins in the human looked area for applied chemical biological body is by far the most productive ap- re search. By upgrading the capacity for proach to drug design. Second, there exist risk-bene½t analysis at agencies such as at least 10,000 diseases identi½ed by Inter- the fda, inno vative medicines could be national Classi½cation of Diseases (icd)– brought to pa tients who need them the including more than 6,500 “orphan con- most, more cheaply and quickly than is ditions” (disorders, often rare, for which currently possible. drug de velopment is commercially non- Perhaps the most far-reaching impact of viable)–which lack effective therapies. the convergence between chemistry and Together, these numbers suggest that, at human biology will be at a pedagogical lev- the present pace of drug discovery and de- el. Recent years have witnessed a gradual velopment, the road to even a moderately de-emphasizing of organic chemistry in comprehen sive arsenal of human-grade pre-medical education. While it is dif½cult drug treatments may be a long one. More

143 (4) Fall 2014 45 Book_Fall 2014_Shinner.qxd 9/8/2014 10:15 AM Page 46

The fundamentally, in sharp contrast to bio- The ½rst relates to the ease of sequencing Conver - logical studies on virtually every other the entire genome of a human being. The gence of Chemistry model organism, human biology remains National Human Genome Research Insti - & Human predominantly an observational science. tute estimates that the present cost of se - Biology Given that genetic manipulation of human quencing an individual’s genome is under beings is likely to remain severely con- $10,000; this number is expected to fall by strained on ethi cal grounds, the nexus be - at least an order of magnitude in the fore- tween chemistry and human biology needs seeable future. It is therefore very likely to be strengthened in order for human that, before too long, every healthcare con- biology to advance from its present status sumer will be able to have his or her entire as a prin ci pally observational science into genome sequenced for the price of an mri. an inter ventional one. The question now becomes: how can one Another argument supporting a serious exploit this information to enhance dis- re-evaluation of the chemistry–human ease management or, better yet, healthy liv - biology interface is the current state of ing? Cost-effective decoding of this data the pharmaceutical industry. The high cost may well be one of chemical biology’s of bringing a new drug to market (which, greatest contributions to our society since by some accounts, can run as high as $500 the sequencing of the genome. million) has forced the industry to prior- Biologists have independently developed itize discovery of the highest-priced med - ways to produce induced pluripotent stem icines over new paradigms for affordable cells (ipscs): cells derived from adult hu - healthcare. The time is ripe for the emer- mans that have been genetically repro - gence of new ideas and technologies that grammed into an embryonic stem cell– could spawn complementary business like state. Notwithstanding the infancy of models and public-private partnerships to this technology, ipscs have the potential restructure the healthcare industry. The to revolutionize human biology, and lead- emergence of a thriving molecular bio- ing medical centers are rushing in to es - marker industry is one example. Chemical tablish facilities for routine generation of biology could foster other analogous op- patient-derived ipscs. The ability to use portunities in the not-too-distant future. stem cells as individualized test tubes to For example, innovations in molecular understand, prevent, or treat disease rep- toxicology will be pivotal to the success resents an extremely promising opportu- of personalized medicine. Similarly, com - nity for chemical biology. panies that harness the creativity of both chemistry and human immunology to de - Although its promise is clear, the ½eld of velop fundamentally new approaches to chemical biology has not, to date, reach ed pre ventive medicine will likely blaze new its full potential. Much effort focuses on trails not too different from those forged harnessing robust chemistry in conjunc - by the pioneers of the Internet era. tion with high-speed engineering plat forms Meanwhile, two technological advances in order to quickly translate emerg ing bio- –genome sequencing and stem cell culture logical knowledge into new chemical tools. –are not only propelling the emergence of This is important work from which new Homo sapiens as one of biology’s most at - drugs will inevitably emerge.2 However, tractive targets of investigation but are also the real promise of chem ical bio logy lies in making a strong case for a closer, intellec- two other pursuits. At a fundamental level, tually deeper alliance between chemistry chemical bio logy can help elu cidate what and human biology. causes derangement of hu man physiolo-

46 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:15 AM Page 47

gy in the ½rst place. There is growing con- mysteries in a manner that lends itself to Chaitan sensus that com plex diseases such as au - chemical analysis or engineering. Bringing Khosla tism and autoimmunity are caus ed by ge - these researchers together is essential in netic as well as environmental fac tors. If so, order to encourage cross-fertilization of chemical biology may be able to shine light ideas and cultures. Environments where on the interplay between these triggers. such collaborations occur will inevitably And at the technological level, radically emerge as spectacularly powerful training new molecular tool-making approaches grounds for a new breed of young “physi- are needed to transform knowledge of the cian-scientist-engineers.” These research- biology of seemingly intrac table diseases ers will speak about human biology in the into practical treatments. Con sider cystic language of chemistry, tinker with objects ½brosis and von Gierke’s disease (a glyco - on a length-scale one million times smal - gen storage disorder): the mutations re - ler than the width of a human hair, and sponsible for these debilitating conditions only show deference to the laws of thermo- were identi½ed more than twenty years ago. dynamics. Their talent for molecular tool- Yet today we are no clos er to translating making, coupled with their passion for hu- these genetic insights into cures or even man biology, will allow them to evolve good treatments. The defective molecules new industries and business models where in cystic ½brosis and von Gierke’s disease health, not sickness, drives the bottom line. are just two examples of scores of clinical - Each fall, I ½nd myself facing a class of ly relevant targets categorized as “undrug- around two hundred of Stanford’s most gable” by today’s drug discoverers. The hu - accomplished undergraduates taking their man chemical biologist, on the other hand, ½rst course in biochemistry. I begin my recognizes that the reason why these genet - ½rst lecture with the reminder that, up un- ic discoveries have fallen short is because til today, my students’ chemical and bio- the right kind of molecular tool has not yet logical educations have been orchestrated been invented–but it can be done. from separate universes, but this is about Several prominent academic institutions to change. After all, biology is chemistry, (including my own, Stanford University) and chemistry would be not nearly as in - have launched major initiatives at the teresting were it not for biology. Many of chemistry-biology interface within the my students go on to become successful past decade. In most cases, these programs doc tors, scientists, or engineers. I can only are collaborative efforts between existing wonder whether any of my students will chemical and biological science depart- someday return to Stanford as clinician- ments within the institution, although a scientist-engineers. If so, what problems few examples of cross-institutional efforts will they see that none of us do? And will have also gained momentum. The ideal they see fundamentally new solutions to envi ronment would bring together clini- problems that the rest of us consider un - cians, scientists, and engineers, all of whom solvable? I look forward to that day. share an interest in strengthening the chemical foundations of human biology. These scholars will likely be either gifted molecular scientists or engineers who have turned their attention to important challenges in human biology, or they will be insightful biologists or physicians who can frame human biology’s most fascinating

143 (4) Fall 2014 47 Book_Fall 2014_Shinner.qxd 9/8/2014 10:15 AM Page 48

The endnotes Conver - 1 gence of President’s Council of Advisors on Science and Technology, “Report to the President on Pro - Chemistry pelling Innovation in Drug Discovery, Development, and Evaluation,” Executive Of½ce of the & Human President of the United States of America, September 2012. Biology 2 For a review of academic drug discovery operations, see Julie Frearson and Paul Wyatt, “Drug Discovery in Academia–The Third Way?” Expert Opinion on Drug Discovery 5 (2010): 909– 919.

48 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 49

Using Computational Chemistry to Understand & Discover Chemical Reactions

K. N. Houk & Peng Liu

Abstract: Chemistry, the “science of matter,” is the investigation of the fabulously complex interchanges of atoms and bonds that happen constantly throughout our universe and within all living things. Com- putational chemistry is the computer modeling of chemistry using mathematical equations that come from physics. The ½eld was made possible by advances in computer algorithms and computer power and continues to flourish in step with developments in those areas. Computational chemistry can be thought of as both a time-lapse video that slows down processes by a quadrillion-fold and an ultramicroscope that provides a billion-fold magni½cation. Computational chemists can quantitatively simulate simple chemistry, such as the chemical reactions between molecules in interstellar space. The chemistry inside a living organism is dramatically more complicated and cannot be simulated exactly, but even here computational chemistry enables understanding and leads to discovery of previously unrecognized phenomena. This essay describes how computational chemistry has evolved into a potent force for progress in chemistry in the twenty-½rst century.

In chemistry class, we learn that chemists study mat - ter and its properties; they wear lab coats and safety glasses and mix chemicals together and observe the amazing things that happen. But there is no need to go into a chemical laboratory to ½nd chemistry. In fact, chemistry is literally everywhere: it is the thou - sands of chemical processes that result in the emer- K. N. HOUK, a Fellow of the Amer- gence of a growing plant from a seed, the transfor- ican Academy since 2002, is the Saul mation of flower nectar into the flight of a humming- Winstein Chair in Organic Chemis- bird, or the conversion in chemical factories of oil try in the Department of Chemistry from decayed ancient life into polymers that are made and Biochemistry at the University into stylish fabrics or spacesuits. How do these things of California, Los Angeles. happen? Chemists learn how chemi cal reactions oc- PENG LIU is an Assistant Professor cur and how to control them for human purposes. In of Chemistry at the University of the twenty-½rst century, computa tional chemistry Pittsburgh. plays a major role in chemical discovery. (*See endnotes for complete con- Before the twentieth century, knowledge about the tributor biographies.) properties and transformations of matter was gained

Using through experimenta tion. Early chemical to obtain ever more accurate solutions to Compu - theories and rules, such as Mendeleev’s pe- small problems. tational Chemistry riodic table, were empirically derived from to Under- observations of chemical phenomena. xperiments yield facts, such as which stand & E Discover Some theories were wrong (for example, products are formed when various chem- Chemical the phlogiston theory, which posited the icals come into contact or how much elec- Reactions existence of an element called phlogiston tricity is generated when sunlight shines on in order to explain combus tion), while oth- a chunk of silicon or sandwich of organic ers were very crude models. The discovery polymers. However, experiments do not of quantum mechan ics in the 1920s revolu- tell us why such results occur. For example, tionized science. Heisenberg, Schrödinger, why are certain products formed and not Dirac, and other physicists developed a others, or why is only a few percent of the theo ry based on pure mathematics that energy in sunlight converted to electricity? explains how chemistry arises from the in- Both theory and computation are needed teractions of nu clei and electrons.1 Paul Di - to answer these questions: theory to pro- rac, one of the Nobel Laureates for quan- vide the general framework and simple tum me chan ics, noted in 1929: mod els for a qualitative conceptual under - pinning of experimental phenomena, and The underlying physical laws necessary for computation to flesh out an accurate mi- the mathematical theory of a large part of croscopic account of them. Today’s chem- physics and the whole of chemistry are thus ists attempt to employ computations to completely known, and the dif½culty is only explain phenomena and guide new exper - that the exact application of these laws leads iments, but quantitative modeling of to equations much too complicated to be chem ical reactions is very challenging due soluble. It therefore becomes desirable that to problems of scale. The chemical phe- approximate practical methods of applying nomena that we observe are the outcomes quantum mechanics should be developed, of rearrangements of the atomic structures which can lead to an explanation of the main of a huge number of very small molecules. features of complex atomic systems without A water droplet contains around one sex- too much computation.2 tillion (1021) molecules, each with a slight ly Exactly as Dirac envisioned, a hierarchy different shape, velocity, and energy at any of mathematical models, with different given moment. The atoms in each water lev els of approximation, has been devel- molecule are moving rapidly inside the oped over the last century.3 But Dirac could droplet: the atoms change to a new ar- not foresee the discovery and development rangement 1014 times per second. To com - of powerful computers with which we can pletely reproduce the properties of that solve some of these highly complex prob- droplet and predict how it will change up - lems of applied mathematics. While we still on heating or mixing with other chemicals cannot obtain exact solutions to the quan- would require simulating all sextillion of tum mechanical equations for chemical the droplet’s fast-moving molecules, were systems with very large numbers of atoms, we to compute everything from exact we can calculate answers as close as de - quan tum mechanical equations (or “½rst sired to the exact mathematical solution, principles”). Modern computers can calcu- given enough computer time. When more late how one molecule changes over time, powerful computers become available, but to calculate all sextillion or even a sig- com putational chemists will set out to ni½cant fraction of them is not practical, solve bigger and bigger problems and try nor will it be anytime in the foreseeable

50 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 51

future. However, approximate equations– this, calculations on such large mole cules K. N. Houk model systems calibrated with empirical must involve shortcuts that make the cal- & Peng Liu data to capture the average properties of a culations faster but less accu rate. water molecule and its interactions with other molecules–can be computed to al- Computational modeling is the simula- low us to understand what occurs in the tion of chemical structures, properties, and drop of water and to estimate its proper- reactions with a computer. Simulation is ties: density, surface tension, viscosity, and sometimes described as the third form of even chemical reactivity. science. The ½rst form, experimental sci- Aside from the daunting numbers of cal - ence, starts with empirical observations culations that must be performed to mim- and models created from inductive logic. ic reality, there is also the issue of the size of The second form is theoretical science, for - some important molecules: smaller mol - mulated in equations that describe the e cules are, of course, much simpler to mod - phenomena of the natural world. Simula- el. As the number of electrons in a molecule tion is a third form where mathematical increases, the time needed to perform cal- equations are coded into computer pro- culations on it goes up rapidly. A hydrogen grams to predict what happens in various H molecule ( 2) consists of two of the light- hypothetical chemical situations. est atoms bonded together and only two The fundamental theories used in these electrons; natural gas con sists pri marily of computer programs are based on classi- CH meth ane ( 4), which has only ½ve light cal and quantum mechanics. Galileo, Kep- atoms and ten electrons. Everything about ler, Newton, and other scienti½c revolu- individual hy dro gen and meth ane mole- tionaries of the late seventeenth century cules can be computed nearly exactly in a developed what we now call classical me - short time. How ever, many mol ecules of chanics, which describes the physics of crucial importance for life, and those that relatively large objects moving on a human make up com mon materials, are much timescale. Newton’s equations of motion larg er. Consider a nucleic acid mol ecule, (as these classical mechanics equations are such as a strand of dna, or a protein that often called) are used for molecular dy - controls so many of the pro cesses of life, namics simulations to derive the motion of or a polymer molecule in a poly styrene cup: atoms in molecules or larger ob jects over each molecule contains thousands of at- time. Classical mechanics can also be used oms and can ex ist in many different three- to study structures of molecules by ½tting dimensional ar rangements that in ter con - equations to empirical data–what chem- vert very quickly. To simulate the be hav - ists call “molecular mechanics.” However, ior of chemicals with so many atoms takes classical mechanics cannot predict chemi- many computer re sources. Depend ing on cal reactions and reactivity, because the how accurate the calculations need to be, mo tions of electrons are wave like, quan- the number of hours needed to perform tized, and described correctly only by quan - com putations on molecules scale be tween tum mechanics. For more mas sive and the third and the seventh power of the slowly moving systems, classical and quan- number of electrons contained within tum mechanics converge, but quantum them! This means, for example, a cal cu la - mechanics is uniquely capable of describ- tion involving a benzylpenicillin mole cule ing the electronic structure of atoms and with forty-one atoms can be up to two mil - molecules and thus chemical properties lion times slower than the same calcu la tion and reactions. Multiscale computational done with a methane molecule. Be cause of methods, which employ both classical and

143 (4) Fall 2014 51 Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 52

Using quantum mechanics, have been developed exploring real chemistry, and software Compu - for calculations of complex chemical and companies have been formed to further de- tational Chemistry biological systems, such as proteins. Here, velop and market these programs commer- to Under- quantum mechanics is applied to study the cially.5 stand & Discover central part of the sys tem: for example, at - The progress described here was stimu- Chemical oms that are close to the forming or break - lated by the development of computers. Reactions ing chemical bond in a reaction. The re - The eniac (Electronic Numerical Integra- maining atoms are treat ed with classical tor and Computer) and other general-pur- mechanics so that such calculations can be pose computers in the 1940s occupied space applied to very large systems. In 2013, three equal to a comfortable house for four peo- of the pioneers in this ½eld, Martin Kar - ple. Now, the computers that reside in our plus, Michael Levitt, and Arieh Warshel, smartphones are about a trillion times were awarded the Nobel Prize in Chemis- more powerful. Furthermore, computa- try for their studies in the early 1970s that tional chemists have access to computers established what is now called the qm/ all over the world, and rapid Internet con- mm method. nections give computational research Based on these underlying theories, groups in the United States access to a many computer algorithms to calculate whole network of powerful computers sup- pro perties and reactions were written in ported by the National Science Foundation the last century, and these developments and other federal research agencies. Other continue to this day. While the fundamen- countries have similar networks, and there tal equations of quantum mechanics are is international competition to produce the de ceptively simple, the computer programs most powerful computer. written in order to use them in simulations The impact of these developments on are extremely complicated. In 1998, the No- the capabilities of computational chemis - bel Prize in Chemistry was awarded to try has been profound, and the ½eld has be- John Pople, a mathematician and chemist come an increasingly important aspect of whose research group developed many of science. In the flagship journal of chemis - these algorithms and computer programs, try, the Journal of the American Chemical Soci- and Walter Kohn, a physicist who, with his ety, the number of computational papers coworkers, developed an alternative meth - has risen from very few in the 1960s to over od of solving the Schrödinger equation, three hundred papers per year. Along with now known as density func tional theory the growth of computational chemistry in (dft). Pople and Kohn were at the fore- mainstream journals, there has been a pro - front of using computational methods in liferation of journals speci½cally devoted to chemistry, inspiring many mathemati- the subject. There is the Journal of Computa - cians, mathematical chem ists, and physi- tional Chemistry, the Journal of Chem ical Theo - cists to devise algorithms and computer ry and Computation, and at least two dozen methods for studying chemical phenom- other journals that concentrate on the study ena. For example, one of the main proof chemistry using computation. Chem- grams now used for these calculations has istry is not unique in this regard: physics seventy-four authors from all over the has a dozen such journals; and bio logy has world!4 These authors and many other sci- Computational Biology and many other pub- entists worked over the last ½fty years on lications that emphasize computation. various computational chemistry programs The most successful computations also that are now in general use by chemists. lead to the development of general con- The programs have become very useful for cepts that can be used to guide future ex -

52 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 53

periments and make predictions. This is, of was predicted by a quantum mechanical K. N. Houk course, helpful in the ½eld of organic chem - cal culation. The calculation took ten min - & Peng Liu istry, the area of expertise of the authors of utes today using a powerful desktop com- this article. Organic chemistry may be no - puter, but thirty years ago when the study torious as a gatekeeper for future doctors, began, the same calculations took one week but it is really an intellectually rich and and involved a small roomful of equip- chal lenging branch of chemistry that in - ment. Chemists have developed these types volves chemical compounds containing of pictures for the rapid visual represen- carbon atoms, along with any of the other tation of what is actually a very compli- atoms of the periodic table. Organic chem- cated mathematical result in a computer. istry touches all of our existence, from life- Experiments already showed that these sav ing and -enhancing pharmaceuticals to reactions typically occur with the rota- fuels, insecticides, and organic electronic tion of both termini in the same direction materials. We describe in the following (this motion was called “conrotatory” by pages how computations are used to ex - Woodward and Hoffmann), rather than plore and understand organic chemistry. op posite directions (called “disrotatory”). As shown in Figure 1, each of the two mo- In 1965, R. B. Woodward and Roald Hoff - tions leads to a distinct product called a ste- mann published one of the most influen- reoisomer. Different stereoisomers have tial conceptual developments that thrust the same atoms and bonds, but they are theory, and eventually computation, into connected together in different three-di - the forefront of organic chemistry.6 Al- mensional spatial arrangements. This dif- though these concepts were grounded in fer ence in shape gives stereoisomers dis- previous developments by many scientists, tinct chemical prop er ties: molecules that they came to be known by chemists as the have the same “structure” but a different Woodward-Hoffmann rules. Based on stereochemistry and shape may turn out to quan tum mechanical principles, these rules be a life-saving drug or a poison, depend- give predictions about a particular class of ing on their three-dimensional shape. It organic chemical reactions in which bond is therefore important to understand and formation and bond breakage occur simul- control which stereo iso mer is formed in taneously in a ring of atoms. One exam- a reaction used to make the molecule. ple of such a reaction is the ring opening Using qualitative reasoning and support- of a molecule known as cis-3,4-dimethyl- ed by the very approximate calculations cyclobutene, shown in Figure 1. possible at the time, Woodward and Hoff - With these images, we launch into real mann provided an elegant quantum me - organic chemistry and hope to introduce chanical interpretation of the selectivity the reader to the visual world that organic shown in Figure 1. The conrotatory process chemists occupy and that computational maximizes bonding all along the reaction organic chemists study. The ½rst two pic- pathway (it is “allowed,” or occurs rapidly), tures shown in Figure 1 are computer draw - while the disrotatory opening in volves a ings of the three-dimensional structure of motion that would require a very high en- the cis-3,4-dimethylcyclobutene molecule. ergy to occur (it is therefore “for bid den”). While it can also be represented by its for- They described these principles in terms of C H mula, 6 10, there are many different mol- the symmetries of orbitals, the regions in ecules with that same formula, each of which electrons are located according to which has unique properties. The exact way the usual form of quantum mechanics used that the atoms are arranged in this ½gure to describe molecules. The insights led to

143 (4) Fall 2014 53 Book_Fall 2014_Shinner.qxd 9/18/2014 2:46 PM Page 54

Using Figure 1 Compu - Three-Dimensional “Space-Filling,” “Ball-and-Stick,” and Schematic Representations tational of cis-3,4-dimeth ylcyclobutene and the Conrotatory and Disrotatory Reaction Products Chemistry to Under- stand & Discover Chemical Reactions

conrotatory (allowed)

cis,trans-2,4-hexadiene

cis-3,4-dimethylcyclobutene disrotatory (forbidden) trans,trans-2,4-hexadiene

In the two structures on the far left, spheres represent the positions of the atoms in the reactant molecule. The larger spheres show the carbon atoms, and the smaller spheres show the hydrogen atoms. The size of the atoms in the “space-½lling” picture represents their van der Waals radii (which measure how close two atoms can ap - proach). Smaller spheres were used for the “ball-and-stick” picture to illustrate the chemical bonds (indicated by bold lines) that are formed by a buildup of electrons between the nuclei. The third picture is a sketch of the same molecule, with the atoms represented by letters and the large balls representing the larger size of the “sub- CH H stituent” methyl groups ( 3; the bold lines indicate that the atoms are in the foreground, and the dotted lines CH indicate that the 3 groups recede backward). The sketches in brackets show the changes occurring in the reactions. The dashed lines indicate bonds that are breaking in the reaction. The products of the reactions are shown in the “ball-and-stick” and sketch renditions to the right of the arrows. Source: Figure prepared by the authors using data in R. Hoffmann and R. B. Woodward, “Stereochemistry of Electrocyclic Reactions,” Journal of the American Chemical Society 87 (1965): 395–397.

new understanding of a broad segment of inward-rotating reaction ten billion times organic chemistry and to predictions of faster than the non-observed outward- new reactions that were subsequently dis - rotating reaction. The result was very puz - CF covered experimentally. zling, since the larger 3 group bumps into Chemists found through experiments the other CF3 in the faster reaction, and that their understanding was incomplete. nature usually minimizes such bumping In examples such as that shown in Figure (which we call steric clashes)–but not 2, there are two different “allowed” con- here. Our group used quantum mechanics rotatory processes: namely, rotation in a and computational chemistry to under- clockwise or in a counterclockwise fash- stand why. ion. The Woodward-Hoffmann rules did Quantum mechanical simulations of not differentiate between the two, but re - these reactions showed the motions of the searchers found a huge preference for one nuclei and electrons in these molecules as direction of rotation in several cases stud- they change from reactants to either of the ied experimentally.7 The difference be- two possible products. These calculations tween the activation energies required for also determine how much energy it takes these processes to occur (30.5 kcal/mol for for the bonds of one molecule (the reac- the inward conrotatory rotation and 49.7 tant) to be reorganized through nuclear kcal/mol for the outward conrotatory and electron rearrangements to form an - rotation) is enough to make the observed other molecule (the product). The “transi-

54 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 55

Figure 2 K. N. Houk Two “Allowed” Reactions and the Large Activation Energy Differences in the Two Modes & Peng Liu of Ring Opening of trans-perfluoro-3,4-dimethylcyclobutene

The horizontal lines represent the relative energies of the reactants (left), transition states (“TS,” center), and prod- E ucts (right), and the lines show how these are interrelated. The experimentally measured activation energies ( a) are shown next to the corresponding transition states. Lower activation energy means a faster reaction, and so the reac - tion follows the path with the lowest activation energy (indicated by the solid lines), even though the product of that path is less stable. Source: Figure prepared by Peng Liu using data from W. R. Dolbier, Jr., H. Koroniak, D. J. Burton, A. R. Bailey, G. S. Shaw, and S. W. Hansen, “Remarkable, Contrasteric, Electrocyclic Ring Opening of a Cyclobu - tene,” Journal of the American Chemical Society 106 (1984): 1871–1872.

tion state” is the highest-energy point along like (rough sketches are given in Figure 2) the best path from reactant to product as so that we could analyze them to under- marked in Figure 2,8 and it is the energy of stand why one transition state is much this transition state relative to the reactants lower in energy than the other. E (activation energy, a) that determines how Calculations of this type could also be fast or slow the reaction will occur. By performed for other atoms and groups be- F CF quan tum mechanical calculations, we sides and 3, and we eventually learn ed found out what the transition states look that certain types of substituents–those

143 (4) Fall 2014 55 Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 56

Using we call electron-donors (D in Figure 3)– This example illustrates that when Compu - always rotate outward away from the quan tum mechanics is applied to a small tational Chemistry breaking bond, but strong electron-accep- enough molecule, an accurate prediction to Under- tors (A in Figure 3) rotate inward toward of the product of a new chemical reaction stand & Discover the breaking bond. Donors are already sur- is possible. These calculations also led to Chemical rounded by electrons and thus avoid inter - the development of the theory of torquo - Reactions acting with the electrons of the bond that selectivity that is applicable to every other breaks, but acceptors seek electrons and reaction of this type. tend to move toward the breaking bond (see Figure 3). The shaded shapes are meant Another signi½cant application for com- to represent the regions where electrons putational chemistry is the development are localized and would have repulsive in - of catalysts (substances that speed up a re - teractions with other ½lled orbitals nearby. action but are not consumed during the The empty shapes represent regions that reaction). Catalysts cause reactions that do not have electrons but would like to; normally do not occur at all to take place these empty orbitals cause an atom to be under conditions that are easily achiev- electron-loving, or “electro philic.” Quan- able. Chemists aim to develop catalysts to tum mechanics shows that the interaction achieve new chemical transformations or of a ½lled orbital with a va cant orbital is to increase the ef½ciency of valuable reac - favorable, while the interaction of two tions. ½lled orbitals is repulsive. This is the basis Many reactions used in the chemical in- of bonding and steric effects, respectively. dustry involve transition metal catalysts Since the bond twists–or torques–as (transition metals are so called because it breaks, we describe this selective twist- they have partially occupied d orbitals and ing as “torquoselectivity.”9 Computations therefore special properties). Three Nobel were used ½rst to reproduce the initial ex- Prizes in Chemistry have been awarded in periment, then to analyze the results, and the last ½fteen years (in 2001, 2005, and then to develop concepts and rules to pre - 2010) for discoveries about organic reac- dict the results of similar future exper i - tions using these catalysts.11 The study of ments. The principle here has been applied chemicals containing carbon and metal at- to predict the course of reactions of new oms in the same molecule is called organo - substances synthesized for the ½rst time. metallic chemistry and is now a prominent For example, Figure 4 shows a simple mol- ½eld of chemistry. The carbon atoms are ecule, 3-formylcyclobutene, which was part of the ligand attached to the metal. made in our laboratory in 1987 for the ½rst Catalytic reactions generally involve time to test the prediction made about the many steps and intermediates that are usu- unexpected stereochemistry of this reac- ally dif½cult to detect or identify experi- tion.10 Based upon our torquoselectivity mentally, although new spectroscopic and theory, we expected that the formyl group imaging tools are being developed to try (labeled “CHO” in Figure 4), an acceptor- to achieve this. In the glory days of mech- type substituent, would rotate inward, and anistic physical organic chemistry, the we used a quantum mechanical simulation masters of the ½eld, such as Saul Winstein to predict exactly how much this was pre- at ucla and Paul D. Bartlett at Harvard, ferred over an outward rotation. Then we devised many clever experiments using the did the experiment, and it worked! Only instrumentation available at the time to the less stable product (shown on the top try to deduce how reactions occurred in line of Figure 4) is formed. solutions. Major controversies often devel-

56 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 57

Figure 3 K. N. Houk Orbital Interactions in Two Conrotatory Transition States in the Electrocyclic Ring & Peng Liu Opening of a cis-3-donor-4-acceptor-cyclobutene

The torquoselectivity model developed from calculations predicts (correctly) that the counterclockwise motion shown on the top line of the ½gure is highly preferred. Source: Figure prepared by the authors using data from N. G. Rondan and K. N. Houk, “Theory of Stereoselection in Conrotatory Electrocyclic Reactions of Substituted Cyclobutenes,” Journal of the American Chemical Society 107 (1985): 2099–2111.

Figure 4 Computations Predicted the Torquoselectivity of Electrocyclic Ring Opening of 3-Formylcyclobutene

Source: Figure prepared by the authors using data from K. Rudolf, D. C. Spellmeyer, and K. N. Houk, “Predic- tion and Experimental Veri½cation of the Stereoselective Electrocyclization of 3-Formylcyclobutene,” Journal of Organic Chemistry 52 (1987): 3708–3710.

143 (4) Fall 2014 57 Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 58

Using oped about the interpretation of the ex- Determining why this ruthenium cata- Compu - periments. Nowadays we try to gain this lyst produced Z-ole½ns was a dif½cult chal- tational Chemistry mechanistic information about catalytic lenge to computational chemistry. The to Under- reactions by looking directly at molecules molecules involved in this reaction are stand & Discover as they react using computer simulations. large and contain metals, which have high Chemical Shown in Figure 5 is ole½n metathesis, a atomic numbers and dozens of electrons. Reactions very important reaction in industry and Consequently, hundreds of computer lab oratory synthesis. Metathesis means hours are needed for each computation. transposition, in this case of the atoms In addition, there are many structures to from one ole½n to another. An ole½n has compute due to the great number of ways two carbons joined by a double bond; each the reaction could occur and the multiple single bond is made of one pair of electrons structures involved in each case. Extensive (a double bond, comprising two pairs of experimental and computational studies electrons, is represented by two lines). The of ole½n metathesis with previously re- metathesis reaction shown in Figure 5 ported ruthenium catalysts have been car - swaps the atoms making up the ends of ried out all over the world in the last few each double-bonded ole½n. This reaction decades. Based on those studies, our group provides one of the most powerful strate- had many clues about what types of reac- gies for making new carbon-carbon double tions we needed to investigate. We were bonds and is important for the synthesis of able to limit the number of structures to many complex organic compounds and evaluate, rather than having to compute polymeric materials like those used for every possibility for this complex reac- many familiar objects, from Norsorex pants tion.14 Given previous results, there were for motorcyclists to gigantic wind turbine really only two plausible pathways, distin- (modern windmill) blades. guished from one another by the direction A major challenge that developed during from which the ole½n molecule approach - the study of ole½n metathesis was to ½nd es the catalyst. The approach can be either catalysts that form “Z-ole½ns,” in which ad jacent or opposite to the ligand. These the two substituents (X and Y) in the prod- two pathways are shown in Figure 7 and uct are on the same side of the double are called “side” and “bottom” approaches. bond. Many years after the discovery of Our computations revealed a major sur- ole½n metathesis, chemists were still try- prise and a crucial discovery: in contrast to ing to learn how to make Z-ole½ns this everything known before, this reaction way so that new compounds and materi- with the new ruthenium catalysts involves als could be synthesized via the ole½n me - side approach of the ole½n to the catalyst.15 tathesis process. In 2009, chemists Amir Computational technology allowed us to Hoveyda and Richard R. Schrock found the render visualizations of the three-dimen- ½rst molybdenum- and tungsten-based sional structures of the transition states ole ½n me ta thesis catalysts that selectively (Fig ure 7). Scientists use microscopes to produce Z-ole½ns,12 and two years later, see microbes and employ atomic force chemist Robert H. Grubbs, one of the No- micro scopes (afm) to study materials at bel Laureates in this ½eld (along with Rich- the atomic level (for example, in the bur- ard R. Schrock and Yves Chauvin), discov- geoning ½eld of nanochemistry). By con- ered a new type of ruthenium catalyst that trast, there are currently no established performed the same function (Figure 6). ex perimental tools to visualize transition Why this catalyst produced Z-ole½ns, how - states, since they are only about 10−9 me- ever, was not known.13 ters (1 nanometer) in diameter and exist for

58 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/18/2014 2:47 PM Page 59

Figure 5 K. N. Houk The Ole½n Metathesis Reaction Swaps the Ends of Ole½ns & Peng Liu

The usual products of these reactions, called E-ole½ns, have the X and Y groups on opposite sides of the double bond. Source: Figure prepared by the authors using data from R. H. Grubbs and S. Chang, “Recent Advances in Ole½n Metathesis and Its Application in Organic Synthesis,” Tetrahedron 54 (1998): 4413–4450.

Figure 6 The Ole½n Metathesis to Give Z-Ole½ns

Source: Figure prepared by the authors using data from K. Endo and R. H. Grubbs, “Chelated Ruthenium Cata- lysts for Z-Selective Ole½n Metathesis,” Journal of the American Chemical Society 133 (2011): 8525–8527.

less than 10−13 seconds! (Chemists like Ah- lyst complex, as shown in Figure 7. The ole- med Zewail at Caltech are working to de- ½n approach ing in this way clashes with vel op such experimental tools.) Computa - the ligand and places the substituents on tions, however, are able to bring the tran- the ole½n on the same side of the newly sition state to life by taking snapshots of formed double bond to form the Z-ole½n. simulated reactions as they happen (imag- This is very different from previous reac- ine a camera with a shutter speed of one tions using other ruthen ium catalysts, in femtosecond, or 10−15 seconds!) and by which the ole½n approaches from the bot- func tioning like a super–high power mi - tom, far away from the ligand, causing the croscope (with 109 times magni½cation!). formation of more stable E-ole½ns rather Although it is a prediction that cannot cur- than Z-ole½ns. rently be veri½ed directly, this picture en - These computations provided important ables us to interpret important occurrences insights for further catalyst development. in this reaction, such as how individual at- Armed with the knowledge that Z-selec- oms attract or repel each other, that would tivity in the new catalysts arises from the otherwise be impossible to ob serve. Such repulsions with the ligand on the catalyst, an analysis revealed that the Z-ole½n is researchers began experimental stud ies of selectively formed due to the “side” ap - catalysts with even larger ligands. This led proach of the ole½n molecule in the cata- to the discovery of an improved Z-selective

143 (4) Fall 2014 59 Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 60

Using Figure 7 Compu - Three-Dimensional Renditions of the Computed Transition State Structures tational of the Possible Approaches of the Ole½n Molecule Chemistry to Under- stand & Discover Chemical Reactions

a) Shows the ole½n approaching the ruthenium catalyst adjacent to the ligand (the “side” approach); b) shows the ole½n approaching opposite the ligand on the ruthenium catalyst (the “bottom” approach). A qualitative rendering is shown at the right. Source: Figure prepared by the authors using data from P. Liu, X. Xu, X. Dong, B. K. Keitz, M. B. Herbert, R. H. Grubbs, and K. N. Houk, “Z-Selectivity in Ole½n Metathesis with Chelated Ru Cata- lysts: Computational Studies of Mechanism and Selectivity,” Journal of the American Chemical Society 134 (2012): 1464–1467.

catalyst by the Grubbs group.16 Compu- different combinations of metals produce tational investigations of this type have be- useful metal alloys as catalysts.17 come a standard way to accelerate under- Nature generally uses proteins, and standing and discovery, and many exper- sometimes ribonucleic acids (rna), to cat - imental groups have become involved in alyze the reactions necessary for metabo- computational work to complement their lism at the rates required to sustain life. experiments. Proteins are poly-amino acids of the general structure shown in Figure 8 (a), where The Z-selective catalyst was discovered “R” can be any of the twenty different side accidentally through experiments; now chains of natural amino acids. The amino computations have helped to determine acid fragments are connected in a speci½c precisely which experiments might im- sequence that determines the structure prove such catalysts. Similar computation- and properties of a protein. al approaches are also being used to pre- Our research group at ucla collabo- dict new catalysts for pharmaceuticals, rates with David Baker’s group at the Uni- fuels, and materials. For example, compu- versity of Washington and Stephen Mayo’s tational materials scientists calculate how group at Caltech to design new enzymes

60 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 61

Figure 8 K. N. Houk The General Structure or “Primary Sequence” of a Protein (a); the Three-Dimensional Representa- & Peng Liu tions of a Protein, Cytochrome P450cam, Showing All Atoms in a Space-½lling Display (b); and a “Ribbon Diagram” of Protein Architecture (c).

Source: The three-dimensional protein structures are illustrated using PyMol (Version 1.3 Schrödinger, llc) with crystal structure obtained from the Protein Data Bank (PDB ID: 2ZWT); and K. Sakurai, H. Shimada, T. Hayashi, and T. Tsukihara, “Substrate Binding Induces Structural Changes in Cytochrome P450cam,” Acta Crystallographica Section F 65 (2009): 80–83.

that catalyze “non-natural reactions”: the ed) needed, it is a simple matter for chem - many reactions not catalyzed by naturally ists to use automatic machines to synthe- occurring enzymes. We do this by using size the desired dna and for molecular computer calculations to predict which biologists to incorporate this dna into a pro tein structures will fold into a speci½c microorganism and induce it to produce three-dimensional structure like those these new proteins. shown in Figure 8 (b) and (c). We can then We use quantum mechanical calcula- try to create a fold that will align the cat- tions to design optimal arrangements of alytic groups from the protein in order to protein active site components that are pre- catalyze a desired reaction–perhaps one dicted to catalyze speci½c reactions. Da vid that has known practical or commercial Baker’s group has developed a computer value or perhaps simply one we dreamed program called Rosetta that predicts the up. If we can predict the amino acid se - amino acid sequences of proteins that will quence (the list of individual amino acids fold up into a speci½c three-dimensional and the order in which they are connect- structure. The program is based on classi-

143 (4) Fall 2014 61 Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 62

Using cal mechanics and empirical information; zymes, and even then only a small fraction Compu - because the proteins that are studied are of the computationally designed en zymes tational Chemistry large and can adopt many shapes, accurate are active in the experiment. Nevertheless, to Under- quantum mechanical calculations would designing protein catalysts from scratch stand & Discover take too long to be useful. Although they using only computer calculations is a major Chemical only produce approximate models, Rosetta development, and we envision that this Reactions and other programs can nevertheless offer technology will lead to catalysts for the valuable information about which ar- synthesis of many important compounds range ment of atoms in proteins is most and for therapeutic purposes as well. stable. Realizing the potential of these tools, These examples illustrate a very small por - our group and David Baker’s began about tion of the ½eld of computational chem- ten years ago to create what we call the istry. Computer programs that calculate inside-out approach to enzyme design and predict properties of chemical systems (outlined in Figure 9).18 We start with using a combination of theoretical meth- quantum mechanical models of the tran- ods have been developed for use in many sition states of the chemical reaction we areas of chemistry. One well-established wish to catalyze and calculate which pro- example of the integration of multiple tein side chains will stabilize the transi- com p utational tools to solve important tion state. This becomes a model for the prob lems is the ½eld of computational core of the enzyme where catalytic reac- drug design.21 Calculations in this enter- tions occur. We call this computational prise range from structural evaluation to model a theoretical enzyme, or “theo- quickly screen thousands of candidate mol- zyme.” Then, using Rosetta, we ½nd a sta- ecules for use in drugs to elaborate simu- ble protein structure in the Protein Data lations of substrate-protein binding that Bank (a database of the three-dimensional can predict whether a molecule will act as structures of all known proteins) that can a good inhibitor for a target protein in - be modi½ed to achieve the designed struc - volved in a disease. Such calculations have ture with the necessary catalytic groups prov en their worth in developing new en - aligned in the perfect positions for cataly - zyme inhibitors, although the path from sis.19 After extensive computational tests effective inhibitors to commercial drugs using both quantum mechanics and clas- is still long, expensive, and mostly empir- sical molecular dynamics, the best compu- ical. tationally designed enzymes are selected Yet another innovative use of computa- for experimental testing. The actual pro- tional chemistry is in developing new teins are produced by modi½ed microor- materials for many different industries. ganisms such as E. coli and are then tested Computational chemists are developing for catalysis. Using this procedure, we have programs based on a combination of quan- successfully produced new effective cata- tum and classical mechanics to compute lytic proteins for three different types of the properties of structural materials, solar- reactions.20 The entire process of design- energy conversion devices, and new chem- ing new enzymes through computation ical batteries. Aiming to aid the develop- currently takes years; however, it takes ment of computational architecture and much longer (billions of years) for nature methodology for materials chemistry, the to evolve enzymes for metabolism. Right White House approved the Materials Ge - now we must do many thousands of cal- nome Initiative in 2011.22 The name evokes culations to make predictions of new en- the remarkably successful Human Genome

62 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 63

Figure 9 K. N. Houk Overview of the “Inside-Out” Computational Approach to Designing Unnatural Enzymes & Peng Liu

Source: G. Kiss, N. Çelebi-Ölçüm, R. Moretti, D. Baker, and K. N. Houk, “Computational Enzyme Design,” Ange- wandte Chemie International Edition 52 (2013): 5700–5725.

Project and extends the idea to the world of Innovations in computer hardware de - materials. It has therefore been recognized sign will continue to enhance the scope of at the highest policy level that computa- computational chemistry. For example, the tional methods can accelerate the discov- development of graphics processing units ery of advanced materials and shorten the (gpus) by the computer industry has en - process of deploying them to the commer- ergized the entertainment industry. The cial market. success of computer gaming has made

143 (4) Fall 2014 63 Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 64

Using these devices inexpensive, and computa- ways, not from simple extensions of known Compu - tional chemists are rushing to adapt their phenomena. Quantum mechanics can pre- tational Chemistry programs to these commodity devices in dict things that have never been observed. to Under- order to enhance their modeling capabil- To predict what reaction happens when a stand & 23 Discover ities. new combination of chemicals is tested Chemical What will we be able to do with these re quires the evaluation of every possibility. Reactions com puters of the future? We have dis - Computational chemists are working on cussed in this article how computations are methods to predict reactions and their applied to study the way chemical reac- rates based solely on the information about tions occur and to improve and extend the separated reactants, catalysts, solvents, them. In the future, this will be done more and reaction conditions, essentially forcing accurately, more quickly, and on much molecules together and seeing what hap- larger systems, producing more realistic pens in the computer.24 models of chemistry. Predicting complete- We have described how computational ly new chemical transformations is likely chemists go about exploring chemistry and to re main challenging, because so many developing new models and theories to differ ent bonds may be made or broken un derstand nature and predict useful in a chemical reaction and, as we stated things. Better algorithms and increasing above, the combinations of even relatively computer power make ever larger and simple pure chemicals can lead to a huge more accurate calculations possible, and num ber of reaction pathways that grows this challenges computational chemists to exponen tially with the number of atoms take on larger and more complex prob- involved. Experimental knowledge about lems. Computational chemistry has grown existing reactions may help chemists guess from the breakthrough theory of the early the outcome of an unknown reaction, but twentieth century into a ubiquitous and im por tant discoveries in chemistry often powerful engine for chemical discovery in re sult from discoveries of new types of the twenty-½rst. transfor mations that occur in unexpected

endnotes * Contributor Biographies: K. N. HOUK, a Fellow of the American Academy since 2002, is the Saul Winstein Chair in Or ganic Chemistry in the Department of Chemistry and Biochemistry at the University of California, Los Angeles. He taught earlier at Louisiana State University and the University of Pittsburgh, and was Director of the Chemistry Division of the National Science Foundation. Over his career, he has “evolved” from an experimental physical organic chemist to a computational chemist, parallel to the developments of the research ½eld de - scribed in this article. PENG LIU is an Assistant Professor of Chemistry at the University of Pittsburgh. He obtained his Ph.D. in Chemistry at the University of California, Los Angeles, where he was a Post- doctoral Scholar in Professor K. N. Houk’s research group. His research interests include computational studies of organometallic and organic reactions. 1 Graham Farmelo, The Strangest Man: The Hidden Life of Paul Dirac, Mystic of the Atom (New York: Basic Books, 2009). 2 Paul A. M. Dirac, “Quantum Mechanics of Many-Electron Systems,” Proceedings of the Royal Society of London A 123 (1929): 714–733.

64 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 65

3 Christopher J. Cramer, Essentials of Computational Chemistry: Theories and Models, 2nd ed. K. N. Houk (Malden, Mass.: John Wiley & Sons, 2004). & Peng Liu 4 M. J. Frisch et al., Gaussian 09, Revision D.01 [electronic structure modeling program] (Wal- lingford, Conn.: Gaussian, Inc., 2009). 5 A. B. Richon, “An Early History of the Molecular Modeling Industry,” Drug Discovery Today 13 (2008): 659–664. 6 See R. Hoffmann and R. B. Woodward, “Stereochemistry of Electrocyclic Reactions,” Journal of the American Chemical Society 87 (1965): 395–397; R. Hoffmann and R. B. Woodward, “Se - lection Rules for Concerted Cycloaddition Reactions,” Journal of the American Chemical Society 87 (1965): 2046–2048; R. Hoffmann and R. B. Woodward, “Selection Rules for Sigmatropic Reactions,” Journal of the American Chemical Society 87 (1965): 2511–2513; and R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry (New York: Academic Press, 1970). 7 W. R. Dolbier, Jr., H. Koroniak, D. J. Burton, A. R. Bailey, G. S. Shaw, and S. W. Hansen, “Re mark able, Contrasteric, Electrocyclic Ring Opening of a Cyclobutene,” Journal of the Amer - ican Chemical Society 106 (1984): 1871–1872. 8 Henry Eyring, “The Activated Complex in Chemical Reactions,” The Journal of Chemical Physics 3 (1935): 107–115. 9 W. Kirmse, N. G. Rondan, and K. N. Houk, “Stereoselective Substituent Effects on Conrotatory Electrocyclic Reactions of Cyclobutenes,” Journal of the American Chemical Society 106 (1984): 7989–7991. See also N. G. Rondan and K. N. Houk, “Theory of Stereoselection in Conrota- tory Electrocyclic Reactions of Substituted Cyclobutenes,” Journal of the American Chemical Society 107 (1985): 2099–2111. 10 K. Rudolf, D. C. Spellmeyer, and K. N. Houk, “Prediction and Experimental Veri½cation of the Stereoselective Electrocyclization of 3-Formylcyclobutene,” The Journal of Organic Chemistry 52 (1987): 3708–3710. 11 These Nobel Prizes in Chemistry were awarded for asymmetric hydrogenations and oxidations (William S. Knowles, Ryoji Noyori, and K. Barry Sharpless; 2001), ole½n metathesis (Yves Chauvin, Robert H. Grubbs, and Richard R. Schrock; 2005), and palladium-catalyzed cross couplings (Richard F. Heck, Ei-ichi Negishi, and Akira Suzuki; 2010). 12 A. J. Jiang, Y. Zhao, R. R. Schrock, and A. H. Hoveyda, “Highly Z-Selective Metathesis Homo - coupling of Terminal Ole½ns,” Journal of the American Chemical Society 131 (2009): 16630– 16631; and S. J. Meek, R. V. O’Brien, J. Llaveria, R. R. Schrock, and A. H. Hoveyda, “Catalytic Z-Selective Ole½n Cross-Metathesis for Natural Product Synthesis,” Nature 471 (2011): 461–466. 13 K. Endo and R. H. Grubbs, “Chelated Ruthenium Catalysts for Z-Selective Ole½n Metathesis,” Journal of the American Chemical Society 133 (2011): 8525–8527; and B. K. Keitz, K. Endo, M. B. Herbert, and R. H. Grubbs, “Z-Selective Homodimerization of Terminal Ole½ns with a Ruthe- nium Metathesis Catalyst,” Journal of the American Chemical Society 133 (2011): 9686–9688. 14 Even though we evaluate only the reasonable possibilities, we use a lot of computer time. Last year we used about ten million hours of fast computer time, equivalent to one thousand years on one fast computer. 15 P. Liu, X. Xu, X. Dong, B. K. Keitz, M. B. Herbert, R. H. Grubbs, and K. N. Houk, “Z-Selec- tivity in Ole½n Metathesis with Chelated Ru Catalysts: Computational Studies of Mecha- nism and Selectivity,” Journal of the American Chemical Society 134 (2012): 1464–1467. 16 L. E. Rosebrugh, M. B. Herbert, V. M. Marx, B. K. Keitz, and R. H. Grubbs, “Highly Active Ru- thenium Metathesis Catalysts Exhibiting Unprecedented Activity and Z-Selectivity,” Journal of the American Chemical Society 135 (2013): 1276–1279. 17 J. K. Nørskov, T. Bligaard, J. Rossmeisl, and C. H. Christensen, “Towards the Computational Design of Solid Catalysts,” Nature Chemistry 1 (2009): 37–46.

143 (4) Fall 2014 65 Book_Fall 2014_Shinner.qxd 9/8/2014 10:04 AM Page 66

Using 18 For a review of this procedure, see G. Kiss, N. Çelebi-Ölçüm, R. Moretti, D. Baker, and K. N. Compu - Houk, “Computational Enzyme Design,” Angewandte Chemie International Edition 52 (2013): tational 5700–5725. Chemistry to Under- 19 See http://www.pdb.org/; and H. M. Berman, J. Westbrook, Z. Feng, G. Gilliland, T. N. stand & Bhat, H. Weissig, I. N. Shindyalov, and P. E. Bourne, “The Protein Data Bank,” Nucleic Acids Discover Research 28 (2000): 235–242. Chemical Reactions 20See D. Rothlisberger, O. Khersonsky, A. M. Wollacott, L. Jiang, J. DeChancie, J. Betker, J. L. Gallaher, E. A. Althoff, A. Zanghellini, O. Dym, S. Albeck, K. N. Houk, D. S. Taw½k, and D. Baker, “Kemp Elimination Catalysts by Computational Enzyme Design,” Nature 453 (2008): 190–195; L. Jiang, E. A. Althoff, F. R. Clemente, L. Doyle, D. Rothlisberger, A. Zanghellini, J. L. Gallaher, J. L. Betker, F. Tanaka, C. F. Barbas III, D. Hilvert, K. N. Houk, B. L. Stoddard, and D. Baker, “De Novo Computational Design of Retro-Aldol Enzymes,” Science 319 (2008): 1387–1391; and J. B. Siegel, A. Zanghellini, H. M. Lovick, G. Kiss, A. R. Lambert, J. L. St. Clair, J. L. Gallaher, D. Hilvert, M. H. Gelb, B. L. Stoddard, K. N. Houk, F. E. Michael, and D. Baker, “Computational Design of an Enzyme Catalyst for a Stereoselective Bimolecular Diels-Alder Reaction,” Science 329 (2010): 309–313. 21 William L. Jorgensen, “The Many Roles of Computation in Drug Discovery,” Science 303 (2004): 1813–1818. 22 National Science and Technology Council, “Materials Genome Initiative for Global Compet - itive ness” (Washington, D.C.: Executive Of½ce of the President of the United States, 2011). 23 Andreas W. Götz, Mark J. Williamson, Dong Xu, Duncan Poole, Scott Le Grand, and Ross C. Walker, “Routine Microsecond Molecular Dynamics Simulations with amber on gpus. 1. Generalized Born,” The Journal of Chemical Theory and Computation 8 (2012): 1542–1555. 24 Satoshi Maeda and Keiji Morokuma, “Communications: A Systematic Method for Locating Transition Structures of A+B g X Type Reactions,” The Journal of Chemical Physics 132 (24) (2010): 241102.

66 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 67

From the Atom to the Universe: Recent Astronomical Discoveries

Jeremiah P. Ostriker

Astronomy starts at the point to which chemistry has brought us: atoms. The basic stuff of which the planets and stars are made is the same as the terres- trial material discussed and analyzed in the ½rst set of essays in this volume. These are the chemical ele - ments, from hydrogen to uranium. Hydrogen, found with oxygen in our plentiful oceanic water, is by far the most abundant element in the universe; iron is the most common of the heavier elements. All the combinations of atoms in the complex chemical compounds studied by chemists on Earth are also possible components of the objects that we see in the cosmos. Although almost all of the regions that we astronomers study are so hot that the more compli - JEREMIAH P. OSTRIKER, a Fellow cated compounds would be torn apart by the heat, of the American Academy since some surprisingly unstable organic molecules, such as 1975, is the Charles A. Young Pro - cyanopolyynes, have been detected in cold re gions of fes sor Emeritus of Astrophysics at space with very low density of matter. Nev ertheless, Princeton University and Professor the astronomical world is simpler than the chemical of Astronomy at Columbia Univer- world of the laboratory or the real biolog ical world. sity. His research interests concern dark matter and dark energy, gal - But the enormous spatial and temporal extent of axy formation, and quasars. His the cosmos allows us–and in fact forces us–to ask publications include Heart of Dark- questions that would seem offbeat to a chemist. ness: Unraveling the Mysteries of the Where do the chemical elements come from? Pre- Invisible Universe (with Simon Mit- cisely how, where, and when were they made? Do ton, 2013) and the volumes Forma- the abundances of the elements change with time? tion of Structure in the Universe (edited Does alternative “matter” that is not made of the or- with Avishai Dekel, 1990) and Un - solved Problems in Astrophysics (edited dinary chemical elements exist and exert gravity in with John Bahcall, 1997). He was the the universe? We in the trades of astronomy and as - recipient of the U.S. National Med- trophysics must ask ourselves these questions–and al of Science in 2000. they are only the beginning.

From the In our ½rst essay, “Reconstructing the Amazingly, much of the discovery work Atom to the Cosmic Evolution of the Chemical Ele- has been done with relatively small tele- Universe ments,” Anna Frebel asks precisely the ½rst scopes, using the astronomical equivalent of these questions. A discoverer of some of crowdsourcing (a movement that Bakos of the oldest and most metal-poor stars, has helped lead). The search may soon she tells us how we have found out where reach the point where we will be able to and when the nuclear cooking of the ele- use large telescopes to analyze the spectra ments occurred and precisely which cos- from the most interesting newly discov- mic explosions spewed out which of our ered planets to tell if any of them have familiar elements, from the sodium in salt atmospheres like ours on Earth; and then to the gold in our jewelry. She also intro- the question of “life on other worlds” will duces some of the remaining mysteries of become the province of science rather than element creation in the early universe: science ½ction. what do we not know? Next up the scale from planets is, of Let us move beyond standard units of course, the domain of the stars–those ½xed ordinary matter to some larger objects in points of light in the sky that the ancients the universe. The Earth, our beloved plan- cataloged and arranged into houses or et, is but a grain of sand on the scale of the constellations. By the eighteenth century cosmos; however, it is a respectable body it was known that they were not, in fact, in our solar system of eight normal planets. ½xed, but varied in brightness and moved For literally thousands of years the (albeit slowly) on their own paths across bright er of these “wanderers” (the mean - the sky, their current positions by then ing of the Greek word “πλανεται,” or being signi½cantly different from those “plan etai”) puzzled our ancestors, who be - recorded by the classical astronomers. By lieved that their motions among the ½xed the twentieth century, the enigmatic “neb- stars foretold events on earth. The revolu- ulae,” including the common spiral nebu- tionary discoveries of Tycho, Kepler, Ga - la, were found to be simply giant assem- lileo, and Newton identi½ed our position blages of stars and galaxies–“billions and as the third orbiter of the sun, follow ing, billions of stars,” as incanted by the won- along with our companion planets, pre- derful popularizer of science of the last cisely the paths predicted by Newton’s century, Carl Sagan. We live in such a gal- laws of gravity and motion. But are there axy–the Milky Way–and our neighbor other planets outside of our solar system? Andromeda, seen in the northern hemi- Are the stars that we see in the night sky sphere in the winter sky, is another ½ne orbited by their own planets? When I was example of a typical spiral galaxy. The a graduate student, this was a subject of greatest classical astronomer, Hipparchus, speculation; there was no factual knowl- constructed a catalog of ½xed stars that edge. But in the last decades several inde- had fewer than one thousand entries. By pendent techniques have been developed the early twentieth century, the standard that tell us without equivocation that ex - HD catalog of bright stars (named for the trasolar planets are common! In fact, most American astrono mer Henry Draper) con- stars probably have planets orbiting them. tained over two hundred thousand entries. In “Exoplanets, 2003–2013,” Gáspár Bakos The Messier catalog of nebulae published lays out the dramatic tale of how we have in 1775 contained slightly more than one recently found a startling variety of new hundred objects; by the end of the nine- planets: fat and thin, in round and in el- teenth century, the similar ngc catalog liptical orbits, massive and lightweight. contained nearly ten thousand galaxies

68 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 69

and clusters. But now we live in the age of whose individual masses typically range Jeremiah P. electronic detectors, huge telescopes, giant from four million solar masses at the center Ostriker computers, and enormous databases. A of our own galaxy to six billion solar masses gigantic explosion of information has oc - at the center of the giant elliptical galaxy curred in our age of “big data,” as outlined M87. The processes by which these mega- in the essay by Michael Strauss, “Map - monsters were formed are under intense ping the Universe: Surveys of the Sky as investigation and are still quite uncertain. Discovery Engines in Astronomy.” Strauss, But the saga of how we discovered these a leader of the Sloan Digital Sky Survey extraordinary objects and what we now (the largest such survey completed to date), know about them can be told; and we are notes that catalogs now contain over a bil- fortunate to have had one of the discover- lion stars and gal axies; and they are grow- ers of quasars, Scott Tremaine, author the ing–if the reader will forgive the pun–at essay “The Odd Couple: Quasars and Black an astronomical rate! Couple this with the Holes.” We are just learning that there ap- new tools available for querying databases pear to be close relations between galaxies (such as Goo gle) and one can imagine the and their resident central massive black rate of discovery. holes, but how and why these relations Massive black holes are much heavier were formed remains a total mystery. (and stranger) than any star. The ½rst so - Moving farther up the cosmic scale, we lution indicating the possibility of the ex - ½nd galaxies, which are typically a thou- istence of black holes was obtained shortly sand times the mass of the black holes after Einstein invented general relativity that they harbor at their centers. Galaxies in 1915, but it was decades before their are the basic building blocks of the uni- character was understood and still longer verse. While it is true that they are collec- before black holes were found in nature. tions of stars, they also seem to be em bed - Given their common name by the visionary ded in massive halos of mysterious “dark physicist John Wheeler in 1967, they can matter,” the total weighing in typ ically at form when massive stars collapse to such a roughly a trillion solar masses with most of small size that gravity overwhelms any it in the mysterious dark component. The pressure or nuclear forces, crushing the visible galaxies were taken for granted by star into a singularity from which nothing, Hubble and the early twentieth-century as- not even light, can escape. But gaseous tronomers as being simply “there,” but by matter falling into black holes is heated as the 1960s, the realization had spread that it is compressed and will copiously emit they must be evolving with time, and in light before it disappears into the abyss. fact their formation itself was a subject that This makes black holes visible to astron- must be addressed. Luckily, our increas - omers, and many have been found in our ingly powerful telescopes can see farther galaxy in binary star systems, each one and farther out and consequent ly look back with a mass roughly ten times that of the to earlier and earlier times due to the ½nite sun. When quasars–enormously lumi- speed of light. The most distant galaxies nous objects at the centers of distant that we can ½nd are thus seen as they ex - galaxies–were discovered, their variability isted several billion years ago. With major and incredible luminosity immediately telescopes, we can use the universe as a led to speculations that they were much time machine and directly study the evolu- more massive black holes. We now know tion–and perhaps even the formation–of that the centers of most massive galaxies galaxies in the distant past. Pieter van in fact harbor these enigmatic beasts Dokkum has done just that; in his essay,

143 (4) Fall 2014 69 Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 70

From the “The Formation and Evolution of Galax- over from the Big Bang. The angular fluc- Atom to the ies,” he will limn out what we have learned tuations seen in the sky by ever more sen- Universe through the use of powerful telescopes and sitive and precise satellites studying the giant computers that simulate the physics cbr (“cobe,” launched in 1989; “wmap,” of galaxy evolution. The discoveries are launched in 2001; and now “Planck,” piling up at a great rate in the last decade launched in 2009) have essentially ban- and we now know that for the most mas- ished doubt about the essential correct- sive galaxies, a two-phase evolution seems ness of the standard model of cosmology. to occur: when the galaxy ½rst forms, cold David Spergel, a leader of the wmap satel- streams of gas converge, flowing into deep lite team, authors the ½nal essay in this wells where the cosmic gravity is greatest volume, “Cosmo logy Today,” which tells due to dark matter ac cumulations, and the story of how these results came huge numbers of stars are formed in rela- about, what they tell us about the universe, tively small regions. Then, at a later epoch, and what (large) puz zles still remain. these monster systems eat their neighbor - These six essays lay out for the interested ing smaller galaxies (massive black holes reader the extraordinary renaissance that and all) and grow further–perhaps by a astronomy has undergone in the last de- factor of two in mass and four in size with- cades. We are fortunate indeed to have out much additional star formation. Can- ½rsthand discoverers’ tales of these ad- nibalism among the gal axies! The evolu- ven tures to entertain and inform us. tion for lower-mass galaxies like the Milky Way and our compan ion Andromeda is less well un der stood. Stay tuned. Finally, climbing up the ladder of distance and time, we approach the largest known scale: the universe itself. During the last two decades, the knowledge of the universe accumulated over the last century has been synthesized into a well-de½ned global model that ½ts all cosmic observations to an uncanny degree. While the na - ture of the two chief ingredients in this model–dark matter and dark energy– re mains a mystery to us, the model has passed all tests given to it so far. The existence of dark matter has been con½rmed by both gravitational lensing and the growth patterns of various cosmic structures. Simi larly, dark energy seems to have produced the amply observed acceleration of the uni verse (it is expanding faster and faster rather than slower and slower, as had been ex pected). But the primary tool for re½ning and precisely testing the new model has been the analysis of the microwave background (Cosmic Background Radiation, or cbr): the relic radiation left

70 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 71

Reconstructing the Cosmic Evolution of the Chemical Elements

Anna Frebel

Abstract: The chemical elements are created in nuclear fusion processes in the hot and dense cores of stars. The energy generated through nucleosynthesis allows stars to shine for billions of years. When these stars explode as massive supernovae, the newly made elements are expelled, chemically enriching the surrounding regions. Subsequent generations of stars are formed from gas that is slightly more element- enriched than that from which previous stars formed. This chemical evolution can be traced back to its beginning soon after the Big Bang by studying the oldest and most metal-poor stars still observable in the Milky Way today. Through chemical analysis, they provide the only available tool for gaining information about the nature of the short-lived ½rst stars and their supernova explosions more than thirteen billion years ago. These events set in motion the transformation of the pristine universe into a rich cosmos of chemically diverse planets, stars, and galaxies.

One beautiful afternoon I went for a run along the river. I was breathing plenty of fresh air, my face was all flushed, and I felt my heart pounding and blood flowing through my body. As air was ½lling my lungs, I was reminded of Carl Sagan’s saying: “We are all made from star stuff.” Indeed we are. When quench - ing my thirst with water, I was consuming hydrogen H O and oxygen in the form of 2 . When breathing, I had been taking in air made from nitrogen, oxygen, ANNA FREBEL is the Silverman and tiny traces of other elements such as argon and (1968) Family Career Development neon. The red liquid of life owes its color to iron, Professor in the Physics Depart- ment at the Massachusetts Insti- which is embedded in our hemoglobin. But these el - tute of Technology and a member ements do not just circulate within our carbon-based of the mit Kavli Institute for As - bodies: before they became part of humans, each of trophysics and Space Research. these atoms was created in a grand cosmic cycle Her research interests include stel- cal led chemical evolution that took place long before lar archaeology and near-½eld cos- biological evolution led to life on Earth. mology. Her work has recently ap - Most of the universe’s iron, for example, is the end peared in Nature, The Astronomical Journal, and Astronomy and Astrophys - result of a binary star system in which one star ac - ics, among other journals; and in the quires enough material from its companion that it volume Planets, Stars, and Stellar Sys- reaches a critical mass and erupts in a huge thermo - tems (ed. Gerard Gilmore, 2012). nuclear explosion, forging new elements in the pro-

Recon - cess. On the other hand, the hydrogen at- Immediately after the Big Bang 13.8 bil- structing oms that make up water are probably lion years ago, there was a time without the Cosmic Evolution nearly fourteen billion years old and were stars and galaxies. The hot gas left over of the created as part of the Big Bang. And all the from the Big Bang had to cool enough be - Chemical Elements carbon upon which life as we know it is fore the ½rst cosmic objects were able to based was synthesized in evolved stars near form. This process took a few hundred mil- the end of their lives. lion years, but eventually the very ½rst stars The fact that all elements except hydro- lit up the universe. The universe at that gen, helium, and lithium are made in stars time was made from just hydrogen and and their subsequent explosions has only helium: heavier elements did not exist yet. been known for less than sixty years. A As a consequence of a variety of gas chem- seminal paper from 1957, often referred to istry and cooling processes that govern as “B2FH” following the initials of the star formation, the ½rst stars are thought auth ors, provided the ½rst comprehensive to have been rather massive. Recent com- summary of “the synthesis of the elements putations suggest these behemoths may in stars.”1 This came after decades of work have had up to one hundred times the mass directed at ½nding the energy source of of the sun.2 In comparison, most stars to- stars. With the elucidation of how and day are low-mass stars with less than one where the chemical elements are forged in solar mass. stars came the realization that there is a Stars are powered by the nuclear fusion chem ical evolution in the universe, caus- taking place in their cores; it is the energy ing a net increase in the amount of elements source that sustains their enormous lumi- over time. Most important, this model pro - nosities. In the ½rst and by far the longest vided observationally testable support for burnin g phase, hydrogen is fused into he - the Big Bang theory and the theory of a lium. At about ten million degrees F, four time-dependent chemical evolution of the protons (or hydrogen nuclei) are fused to - universe. gether in a series of nuclear reactions to While the nature of chemical evolution make a helium nucleus. Subsequent burn - of galaxies is now well established, many ing stages, which occur only in the last ten details of the complex circle of nucleosyn- percent of stars’ lives, result in three heli- thesis in stars, later chemical enrichment um nuclei (“α-particles”) being converted of interstellar gas, and subsequent star for- to beryllium, which then captures another mation remain poorly understood and particle to become carbon in the so-called thus continue to be subject to ongoing re - triple-α process. search. Many questions center around After that, through additional particle what the exact abundance yields of indi- captures, carbon nuclei are converted to vidual supernova explosions may be, as oxygen; through yet more nucleosynthe- well as how the nature of the exploding sis processes, all elements in the periodic stars themselves and the astrophysical en- table up to iron are built up. The fusion of vironment influences nucleosynthesis and lighter elements into heavier ones results the production of the elements through- in a conversion of a small amount of mass out the periodic table. Because old stars into energy. For example, a helium nucle- that formed in the early phases of chemi- us is 0.7 percent lighter than four individ- cal evolution can help with this quest, we ual hydrogen nuclei. It is this mass differ- will start the tale of the origin of the ele- ence that, as described by E = mc2, fuels ments from the very beginning of the uni - the star and sustains its luminosity for verse. long periods of time. However, once the

72 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 73

star has created an iron core at its center, formation of smaller stars. Lower-mass Anna nucleosynthesis stops. No more energy can stars like the sun could therefore form for Frebel be gained by fusing iron into even heavier the ½rst time. The ½rst low-mass stars nuclei: the star’s energy source has ceas ed (those with 60 to 80 percent of the mass for good. As a consequence, the star can no of the sun) have long lifetimes of ½fteen to longer maintain equilibrium and begins twenty billion years due to their sparse con- to collapse due to its own gravity. As a sumption of the nuclear fuel in their cores. result of the huge pressures, the iron core Born soon after the Big Bang as second- or is converted into an extremely dense neu- third-generation stars, they are still shin- tron star. The collapsing mass of the star ing today. Many of these ancient survivors bounces off the hard neutron star and leads are suspected to be hiding in our Milky to a gigantic supernova, leaving the neu- Way galaxy and, indeed, astrono mers have tron star behind. This was also the fate of discovered dozens of them over the past the massive very ½rst stars. To sustain their three decades. What makes these extreme - great luminosities, massive stars (those ly rare objects so valuable is that they pre- with more than ten solar masses) require serve in their atmospheres information large amounts of nuclear energy. Conse- about the chemical composition of their quently, they burned through the hydro- birth cloud, which existed soon after the gen and subsequently created heavier-ele- Big Bang. Hence, studying their chem ical ment fuel much more quickly than stars composition allows astronomers to recon - with lower masses, therefore limiting their struct the early era of their births. lifetimes to just a few million years. In the earliest stages of the universe’s During the explosion of a star, all the de velopment, massive stars exploding as newly created elements are released into supernovae dominated the production of the surrounding gas. The death of the ½rst iron in the universe. However, this changed stars marked an important milestone in after about a billion years. Through the the evolution of the universe: it was not existence of the ½rst lower-mass stars with pristine anymore, but “polluted” with car - longer lifetimes, a different pathway for bon, oxygen, nitrogen, iron, and other ele - iron production emerged. At the end of ments. Thus, over time, the universe be - their long lives, low-mass stars turn into came more and more enriched in the ele- compact white dwarf remnants. If a star ments heavier than hydrogen and helium, and a white dwarf are in a binary system which are collectively called “metals” by and enough mass is transferred from the astronomers. In contrast, the very ½rst stars star to the white dwarf, the latter will un - were the only ones that formed from com - dergo a thermonuclear explosion. Given pletely metal-free gas. All stars in sub se - the dominance of low-mass stars in the quent generations would then form from universe today, iron is thus mainly pro- gas clouds that contained some metals produced by this process rather than by ex - vided by at least one previous generation of ploding massive stars, as was exclusively stars exploding as supernovae. the case in the early universe. The sudden existence of metals in the After about nine billion years of this early universe following the death of the chemical evolution, driven by different ½rst stars changed the conditions for sub- types of stars at different times, our sun, sequent star formation. Gas clouds can cool together with its planets, ½nally formed. down more ef½ciently when metals or dust Its birth gas had been enriched by per- made from metals are present, leading to haps a thousand generations of stars and the collapse of smaller clouds, and thus the supernova explosions. That evolution pro -

143 (4) Fall 2014 73 Book_Fall 2014_Shinner.qxd 9/18/2014 2:47 PM Page 74

Recon - vided the gas with enough metals to enable they formed from gas enriched with only structing the formation of planets–something that a trace amount of heavy elements, created the Cosmic Evolution may not have been possible much earlier by the ½rst few stellar generations after the of the on in the universe. Consequently, when as- Big Bang.3 In contrast, “metal-rich” stars Chemical Elements tronomers look for extrasolar planets, they like the sun must have formed at a much focus their search on stars that are close later time when the universe was sig- in age to or younger than the sun. ni½cantly enriched with metals by many stellar generations. Through spectroscopic observations, as - Our study of early star formation and tronomers can determine which elements chemical evolution relies on our ability to are present in a star’s outer layers and what measure stars’ metallicity, or metal con- their respective abundances are. Spectros- tent. The main indicator used to determine copy is a technique in which star light is stellar metallicity is iron abundance, split up into its components, just as sun- which, with few exceptions, reflects a star’s light is split when we see a rainbow. The overall metallicity fairly well. Absorption different elements (hydrogen, helium, and lines of iron (Fe) can be found through- metals) in the star’s atmosphere ab sorb out stellar spectra, often covering large light at very speci½c colors, or wavelengths. wavelength ranges from 350 to 900 nano- When carrying out high-resolution spec- meters, which makes measuring iron abun - troscopy, the starlight is signi½cantly dance relatively straightforward. The iron stretch ed out over all visible wavelengths abundance of a star is given as [Fe/H], to enable the detection of even very weak which is used in the logarithm of the ratio ab sorption lines left behind by all the ele- of iron atoms to hydrogen atoms in com- ments in the stars. The existence and parison to that of the sun. The formal de½nition reads [Fe/H] = log NF NH strength of absorption lines corresponding 10( e/ )* to speci½c elements are measured and an- −log10(NFe/NH)☉ with N being the num- alyzed with computer programs that re- ber of Fe and H atoms, respectively, and construct the stellar atmosphere. This way, * and ☉ representing the star being eval- astronomers can calculate how many at - uated and the sun, respectively. The con- oms of a given element are present in the sequence of this logarithmic de½nition is star. that metal-poor stars will have negative High-resolution spectroscopy, especially [Fe/H] values, as those stars have a lower for fainter stars, requires the largest tele- concentration of Fe atoms than the sun. scopes that observe the visible wavelength Stars containing higher concentrations of range. Telescopes like the Magellan-Clay metals than the sun will show a positive Telescope at Las Campanas Observatory, [Fe/H] value. located in Chile’s Atacama desert, are To illustrate the difference between equipped with high-resolution spectro- younger metal-rich and older metal-poor graphs. Thanks to its large 6.5-meter-dia - stars, Figure 1 shows spectra of the sun and meter mirror, the Magellan Telescope is three metal-poor stars. Their decreasing capable of collecting enough light from metallicities are listed. The corresponding faint stars to enable high-resolution spec- number of absorption lines detectable in troscopic measurements. Chemical analy- the spectra decreases with increasing met al- sis then shows how much of each type of de½ciency. In star HE 1327−2326 (bot tom metal is present in a star, which indicates spectrum), only very few metal absorp - the star’s formation time. So-called metal- tion lines are left to ob serve. Their weak- poor stars are assumed to be old because ness is such that determining their metal-

74 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 75

Figure 1 Anna Spectral Comparison of the Sun with Three Metal-Poor Stars with Different Metallicities Indicating Frebel the Course of Chemical Evolution from the Early Universe (Bottom) to the Sun’s Birth ~4.5 Billion Years Ago (Top)

Source: Anna Frebel, “Stellar Archaeology: Exploring the Universe with Metal-Poor Stars,” Astronomische Nachrichten 331 (2010): 474.

licity requires extremely high-quality data that push us toward understanding the on- that can only be obtained with large tele- set of star and galaxy formation in the early scopes.4 universe some thirteen billion years ago. If one wishes to identify the oldest stars, Metal-poor stars are, however, the only the task is to ½nd stars with the lowest tool we have available to learn about the metallicities and thus the earliest forma- nature of the ½rst stars and their supernova tion times. It is those stars that allow astron- explosions. Our study of these stars there - omers to look back in time and recon- fore provides unique constraints on vari- struct the formation and evolution of the ous theoretical concepts regarding the chemical elements and the involved nu - physical and chemical nature of the early cleosynthesis processes that created them. universe. While very distant galaxies are often used Past sky surveys for metal-poor stars for observational studies of galaxy forma- have shown that these ancient objects can tion and cosmology, metal-poor stars are be systematically identi½ed in a three-step the local equivalent of the distant universe process that involves the selection of candi- and thus the object of “near-½eld” cosmol- dates from the survey data and subsequent ogy. Both approaches to cosmology com - follow-up of the best targets with medium- plement each other in providing detailed and high-resolution spectroscopy.5 This information and observational constraints technique has identi½ed large numbers of

143 (4) Fall 2014 75 Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 76

Recon - metal-poor stars on the outskirts of the can be successfully reproduced by scenar- structing Milky Way, the so-called halo of the galaxy. ios in which a massive ½rst supernova ex- the Cosmic Evolution Work done over the last few decades has plosion provided the elements to the gas of the shown that stars with low metallicity are cloud from which the observed object later Chemical Elements much fewer in number compared to more formed. In fact, the most metal-poor stars metal-rich stars, reflecting not only the all display the “½ngerprint” of one single chemical evolution of the universe but also massive ½rst supernova, which allows as- the overwhelming number of stars that tronomers to ascertain the mechanisms have formed since its early stages. and details of the supernova itself and the The most metal-de½cient stars, in par- nature of the long-extinct progenitor star. ticular, are extremely rare and dif½cult to ½nd. Only about ½fty stars are known to With the exception of hydrogen and he- have metallicities of [Fe/H] < −3.5, which lium, all elements up to iron are created corresponds to ~1/3000th of the solar through nuclear fusion during lifetimes met allicity. Of those, only six have [Fe/H] of stars. But these elements (with atomic < −4.0 or < 1/10,000th of the solar value. number Z ≤ 30) make up less than one The current record holder is the star third of the periodic table. So where do smss 0313-6708, with [Fe/H] < −7.0. No the other elements with higher atomic iron lines could be detected, so only an masses, such as silver (Z = 47) and gold (Z = upper limit on the iron abundance could 79), or more exotic rare earth elements, be determin ed, which corresponds to less such as lanthanum (Z = 57) and europium than ~1/10,000,000th that of the sun. (Z = 63), come from? The next most iron-poor star, HE 1327− The study of metal-poor stars has great- 2326, has an iron abundance of [Fe/H] = ly advanced our understanding of this top- −5.4 (~1/250,000th of the solar iron ic. As we now know from nuclear physics, abundance). This translates into an actu- elements heavier than iron are created not al iron mass of just 1 percent of the iron through fusion processes but through mass present in the Earth’s core. This is a neutron-capture by seed nuclei (for ex- very small amount, considering that the ample, iron nuclei). In an astrophysical en- star is approximately 300,000 times vironment that provides a constant flux more massive than the Earth and about of neutrons, heavy elements can thus be one million times larger in size. It also built up. Such conditions are thought to reveals that in the early universe, iron and occur during certain kinds of supernova other elements were rare commodities. explosions in which a strong neutron flux Thus, the few stars with [Fe/H] < −4.0 develops above the newly formed central have opened a new and unique observa- neutron star. For example, if iron nuclei tional window to the time shortly after are extremely rapidly bombarded with the Big Bang when only the very ½rst stars many neutrons before the nuclei β-decay, had enriched the universe. They are fre- their nuclei capture more neutrons, cre- quently employed to constrain theoretical ating heavy, neutron-rich, and unstable iso - studies about the formation of the ½rst topes. Once these have β-decayed to sta- stars, element production and chemical bility, new and heavier elements remain.6 evolution, and supernova yields. The ele- Beta decay is a spontaneous decay of one mental-abundance patterns (chemical element into another through the conver - abundances as a function of atomic num- sion of a neutron into a proton accompa- ber of the respective element) of these stars nied by the emission of an electron and a appear to be highly individual, but in fact neutrino. Due to the rapid bombardment,

76 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/18/2014 2:48 PM Page 77

this process is called the r-process. About universe, the r-process creates its heavy Anna half of all stable isotopes of elements heavi- ele ments in the exact same ratios, indi- Frebel er than zinc are made this way. cating that the r-process is a universal pro - The other half of the isotopes of heavy cess. Since most neutron-capture elements elements are created in the so-called s- are too heavy to be created and studied in process, where a slower neutron bombard- accelerator laboratories on Earth, this has ment (over a longer timescale than the β- been an important empirical ½nding based decay process) leads to the successive build- on stellar astronomy. up of heavy elements. This pro cess occurs in the pulsing outer shells of evolved red The elements produced in the r-process giant stars with masses of less than eight include thorium and uranium, which are solar masses and metallicities of [Fe/H] > very long-lived radioactive isotopes: 232Th −3.0 (indicating that the star formed from has a half-life of 14 billion years while gas already containing a small amount of 238U has a half-life of 4.5 billion years (they iron atoms that could func tion as seed are thus decaying very slowly and are nuclei). Through stellar winds, these ele- near-stable on Earth). Measuring thorium ments are eventually re leased into the sur - and uranium abundance in metal-poor rounding gas. stars whose birth gas cloud was enriched In 1995, a low-mass, metal-poor star, CS by only one or few supernova events en - 22892-052, was discovered to possess a ables astronomers to carry out cosmo- very high abundance of numerous neu- chronometry: dating the oldest stars tron-capture elements (including the with a method analogous to dating archae- rare earth elements) compared to lighter ological ½nds through radiocarbon analy- elements such as iron. Indeed, the star has sis. In the latter technique, the initial ra - a metallicity of [Fe/H] ~−3.0 (or ~1/1000th tio of 12C (the typical stable form of car- of the solar iron abundance), but the neu- bon) to 14C (an unstable isotope) must be tron-capture material is about forty times estimated and then compared to the ratio more abundant. The various neutron- at the time of discovery. Cosmo-chronom- capture elements detected in this star are etry requires astronomers to know the likely the result of an r-process event that initial amount of the heaviest el ements, took place prior to the star’s birth. When which were presumably produced together the star formed, it inherited the chemical in a massive supernova explosion (obtain - signature of this particular nucleosynthe- ing such information is ex tremely challeng - sis event. For the 4.5-billion-year-young ing, but detailed calculations of r-process sun, which formed from gas enriched by nucleosynthesis have yielded estimates). many generations of stars, it is possible to The estimated initial ratios of unstable infer how much of each observed element and stable rare earth elements (such as tho- may have been produced by r-process rium to europium, uranium to osmium, events prior to the sun’s formation. The and thorium to uranium) can then be com - resulting solar r-process pattern can be pared with the currently observed ratios, compared to that of other, more metal- and the degree of decay of the unstable poor stars. A comparison between the sun isotopes thus provides the age of the star. and the metal-poor CS 22892-052, for ex - While thorium is often detectable, ura- ample, revealed that both stars have the nium poses a great challenge. Only one ex- exact same relative pattern of neutron- tremely weak absorption line of uranium capture abundances (see Figure 2). It ap- is available in the optical spectrum, mak- pears that at any time and place in the ing its detection dif½cult, if not impossible,

143 (4) Fall 2014 77 Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 78

Recon - Figure 2 structing Elemental Abundances in Four Metal-Poor Stars with a Relative Overabundance of r-Process Elements the Cosmic (Various Symbols) Compared with the Scaled Solar r-Process Pattern (Solid Line) Evolution of the Chemical Elements

All patterns are arbitrarily offset to allow a visual comparison. Note the remarkable agreement of the metal-poor star pattern and that of the sun for elements heavier than barium (Z ≥ 56). Source: Anna Frebel and John E. Norris, “Metal-Poor Stars and the Chemical Enrichment of the Universe,” in Planets, Stars and Stellar Systems, vol. 5, ed. Gerard Gilmore (Amsterdam: Springer, 2012), doi:10.1007/978-94-007-5612-0_3.

in most cases. In an ideal scenario, both dated with seven different “cosmic clocks”; ra dio active elements are detected so that that is, abundance ratios containing either many ratios of the thorium and/or urani- thorium or uranium and different rare um abundance to those of stable rare earth earth elements. The average age obtained elements can be compared to model pre- through this analysis is 13.2 billion years; dictions for the yields of the r-process this is consistent with the universe’s age event. Indeed, several r-process metal-poor of 13.8 billion years, which has been de- stars with metallicities of roughly 1/1000th duced from observations of the cosmic of the solar value were found to be about background radiation interpreted with 14 billion years old. These include HE 1523− the latest cosmological models. Unfortu- 0901, which is only the third metal-poor nately, the range of uncertainty with star in which uranium can reliably be de- respect to stellar age is often several bil- tected. Moreover, HE 1523−0901 can be lion years. Regardless, cosmo-chronometry

78 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 79

con½rms that HE 1523−0901 and all other inated from dwarf galaxies when analo- Anna metal-poor stars are ancient and formed gous systems were gobbled up by the Frebel soon after the Big Bang during the early Milky Way during its assembly pro cess. phases of chemical evolution. Topics like these inspire astronomers to Through individual age measurements, collect additional information about the metal-poor r-process stars provide an in- nature and structure of the galaxy. To dependent lower limit for the age of the chem ically characterize the galactic halo universe. This makes them vital probes for in detail, including its streams, substruc- near-½eld cosmology. At the same time, tures, and satellites, wide-angle surveys given their rich inventory of very heavy, with large volumes are needed. The Aus- exotic nuclei, these stars also closely con- tralian SkyMapper Telescope is already nect astrophysics and nuclear physics by mapping the Southern sky. It is optimized acting as a “cosmic lab” for both ½elds of for stellar work and is delivering new study. metal-poor star candidates for which high- resolution spectroscopy will be required. Recent searches for metal-poor stars The Chinese lamost spectroscopic sur- have not only focused on the old stellar vey is providing numerous metal-poor halo but also on dwarf satellite galaxies candidates in the Northern hemi sphere. orbiting the Milky Way. The ultra-faint Studying ever-fainter stars fur ther out in dwarf galaxies–whose total luminosities the deep halo of the Milky Way and in far - range from 1,000 to 100,000 solar lumi- away dwarf galaxies may become a reality nosities, making them the dimmest gal - with the light-collecting power of the next axies known–appear to contain almost ex- generation of optical telescopes, including clusively metal-poor stars. These systems the Giant Magellan Telescope, the Thirty ran out of gas for additional star forma- Meter Telescope, and the European Ex- tion billions of year ago. Chemical evo lu - tremely Large Telescope. These telescopes tion and star formation ceased as a result, are currently scheduled for completion and when we observe these systems, we around 2020. At this point, only the Giant can only see the leftover low-mass stars Magellan Telescope is scheduled to be that are still shining today. They, too, tell equipped with the high-resolution spectro- us the story of nucleosynthesis and enrich - graph necessary to study metal-poor stars. ment in the early stages of the universe.7 Further, gaia, an astrometric space mis- In fact, there are recent indications that sion led by the Eu ropean Space Agency these systems are nearly as old as the uni- (esa) that was launched in late 2013, will verse itself: some of them may be among obtain high-precision astrometry for one the ½rst galaxies that formed after the Big billion stars in the galaxy, along with the Bang. Studying these stars thus offers an - physical parameters and the chemical other chance to reconstruct the initial composition of many of them. Together, events of element creation within the ½rst these new data will revolutionize our un- stars and their violent explosions, and the derstanding of the origin, evolution, struc- subsequent incorporation of this material ture, and dynamics of the Milky Way. into next-generation stars. More over, the All of these new observations will be existence of such old satellites may shed accompanied by an increased theoretical light on the existence of metal-poor stars understanding of the ½rst stars and gal- in the halo of the Milky Way. Predating axies, supernova nucleosynthesis, and the our own galaxy, these halo stars must have mixing of metals into gas clouds in the ear- come from somewhere; perhaps they orig- ly universe, as well as cosmic chem ical evo -

143 (4) Fall 2014 79 Book_Fall 2014_Shinner.qxd 9/8/2014 10:05 AM Page 80

Recon - lution. New generations of sophisticated poor stars in the Milky Way’s sa tellite structing cosmological simulations of galaxy foma- dwarf galaxies. This way, studying nucle- the Cosmic Evolution tion and evolution will enable a di rect in - osynthesis and the products of chemical of the vestigation of chemical evolution (in a evolution will reveal whether any of the Chemical Elements ½rst-galaxy simulation, for example). Be - ultra-faint dwarf galaxies are surviving ing able to trace the metal production and ½rst galaxies and whether the metal-poor corresponding spatial distributions will al- galactic halo was assembled from early low astronomers to compare the results an alogs of today’s dwarf satellites bil- with abundance measurements of metal- lions of years ago.

endnotes 1 E. Margaret Burbidge, Geoffrey R. Burbidge, William A. Fowler, and Fred Hoyle, “Synthesis of the Elements in Stars,” Reviews of Modern Physics 29 (1957): 547. 2 Tom Abel, Greg L. Bryan, and Michael L. Norman, “The Formation of the First Star in the Universe,” Science 295 (2002): 93. 3 Anna Frebel and John E. Norris, “Metal-Poor Stars and the Chemical Enrichment of the Uni- verse,” in Planets, Stars and Stellar Systems, vol. 5, ed. Gerard Gilmore (Amsterdam: Springer, 2012), doi:10.1007/978-94-007-5612-0_3. 4 Anna Frebel, “Stellar Archaeology: Exploring the Universe with Metal-Poor Stars,” Astron - omische Nachrichten 331 (2010): 474. 5 Timothy C. Beers and Norbert Christlieb, “The Discovery and Analysis of Very Metal-Poor Stars in the Galaxy,” Annual Review of Astronomy and Astrophysics 43 (2005): 531. 6 Christopher Sneden, John J. Cowan, and Roberto Gallino, “Neutron-Capture Elements in the Early Galaxy,” Annual Review of Astronomy and Astrophysics 46 (2008): 241–288. 7 Anna Frebel and Volker Bromm, “Precious Fossils of the Infant Universe,” Physics Today 65 (4) (2012), doi:10.1063/PT.3.1519. 8 Anna Frebel and Volker Bromm, “Chemical Signatures of the First Galaxies: Criteria for One-Shot Enrichment,” The Astrophysical Journal 759 (2012): 12.

80 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 81

Exoplanets, 2003–2013

Gáspár Áron Bakos

Abstract: Cosmologists and philosophers had long suspected that our sun was a star, and that just like the sun, other stars were also orbited by planets. These and similar ideas led to Giordano Bruno being burned at the stake by the Roman Inquisition in 1600. It was not until 1989, however, that the ½rst exoplanet–a planet outside the solar system–was discovered. While the rate of subsequent discoveries was slow, most of these were important milestones in the research on extrasolar planets, such as ½nding planets around a pulsar (a compact remnant of a collapsed star) and ½nding Jupiter-mass planets circling their stars on extremely short period orbits (in less than a few Earth-days). But the ½rst decade of our millennium witnessed an explosion in the number of discovered exoplanets. To date, there are close to one thousand con - ½rmed and three thousand candidate exoplanets. We now know that a large fraction of stars have planets, and that these planets show an enormous diversity, with masses ranging from that of the moon (1/100 that of Earth, or 0.01M⊕) to twenty-½ve times that of Jupiter (25MJ, or approximately 10,000M⊕); orbital periods from less than a day to many years; orbits from circular to wildly eccentric (ellipses with an “ec - centricity” parameter of 0.97, corresponding to an aspect ratio of 1:4); and mean densities from 0.1g cm−3 (1/10 of water) to well over 25g cm−3. Some of these planets orbit their stars in the same direction as the star spins, some orbit in the opposite direction or pass over the stellar poles. Observations have been immensely useful in constraining theories of planetary astrophysics, including with regard to the formation and evolution of planets. In this essay, I summarize some of the key results.

Several processes have been used to discover exoplanets, but the majority have been found by one of the following four observational methods: 1) radial velocity (rv) variations of the host star; 2) brightness variations due to the transit of the planet in front of its host star(s); 3) brightness fluctuations of a back ground source caused by the gravitational ½eld GÁSPÁR ÁRON BAKOS is an As- of the planet (called microlensing); and 4) direct imag- sistant Professor of Astrophysical ing of the planet.1 Sciences at Princeton University. The rv method measures the periodic change in His research interests include extra- the line-of-sight (radial) velocity of the host star, solar planets, instrumentation (with due to the gravitational pull of the planet as it re - special focus on small telescopes), volves around the star. In other words, the star circles and all-sky variability. He has served as Principal Investigator of around the center of mass of the star-planet system the hatnet and hatsouth extra- because of the planet’s pull, and we observe the solar planet searches, discovering line-of-sight component of this motion. The change ½fty transiting exoplanets so far. in the rv of the star is measured by observing the

Exoplanets, Doppler shift of the stellar spectrum: the bright star in a one-minute exposure. For F 2003–2013 periodic blue-and-red shift of the starlight. bright stars, the primary limitations on Based on the rv signature of the star, the precision include instrument systematics presence and orbital period of the planet and noise due to stellar activity. Recent can be established, and under certain con- record-breaking detections include a ditions, a minimal mass for the planet is planet inducing an rv variation of only a derived. The inclination of the orbit with half-meter per second on its host star respect to the line of sight remains un- (called HD 20794) and a possible Earth- known; that is, the planet may be orbiting mass planet around one of the brightest edge-on, or almost face-on. Typical rv var - and closest stars, ɑ Centauri B, causing a iations (for the star) are: approximately similar rv variation roughly equivalent to 200 ms−1 due to a Jupiter-mass object the speed of a person walking slowly. orbiting a solar-type star on a one-day Signi½cant advances have been made by period orbit, 12 ms−1 for the same con- way of high-precision spectroscopy using ½guration with 5.2-year period orbit (the “laser-combs,”2 which provide a highly period of Jupiter itself ), and 0.09 ms−1 accurate and stable calibration source. due to an Earth-mass planet orbiting a High-precision infrared spectroscopy3 solar type star on a one-year orbit. Another has also been at the frontier of research, key parameter measured is the “eccen- motivated by the enhanced detectability tricity” (ovality) of the orbit. If multiple of potentially habitable Earth-mass plan- planets orbit the same star, these parame- ets around smaller (and cooler) stars. Plans ters can be derived for all the planets. for future instrumentation on the next As shown in Figure 1, the number of gen eration of large telescopes under de - exoplanet detections has been rising steep- velopment are being shaped by the goal ly over the past decade. While, in terms of detecting small planets. (Examples in- of sheer numbers, the rv method was the clude the espresso instrument on the most successful for much of the past de- 8.2m diameter vlt telescope, codex for cade, this changed in 2012, when the tran- the future 39m e-elt telescope, and gclef sit method took over (discussed later). for the future 24m gmt telescope.) This takeover is even more pronounced if To date, rv searches have targeted a few we consider the approximately three thou- thousand relatively bright stars, and have sand planet candidates from the Kepler discovered around ½ve hundred exoplan- space mission. ets in approximately four hundred plane- While the concept is simple, measuring tary systems (some of which are multi- the rv of a star at the ms−1 level has been planet systems). This sample is large a challenge, and only a few astronomical enough to derive meaningful statistics, as facilities have been able to achieve this. has been done by many authors.4 Some of Two notable examples, among a dozen fa - the key results include: Gas giant planets M M cilities, are the High Accuracy Radial veloc- with planetary mass p > 50 ⊕ (50 Earth ity Planet Searcher (harps) on the Euro- masses) on short-period orbits (less than pean Southern Observatory (eso) 3.6m ten days), also known as “hot Jupiters,” are diameter telescope, and the HIgh Resolu- intrinsically rare, present in only around tion Echelle Spectrograph (hires) on the 1 percent of all star systems. However, Keck-I 10m diameter telescope. The preci- there is an extremely strong bias favoring sion of instruments has improved signif - their discovery, which explains why the icantly over a decade: for example, harps ½rst rv detection of a Jupiter-mass planet reaches 1 ms−1 precision for a moderately (around the star 51 Peg) was that of a hot

82 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/18/2014 2:48 PM Page 83

Figure 1 Gáspár Number of rv (black) and Transit (gray) Detections as a Function of Year Áron Bakos

120 RV TR

100

60 N

0 1990 1995 2000 2005 2010 2015 Discovery date

This plot does not show planet candidates from the Kepler mission. Source: Data from exoplanets.org.

Jupiter,5 and why most ground-based days. While giant planets exhibit a wide transit surveys have discovered only hot distribution of eccentricities (even reach - Jupiters. In contrast, “light” planets more ing e ≈ 0.97), small planets exhibit more M M like Earth ( p < 30 ⊕) are abundant, with circular orbits. Hot Jupiters are “lonely,” a very sharp increase in the occurrence with either no detectable companion or a rate as planetary mass decreases. The oc- companion on a very wide orbit. In con- currence rate of giant planets increases trast, small planets are often in multiplan- with the metal content (fraction of ele- etary systems6 like our own solar system. ments heavier than helium; also called These observational facts are extremely im- “metallicity”) and mass of the host star. portant for constraining the various plan - This, however, is not true for light planets et formation and evolution theories. (Neptunes, super-Earths, and smaller), Some interesting numbers on the oc - which have a much weaker dependence currence rate of exoplanets, as based on rv on metallicity. For giant planets, we ob - searches, are as follows: three-quarters of serve a bimodal distribution in the peri- dwarf (solar-like) stars have a planet with od, with a small “pile up” at P ≈ three days a period less than ten years, and one-quar- M (hot Jupiters) followed by a “period val- ter of dwarf stars have a 0.5 to 2 ⊕ Earth- ley” and steep increase in the occurrence mass planet with a period less than ½fty rate for P ≥ one hundred days. Light plan- days.7 It may turn out, when all periods ets, however, tend to have short-period and masses are considered, that essentially orbits, the most typical period being forty all stars have planets.

143 (4) Fall 2014 83 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 84

Exoplanets, Below I list some notable exoplanetary High-precision rv measurements will 2003–2013 systems that have been discovered by the continue to improve in the next decade, rv method. Note that the nomenclature and should reach a precision level of just of exoplanets is such that most exoplanets a few centimeters per second. A serious carry the name of the host star together limitation on the method will be stellar with a suf½x for the planet (“b” for the ½rst, noise, due to either oscillations of the star “c” for the second, and so on). For exam- or spots and other surface irregularities. ple, the ½rst planet discovered around the star 55 Cnc is called 55 Cnc b, the second By chance alignment, our line of sight and third planets are 55 Cnc c and 55 Cnc d, may lie in the orbital plane of an exoplanet. respectively. In this case, the planet periodically transits across the face of the star as seen from • HD 80606 b is a massive planet on an extremely eccentric and long-period orbit, Earth. During transits, the star’s light is and was later found to transit its host star. dimmed by a fraction that is proportional to the ratio of the projected area of the • A Neptune-mass planet orbiting a nearby R R 2 planet to that of the star: that is, ( p/ ⋆) , M dwarf, GJ 436, was later found to tran- R R where p and ⋆ are the planetary and stel- sit its host star. lar radii, respectively. As viewed from such a vantage point, Jupiter transiting the sun • Four planets were discovered around the red dwarf star Gl 876,8 with three of them would cause a 1 percent dip in the light curve in the so-called Laplace-resonance, (the light of the star as a function of time) where the ratio of the orbital periods is lasting roughly thirty hours, and Earth 1:2:4. This con½guration is strikingly would cause a 0.01 percent dip for about sim ilar to the inner three Galilean moons thirteen hours. of our Jupiter. Careful analysis of the light curve during the transit, together with rv obser vations • A system of three to six planets was found of the system, yield the following parame- around Gl 581 (some of which are dis- ters for the system: period, planet-to-star puted), including one or more super- radius ratio, inclination (angle or orbital Earths (planets more massive than Earth, plane with respect to the sky plane), and but less than about ten times the mass semi-major axis of the planet (half of the of Earth) close to or inside the habitable longest diameter of the elliptical orbit, in zone. units of the stellar radius). Further, using Kepler’s third law, the following fundamen - • 55 Cnc is a star visible to the naked-eye with ½ve planets. The innermost planet tal physical pa rameters are determined: (55 Cnc e) is a super-Earth on a very the surface gravity of the planet and the short-period orbit of only 0.73 days, mean density of the star. When coupled and was later found to transit the star. with spectroscopic observations and stellar models, the mean stellar density is, in HD • 10180 has a planetary system of seven turn, an important constraint on the mass (or even more) planets, the largest num - and radius of the star. In other words, the ber yet for exoplanetary systems! transiting planet helps us determine the

• An Earth-mass planet was discovered, properties of its host star. Knowing the inducing only 0.5 ms−1 variation on ɑ host star, then, is essential for determin- B ing the parameters of the planet. Centauri , one of the brightest and clos- tep est stars in the sky.9 (Note: this discov- For transiting exoplanets ( s), if the ery is still debated.) stellar radius is known (which is typically

84 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 85

the case), the planetary radius is also de - planetary transits. Nevertheless, as teps Gáspár termined. If rv measurements of the host became a hot topic in contemporary as- Áron Bakos star are available, and if the stellar mass is trophysics, their study provided a unique known (which is also common), then the opportunity for small telescopes to par- mass of the tep is also derived without ticipate in cutting-edge science. Examples any ambiguity (not only a lower limit, as of such surveys include tres, xo, hat- for pure rv detections). net, wasp, and hatsouth. Together, these Astronomers recognized long ago that projects have revealed approximately 150 teps provide us with an unparalleled op - teps, which are among the best charac- portunity for understanding their physical terized exoplanets. In fact, the majority properties, as one can unambiguously de- (70 percent) of exoplanets with masses termine their masses, radii, mean densi- and radii measured to better than 10 perties, and surface gravities, among other cent accuracy were discovered by wide- mea surements. The chance of detecting ½eld ground-based surveys. transit of a planet around a random star, Figure 2 plots the mass-radius diagram however, was initially thought to be very for around two hundred teps with well- slim, because 1) giant planets were thought determined physical parameters. Broadly to be rare; 2) the chance that we would speaking, the radius increases with increas - fall in its orbital plane is tiny (for example, ing mass, but there are a number of inter- 1 in 1,000 for our Jupiter, as seen from a esting features in this ½gure. One is that– vantage point); 3) the fraction of time as theoretical physics would predict–the spent transiting is small (for example, radii of massive planets are relatively small; 0.0003 for our Jupiter); and 4) the transit that is, the planets are very dense. Also, signature (diminution of starlight) is many transiting gas giant planets on short- small (< 1 percent). Thus, it is no accident period orbits were found to have much that the ½rst tep detection, in 2000,10 was larger radii than that expected for an old that of a giant planet (HD 209458 b), pre- and cool pure hydrogen/helium body (see viously known from rv measurements to the top portion of Figure 2). One example orbit a bright star on a very short-period is hat-p-32 b, with mass equal to but a orbit. Only a couple of years later, in 2003, radius twice that of Jupiter’s–meaning the ogle project11 detected the ½rst teps the planet is 1/8 Jupiter’s mean density! without prior rv measurements.12 The There has been no shortage of theories to enormous scienti½c value of transiting explain this “inflation” of planets. Ground- planets triggered a gold rush for teps, and based surveys yielded a big enough sam- a small armada of projects were under- ple to show that the inflation of radii (rel- taken. These employed primarily wide- ative to those predicted) was connected ½eld instruments observing tens of thou- to the heating by in-falling stellar flux, and sands of stars per exposure and monitor- perhaps by the metal content of the star. ing stellar ½elds every clear night. Certain At present there is still no clear under- challenges were realized, such as the need standing of this matter.13 for 1) robust automation of telescopes During the transit of a planet, starlight and data processing; 2) high-precision passes through the atmosphere of the plan- photometry (stellar flux measurements) et, and a fraction of the light is absorbed, over a wide ½eld in the sky; and 3) exten- depending on the properties of the at- sive resources for following up on the plan- mosphere (chemical composition, scale, et candidates to deal with the large num- height). By comparing the stellar spectrum ber of astrophysical scenarios that mimic in and out of transit, one can infer these

143 (4) Fall 2014 85 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 86

Exoplanets, Figure 2 2003–2013 The Mass-Radius Diagram of Transiting Extrasolar Planets

2.5

1.5 ] Jup [R P R

0.5

0.01 0.1 1 10

MP [MJup]

Circles indicate ground-based discoveries, while squares show space-based discoveries. Point-size scales with the inverse of the planetary surface gravity (higher gravity = smaller points). The gray tone scales with equilibrium temperature, with the very light symbols indicating temperatures in excess of 2000 K. Dashed lines indicate iso- density lines corresponding to 0.4, 0.7, 1.0, 1.33, 5.5, and 11.9 g cm−3, respectively.

properties of the planetary atmosphere. of the planets? It would suggest that The ½rst such measurement was the de - there may be life on the planet, since oxy- tection of sodium in the atmosphere of the gen is very reactive and does not persist ab - planet HD 209458 b using the stis instru- sent living organisms. (Note, however, that ment on the Hubble Space Telescope this is a potential “biosignature,” and not (hst). Since then, this planet has been de½nite evidence.15) subject to intensive studies, detecting an During the occultation of a planet–that extended hydrogen exosphere, for exam- is, when the planet moves behind the star– ple, and water absorption in the near in - the combined light from the star and plan - frared14 via transmission spectroscopy. et drops (as seen from Earth). In the in - Another well-studied system is HD 189733 b, frared, this drop is primarily due to the a gas giant on a short period orbit around plan et’s thermal radiation being eclipsed a nearby star (which star is somewhat by the star. By measuring the depth of the cooler than the sun). Here sodium was de - occultation, one can measure the so-called tected from the ground using the Hobby brightness temperature on the “dayside” Eberly Telescope, and a featureless spec- of the planet. Such measurements have trum in the visible (hst/acs) suggested been performed for more than thirty star haze in the atmosphere. Can you imagine systems. In the case of the aforementioned the excitement if we were to detect mo- HD 189733 b,16 the “phase-curve” (total lecular oxygen in the atmosphere of one observed brightness as the planet moves

86 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 87

on its orbit) was observed from before pri- object spectrographs with wide slits, such Gáspár mary transit to after occultation, and as mmirs on the Magellan Telescope. In Áron Bakos night- and dayside brightness was found general, astronomers made excellent use to reach temperatures of approximately of instruments that were not originally 970 K and 1200 K, respectively. This is a designed to carry out such high-precision relatively small difference, especially be- measurements, occasionally stretching cause we believe that this planet is tidally beyond the capabilities of these tools. Re- locked: the planet shows the same side to sults have sometimes been questioned by the star (just as our moon shows the same competing teams, and were shown to be face to Earth at all times), which there- very sensitive to the data analysis proce- fore receives an enormous radiant flux. A dures.18 plausible solution is that the heat is ef- As hinted at earlier, another quantity ½ciently redistributed to the nightside via that can be measured for teps is the angle circulation of winds. Even more amazing between the orbital plane of the planet and is that we now have a thermal map of this the equator of the star, as projected onto exoplanet showing that the warmest spot the plane of the sky. This angle is revealed on the planet is 16◦ off (to the “east”) through an anomaly in the rv measure- from the point directly facing the star. ments of the star made during transit. This Phase-curves were automatically ac - “Rossiter-McLaughlin” anomaly was pre - quired by the Kepler space mission for dicted and observed for eclipsing binary thousands of transiting planet candi- stars almost a century ago, but was only dates, as it performed nonstop observa- applied to teps in the past decade.19 Ini- tions in the visible band-pass. Occultations tial measurements suggested that all tep in the visible light (as compared to the in - orbits are well aligned (“prograde”) with frared, discussed above) are primarily due the equator of their host stars, and this to reflected light occulted by the star, and almost led to a pause in the investigation provide a handle on the reflectivity of of further systems. Then, highly tilted extrasolar planets. For the planet TrES-2 b, (called high obliquity) and even retrograde Kepler’s measurements established a stun- planets (circling the star in the “opposite” ningly low reflectivity; this planet is “pitch direction) were discovered, such as hat- dark,” reflecting only 2.5 percent of the in- p-7 b.20 This hot Jupiter, on a 2.2-day ret- falling light.17 rograde orbit, clearly violated the prevail- Planetary spectra were also investigated ing theory of giant planet formation, close to their occultation. By comparing which argued that planets form far from the spectrum of the system during and the stellar heart of the system, where ices outside occultation, one can infer the emis- condense from the rotating protoplane- sion spectrum of the planet. Molecules tary disc and then slowly migrate inward CH H O CO such as 4, 2 , and 2 were detected (keeping at least the direction of their an - for a number of exoplanets. gular momentum). Astronomer Joshua Observing transmission spectra or other Winn and colleagues concluded that hot properties of exoplanets can be extremely stars with hot Jupiters have high obliqui- challenging from the ground, due to sys- ties, with the dividing line between well- tematic noise introduced by our own at - aligned and misaligned systems being mosphere. Nevertheless, it has been a somewhere at stellar effective temperature trend in recent years to use ground-based higher than the sun.21 Recently, physicist instrumentation, such as osiris on the Simon Albrecht and colleagues have Gran Telescopio Canarias, and multi- claimed that the star-planet obliquities for

143 (4) Fall 2014 87 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 88

Exoplanets, close-in hot Jupiters were initially random, planet radii scales with the inverse square F 2003–2013 and aligned systems are those where tidal of the planetary radius.25 Approximately R interactions between the star and the plan- 13 percent of stars have small (2–4 ⊕) et are expected to be strong (see Figure 3).22 planets with relatively short (< ½fty days) Space-based transit searches are quali- periods. They also found that the occur- tatively different from ground-based rence of small planets in the Kepler ½eld searches in that they have achieved much increases for cool stars (< 4000 K) by a higher photometric precision than ground- factor of seven when compared to hot stars based surveys, using almost uninterrupted (> 6600 K). It also appears that while smal - observations to observe fainter stars. One ler planets are more frequent, this occur- C R T R key player was the o o satellite. Perhaps rence rate plateaus at about 2 ⊕; that is, one of the top scienti½c results of CoRoT planets smaller than two Earth-radii are was the discovery of CoRoT-7 b,23 a tran - still not more frequent. Another important R siting super-Earth with 1.6 ⊕ radius on a ½nding of Kepler was that multiple plane- twenty-hour-period orbit, and with a mass tary systems are intrin sically frequent: at M ≤ 8 ⊕. least 27 percent of planets are in multi- The other key player in exoplanet dis- transiting systems, and 15 percent of stars covery is the Kepler space mission, which with teps host more than one planet.26 is fully dedicated to tep detection. It was Multiple systems consist pri marily of designed to have the capability of detect- small planets; hot Saturns and Jupiters are ing an Earth-sized planet transiting a sun- “lonely,” with no nearby companions. A like star. Kepler has been transformative shortlist of some truly amazing planetary to the ½eld. Launched in 2009, it has been systems found by Kepler is given be low. continuously monitoring a selected area • Kepler-11: six teps in a densely packed in the sky, roughly the area of the Big Dip- con½guration, ½ve with orbital periods per, yielding exquisite photometry for some between ten and forty-seven days. 150,000 stars. Most stars are obser ved at a thirty-minute cadence, and the per-point • Kepler-36: a pair of planets with orbital precision of the stellar fluxes reaches thirty distances differing by only 10 percent parts per million! (Kepler failed mechani- and with densities differing by a factor of cally in May 2013, and a new mis sion plan, eight. “K2” or “Second Light,” has been adopted • Kepler-47, the ½rst circumbinary plane- to make use of Kepler’s re maining capa- tary system: two super-Earths orbiting at bilities.) To date, Kepler has found some P ≈ 50 days and P ≈ 300 days around a three thousand planetary can didates. As as- pair of stars that are eclipsing each other trophysicist Timothy Morton and astron- about every seven days. omer John Asher Johnson have shown, based on statistical arguments, at least 90 • Kepler-62: a ½ve-planet system with plan- R percent of these should be real planets, ets of 1.4 and 1.6 ⊕ radii orbiting in the even though the classical con½rmation is habitable zone (that is, they may host not available for the majority.24 life).27 Notably, the Kepler space mission found • koi-872 b, c: the detection and character- that small (radius) planets, reaching down ization of a nontransiting planet (koi- to Earth-size, are extremely frequent. 872 c) by variations (due to gravitational Using the Kepler data, Andrew Howard and interaction) induced on the transit times colleagues found that for orbital periods of the transiting planet (koi-872 b). shorter than ½fty days, the distribution of

88 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 89

Figure 3 Gáspár Obliquity of Transiting Exoplanets with Respect to the Spin Axis of Their Host Stars Áron Bakos

180 HAT-P-7 b 150 WASP-8 b WASP-33 b 120 HAT-P-11 b HAT-P-32 b 90 WASP-12 b 60 HD 80606 b proj. obliquity [deg] 30 XO-3 b HD 17156 b 0 10–4 10–2 100 102 104 106

relative tidal-dissipation timescale τmcz / age

The vertical axis shows the obliquity of the planets with respect to the stellar spin axis, projected on the sky plane. Zero degrees means a perfectly aligned prograde orbit, and 180 degrees means retrograde orbit. The horizontal axis shows the estimated timescale required for aligning a planet with the stellar spin axis. Stars with temperatures higher than 6250 K are shown with ½lled symbols. Open symbols show stars with temperatures lower than 6250 K. Source: Simon Albrecht et al., “Obliquities of Hot Jupiter Host Stars: Evidence for Tidal Interactions and Pri- mordial Misalignments,” The Astrophysical Journal 757 (2012): 18, arXiv:1206.6105; used here with permission of the American Astronomical Society.

koi koi • -142: similar case to -872, but also The past decade saw real breakthroughs using variations in the transit duration in the direct imaging of exoplanets. The (for the ½rst time) to determine the sys- task is extremely challenging because stars tem parameters. are bright while planets are faint and ap - pear close to the stars; thus, capturing and The next decade will be extremely separating the light emanating from the promising in terms of scienti½c progress planet requires very-high-contrast and based on transiting planets. The U.S.-based high-spatial-resolution imaging. For these tess mission, with a proposed launch reasons, state-of-the-art instrumentation sometime in 2017, will scan the entire sky has been developed and used on the largest to detect thousands of transiting planets telescopes, employing infrared imaging, around the brightest stars near Earth. adaptive optics, coronography, and novel Among these will be potentially habitable observing and data analysis techniques. super-Earths amenable to follow-up ob- The targets typically are young (a few mil- servations. The James Webb Space Tele- lion years old) stars, because these may scope (jwst) and the European echo have young planetary systems, in which space mission will observe atmospheres the planets still emit excess (infrared) light of teps through transmission and occul- due to their primordial heat. tation spectroscopy at unprecedented One spectacular success was direct im - precision. There is hope that by the next aging of the planets around the young star decade we will actually detect bio-signa- HR 8799.28 Not only were the planets clear- tures in the atmospheres of remote worlds. ly visible, but their face-on orbital motion

143 (4) Fall 2014 89 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 90

Exoplanets, around the star was also apparent on im - via microlensing. An interesting aspect of 2003–2013 ages taken a year apart. Another impor- microlensing is its sensitivity toward low- tant discovery was the planet orbiting the mass planets on wide orbits, even free- very bright and nearby star, β Pictoris.29 floating planets without a host star. Micro- This star has a disk of debris (dust and lensing is sensitive to classes of planets rocks) that is almost edge-on. An approx- that currently cannot be found by rv, tran- M imately 8 J planet around the same star sit, or direct-imaging searches. was detected on archival images from The ½rst microlensing-detected planet 2003, then reobserved in 2009 on the other was a 2.6 Jupiter-mass object at a 5 au orbit side of the star! Finally, a very recent de - (astronomical unit, the mean distance be - M tection is the 13 J mass single gas giant tween the Earth and the sun: 149.6 million around the star “к And,” detected with kilometers).35 Microlensing detected the Subaru/hiciao instrument as part of planets more massive than Jupiter around the seeds survey.30 very small stars,36 which is somewhat sur- Another breakthrough was the ability prising given the scarcity of such objects to take spectra of the directly imaged plan- found by rv and transit searches. It is also ets. This was done for all four planets from microlensing that we learned about around HR 8799 by Project 1640 on the ven- the existence of cold super-Earths that erable Palomar 5m telescope.31 The low- orbit their stars beyond the ice-line (dis- resolution spectra show an unexpected di- tance beyond which it is cold enough for versity among the four planets, with hints ices to form) at several au distance. Mul- CH NH CO of 4, 3, 2, and other molecules. tiple planetary systems, such as a Jupiter/ Massive current efforts (nici, seeds) and Saturn analogue, were also detected by future projects (gpi, sphere) will certain- microlensing.37 Recently, microlensing has ly yield many more directly imaged planets found signals due to free-floating planets, with spectra. and has concluded that such planets are twice as common in our galaxy as main- An object (star or planet), by virtue of sequence stars.38 its ½nite mass, will perturb the light from While progress in the ½eld of exoplanets a background source that falls along the over the past decade has been spectacular, line of sight, creating multiple images and most of the excitement is yet to come. This magnifying the source. As the object includes ½nding analogues of our solar sys- (“lens”) moves with respect to the back- tem, planets similar to our Earth, and ground source, the magni½cation changes moons around exoplanets. We hope to in time, resulting in the brightening of detect biomarkers and–ultimately–signs the background source. The brightening, of intelligent life capable of communica- as the function of time (the light curve), tion with us, however slow the turnaround has a characteristic “bell-shape.” This ef - time may be. fect, predicted by Einstein, has now become a practical astronomical tool. Thou- sands of such microlensing events32 are now detected every year by surveys such as ogle33 and moa.34 Planets around the lensing star cause further perturbation of the light, and appear as anomalies on the microlensing light curve. To date, some thirty or so planets have been discovered

90 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 91

endnotes Gáspár Áron Author’s Note: I thank Joel Hartman and Jeremiah Ostriker for careful reading of this man- Bakos uscript. I apologize to those whose work I could not discuss due to space limitations. 1 Michael Perryman, The Exoplanet Handbook (Cambridge: Cambridge University Press, 2011). 2 Gaspare Lo Curto et al., “Achieving a Few cm/sec Calibration Repeatability for High Reso- lution Spectrographs: The Laser Frequency Comb on harps,” Proceedings of SPIE (2012): 8446. 3 Peter Plavchan et al., “Precision Near-Infrared Radial Velocity Instrumentation and Exoplanet Survey,” American Astronomical Society, aas Meeting No. 221, #109.06, 2013. 4 For example, Andrew W. Howard et al., “The Occurrence and Mass Distribution of Close- in Super-Earths, Neptunes, and Jupiters,” Science (330) (2010): 653. 5 Michel Mayor and Didier Queloz, “A Jupiter-Mass Companion to a Solar-Type Star,” Nature (378) (1995): 355. 6 Michel Mayor et al., “The harps Search for Southern Extra-Solar Planets XXXIV: Occurrence, Mass Distribution and Orbital Properties of Super-Earths and Neptune-Mass Planets,” Astron- omy & Astrophysics (2011), arXiv:1109.2497. 7 Andrew W. Howard et al., “Planet Occurrence within 0.25 au of Solar-Type Stars from Kepler,” The Astrophysical Journal Supplement (201) (2012): 15. 8 Eugenio Rivera et al., “The Lick-Carnegie Exoplanet Survey: A Uranus-Mass Fourth Planet for GJ 876 in an Extrasolar Laplace Con½guration,” The Astrophysical Journal (719) (2010): 890. 9 Xavier Dumusque et al., “An Earth-Mass Planet Orbiting ɑ Centauri B,” Nature (491) (2012): 207. 10 David Charbonneau et al., “Detection of Planetary Transits Across a Sun-Like Star,” The Astro - physical Journal (529) (2000): L45. 11 Andrzej Udalski et al., “The Optical Gravitational Lensing Experiment: Search for Planetary and Low-Luminosity Object Transits in the Galactic Disk. Results of 2001 Campaign,” Acta Astronautica (52) (2002): 1. 12 M. Konacki et al., “An Extrasolar Planet that Transits the Disk of Its Parent Star,” Nature (421) (2003): 507. 13 For a review, see David S. Spiegel and Adam Burrows, “Thermal Processes Governing Hot- Jupiter Radii,” The Astrophysical Journal (2013), arXiv:1303.0293. 14 D. Deming et al., “Infrared Transmission Spectroscopy of the Exoplanets HD 209458 b and XO-1 b Using the Wide Field Camera-3 on the Hubble Space Telescope,” The Astrophysical Journal (774) (2013): 95, arXiv:1302.1141. 15 Hanno Rein, Yuka Fujii, and David S. Spiegel, “Some Inconvenient Truths about Biosignatures Involving Two Chemical Species on Earth-Like Exoplanets,” Proceedings of the National Academy of Sciences 111 (19) (2014): 6871–6875. 16 Heather A. Knutson et al., “A Map of the Day-Night Contrast of the Extrasolar Planet HD 189733 b,” Nature (447) (2007): 183. 17 David M. Kipping and David S. Spiegel, “Detection of Visible Light from the Darkest World,” Monthly Notices of the Royal Astronomical Society (417) (2011): L88. 18 For example, see Deming et al., “Infrared Transmission Spectroscopy of the Exoplanets HD 209458 b and XO-1 b Using the Wide Field Camera-3 on the Hubble Space Telescope.” 19 D. Queloz et al., “Detection of a Spectroscopic Transit by the Planet Orbiting the Star HD 209458,” Astronomy and Astrophysics (359) (2000): L13; and Joshua N. Winn et al., “Measurement of Spin-Orbit Alignment in an Extrasolar Planetary System,” The Astrophysical Journal (631) (2005): 1215.

143 (4) Fall 2014 91 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 92

Exoplanets, 20 Joshua N. Winn et al., “hat-p-7: A Retrograde or Polar Orbit, and a Third Body,” The Astro- 2003–2013 physical Journal (703) (2009): L99. 21 Joshua N. Winn et al., “Hot Stars with Hot Jupiters Have High Obliquities,” The Astrophysical Journal (718) (2010): L145. 22 Simon Albrecht et al., “Obliquities of Hot Jupiter Host Stars: Evidence for Tidal Interactions and Primordial Misalignments,” The Astrophysical Journal 757 (2012): 18, arXiv:1206.6105. 23 D. Queloz et al., “The CoRoT-7 Planetary System: Two Orbiting Super-Earths,” Astronomy and Astrophysics (506) (2009): 303. 24 Timothy D. Morton and John Asher Johnson, “On the Low False Positive Probabilities of Kepler Planet Candidates,” The Astrophysical Journal (738) (2011): 170. 25 Howard et al., “Planet Occurrence within 0.25 au of Solar-Type Stars from Kepler.” 26 Ibid. 27 William J. Borucki et al., “Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone,” Science (340) (2013): 587, arXiv:1304.7387. 28 Christian Marois et al., “Direct Imaging of Multiple Planets Orbiting the Star HR 8799,” Science (322) (2008): 1348; and Christian Marois et al., “Images of a Fourth Planet Orbiting HR 8799,” Nature (468) (2010): 1080. 29 A.-M. Lagrange et al., “A Probable Giant Planet Imaged in the Beta Pictoris Disk: vlt/NaCo Deep L’-band Imaging,” Astronomy & Astrophysics (493) (2009): L21. 30 J. Carson et al., “Direct Imaging Discovery of a ‘Super-Jupiter’ around the Late B-Type Star к,” The Astrophysical Journal (763) (2013): L32. 31 B. R. Oppenheimer et al., “Reconnaissance of the HR 8799 Exosolar System. I. Near-Infrared Spectroscopy,” The Astrophysical Journal (768) (2013): 24. 32 B. Paczyński, “Gravitational Microlensing by the Galactic Halo,” The Astrophysical Journal (304) (1986): 1. 33 Andrzej Udalski, “ The Optical Gravitational Lensing Experiment. Real Time Data Analysis Sys - tems in the ogle-III Survey,” Acta Astronautica (53) (2003): 291. 34 T. Sako et al., “moa-cam3: A Wide-Field Mosaic ccd Camera for a Gravitational Micro - lensing Survey in New Zealand,” Experimental Astronomy (22) (2008): 51. 35 I. A. Bond et al., “ogle 2003-blg-235/moa 2003-blg-53: A Planetary Microlensing Event,” The Astrophysical Journal (606) (2004): L155–L158. 36 Andrzej Udalski et al., “A Jovian-Mass Planet in Microlensing Event ogle-2005-blg-071,” The Astrophysical Journal (628) (2005): L109. 37 B. S. Gaudi et al., “Discovery of a Jupiter/Saturn Analog with Gravitational Microlensing,” Science (319) (2008): 927. 38 T. Sumi et al., “Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing,” Nature (473) (2011): 349.

92 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 93

Mapping the Universe: Surveys of the Sky as Discovery Engines in Astronomy

Michael A. Strauss

Abstract: Astronomers can map the sky in many ways: observing in different regions of the electromagnetic spectrum, obtaining spectra of stars and galaxies to determine their physical properties and distances, and repeatedly observing to measure the variability, explosions, and motions of celestial objects. In this review I describe recent surveys of the sky astronomers have carried out, focusing on those in the visible part of the spectrum. I describe in detail the Sloan Digital Sky Survey, an ongoing imaging and spectroscopic survey of over one quarter of the celestial sphere. I also discuss some of the major surveys planned for the next decade, using telescopes both on the ground and in space.

Astronomy is an observational science. Unlike chemistry or biology, the objects of study in astronomy are far removed, at distances to which we will not have the capability to travel using even the most advanced foreseeable technology. This means that we cannot carry out experiments on the stars and galaxies that are the bread and butter of our discipline; all the information we can glean about them is the result of measuring the tiny fraction of the light that they emit that happens to fall on our eyes and our telescopes. We then interpret these data in the context of the laws of physics to draw conclu- sions about the nature of these distant bodies, allow- MICHAEL A. STRAUSS is Professor ing us to infer, for example, the conditions in the of Astrophysical Sciences and As- cores of stars, or the existence of new forms of matter sociate Chair of the Department of that are unknown from our experience and experi- Astrophysical Sciences at Princeton ments here on Earth. University. His research concerns The range of phenomena in the universe is vast, all aspects of extragalactic astron- and the rate of astronomical discovery today tells omy and observational cosmology. us that we are far indeed from a complete under- He has published over two hundred refereed papers on subjects ranging standing of all that the universe has to teach us. This from the large-scale distribution of essay describes one of the most productive approach- galaxies to the discovery of the most es we have toward astronomical discovery; namely, distant quasars known. using our telescopes to map the heavens and create

Surveys of a census of the objects we ½nd. Astronomi- ent regimes is a powerful tool for explor- the Sky as cal surveys have always been a key aspect ing these phenomena. However, normal Discovery Engines in of our ½eld: such surveys have much to stars (and thus galaxies, which are made Astronomy teach us about the formation and struc- up of stars) emit most of their radiation ture of the Milky Way galaxy in which the at visible and near-infrared wavelengths, sun sits; the expansion, future fate, and making this the most effective regime in origin of the universe as a whole; and the which to survey the sky for these objects. nature of stars and the planets that orbit them. Indeed, astronomy is the study of Go outside on a clear moonless night, origins; we ask (and occasionally answer!) far from the lights of civilization. Once the most fundamental questions about your eyes have adapted to the darkness, where planets, stars, galaxies, and the uni- between two and three thousand stars are verse as a whole come from. With each discernable to the naked eye at any given advance in technology and new way to time. You will also see a silvery band cross- survey the heavens, we uncover new phe- ing the sky: the Milky Way. Galileo Galilei nomena that we did not anticipate, and was the ½rst to point a telescope to the we ½nd ourselves addressing questions heavens, and he discovered that the light that we previously did not have the imag- of the Milky Way comes from countless ination to ask. stars. Since that time, astronomers have With one hundred billion stars in our used ever larger telescopes to map objects Milky Way, and one hundred billion gal- in the sky. axies in the observable universe, our sur- However, the images we make with cam- veys have entered the realm of big data. eras placed at the back of our telescopes The biggest survey telescopes today can are two-dimensional. We have no depth gather terabytes of data in a single night, perception, and the stars look all to be and our catalogs of galaxies and stars in- equidistant, with no sense of which are clude over a billion objects, a number that closer and which are farther away. In fact, will increase by a factor of ten over the the nearest star (other than the sun) is next decade or so. The discovery potential about four light years (or about forty tril- of our surveys is limited by a combina- lion kilometers) from us, a distance that tion of raw computer processing power, is completely outside our everyday expe- the cleverness of our algorithms, and our rience. It would take thirty thousand years imagination. Just as Google allows us to to cover that distance traveling at the speed query human databases to uncover facts of our fastest spacecraft (forty-½ve kilome- and the relations between them, astron - ters per second). While most of the indi- omers have been developing similar tech- vidual stars visible to the naked eye are nology to query the database of the uni- within a few hundred light years of us, the verse. bulk of stars in the Milky Way are much This essay will focus on surveys in visi- farther away, arrayed in a vast flattened ble light, which of course represents only spiral structure some one hundred thou- a small sliver of the full range of electro- sand light years across, containing roughly magnetic waves, from high-energy gamma one hundred billion stars. rays to long-wavelength radio waves. Very One hundred years ago, astronomers different physical phenomena are respon- understood the Milky Way galaxy to be sible for emission at different regions of the full extent of the universe. However, in the electromagnetic spectrum, and com- addition to the myriad stars apparent in parison of maps of the sky in these differ- astronomical images, one also sees fuzzy

94 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 95

extended objects, termed nebulae (the Latin tells us that galaxies are made of stars. Michael A. word for cloud). Edwin Hubble demon- However, there is an important difference: Strauss strated in the 1920s that these nebulae were the absorption lines in galaxy spectra are other “island universes,” as large as our shifted systematically to longer (redder) own Milky Way but at much greater dis- wavelengths. Hubble found that the spec- tances. This discovery enormously expand - tra of essentially all galaxies are redshifted, ed our understanding of the size of the and the degree of redshift is proportional universe. The nearest big galaxy to our own to the distance of the galaxy. is about two million light years distant, This relationship between redshift and and the number of galaxies seen in the distance is a consequence of the expan- deep est images with the epon ymous Hub- sion of the universe and, as explained in ble Space Telescope imply that there are David Spergel’s companion article in this about one hundred billion of them in the volume, it leads directly to our modern observable universe. understanding of the Big Bang. For our purposes, however, this becomes a valu- Surveys of galaxies based on photo- able tool for mapping the universe in three graphic plates show that they are far from dimensions: measuring the spectrum of uniformly distributed in the sky: clusters a galaxy allows us to determine its redshift, a few million light years across containing and thus, by Hubble’s law, its distance. hundreds of galaxies are apparent, with Photographic ½lm records the presence hints of larger structures yet. But to really of only (at best) a few percent of the pho- map the distribution of galaxies in three tons that fall upon it. Thus, even with the dimensions, we need an unambiguous way largest telescopes available in Hubble’s to measure their distances. Hubble’s sec- time, measuring the spectrum of a faint ond great discovery–that the universe is galaxy in order to determine its redshift expanding–gives us the way to do so. was enormously time-consuming, requir- Consider the spectrum of an astronomical ing exposure times of many hours for even object, which measures the intensity of the nearest galaxies. Modern electronic its light as a function of wavelength. This detectors, such as those in your digital spectrum gives much more detailed infor - camera, are far more sensitive, detecting mation about the physical nature of the close to 100 percent of the photons that object than the properties (size, brightness, fall on them. Their development and adop- and color) measureable from an astronom- tion by the astronomical community ical image. For example, the wavelength starting in the late 1970s meant that ap - at which the spectrum of a star peaks is a preciable numbers of galaxy spectra, and measure of its surface temperature, which thus redshifts and distances, could be mea- for most stars in turn indicates their mass. sured. As described in Anna Frebel’s article in The galaxy distribution based on these this volume, there are characteristic wave- early surveys of a few thousand redshifts lengths at which the star is dimmer (absorp- were stunning and surprising. These maps tion lines); these are due to absorption of showed that most galaxies are strung along light by atoms in the atmosphere of the long ½lamentary structures that connect star, and they reflect the star’s chemical the rich clusters. These ½laments, up to comp osition. hundreds of millions of light years across, As illustrated in Figure 1, the spectra of surround enormous volumes, essentially galaxies show many of the same absorp- devoid of galaxies. These pictures raised tion lines as do stars like the sun, which many questions: How large can these

143 (4) Fall 2014 95 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 96

Surveys of Figure 1 the Sky as Spectra of Objects from the Sloan Digital Sky Survey Discovery Engines in Astronomy

The upper panel shows a star similar to the sun; the pair of dips (circled) at a wavelength just below 4000 A is due to absorption by calcium atoms in the atmosphere. The other panels show spectra of galaxies at ever greater distances (marked in each panel in units of millions of light years, or Mly). The calcium absorption feature is circled in each case; the more distant the galaxy, the larger the redshift. Source: The Sloan Digital Sky Survey, http:// www.sdss.org/.

struc tures be? How did they form? What of which was the Sloan Digital Sky Sur- can they tell us both about the nature of vey (sdss).1 This program was the scien- galaxies and the structure of the universe ti½c vision of Princeton University astron- overall? omer James Gunn, who realized in the late 1980s that advances in electronic de - All this motivated the next generation tector technology, telescope design, and of redshift surveys, the most ambitious computer processing power made it pos-

96 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 97

sible to design a dedicated telescope proj- Kirshner’s memorable phrase: there are Michael A. ect to map the universe much more thor- no structures even larger than this; and if Strauss oughly than had been done to date. The we step back far enough, the distribution initial stated goal of the project was to of galaxies appears uniform. measure redshifts for one million galax- As Spergel’s article describes, these data, ies in the local universe in order to char- together with measurements of the cos- acterize the nature of the galaxy distribu- mic microwave background and other cos- tion on the largest scales. The sdss built mological probes, lead to a comprehen- a specialized telescope with a two-and-a- sive model for the structure and makeup half-meter-diameter primary mirror, as of the universe. In particular, we can un- well as an astronomical camera with a derstand the richness of the distribution total of 145 megapixels (the largest ever at of galaxies in a model in which ordinary the time it was built). The telescope swept matter (in the form of atoms, mostly hy - the sky essentially every clear moonless drogen, which make up the stars that cause night for ten years, covering twenty square galaxies to shine) represents just under 5 degrees every hour in ½ve broad ½lters. percent of the mass-energy density of the The resulting color images, which survey universe, with the remainder being in the the region of sky far from the band of the form of dark matter and dark energy. Milky Way (whose dust obscures the light One lesson of astronomy surveys is that of more distant galaxies) now cover about by gathering data necessary to answer one one-third of the celestial sphere and con- question (in this case, wide-½eld imaging tain data on almost half a billion stars, gal- and spectroscopy to measure the large- axies, quasars, and asteroids. scale distribution of galaxies), astronomers On the less than pristine nights, the gain the ability to address other problems imaging camera was replaced with a in the ½eld, many of which were unantic- spectrograph, fed by optical ½bers from ipated at the beginning of the survey. The the focal plane of the telescope, allowing sdss is no exception to this rule, making spectra of 640 objects selected from the fundamental discoveries in the structure images (increased to 1000 objects after a of the Milky Way galaxy, the most distant major upgrade to the system in 2009) to quasars and the nearest stars, and many be measured at a time. To date, the survey other topics.2 has measured the spectra, and thus red- About half the objects visible in the sdss shifts and distances, of over two million images are stars in our own Milky Way, at s galaxies, creating a stunning and detailed typical distances of thousands to tens of r map of the universe in which we live. Fig- thousands of light years, appearing as / ure 2 covers only a few percent of the full sharp points of light; most of the rest are sample; each of the roughly ½fty thou- galaxies, which appear fuzzy in the images. sand dots shown represents an entire Away from the densest part of the Milky galaxy, as large as the Milky Way, con- Way, a flattened rotating disk, stars lie in taining one hundred billion stars. The what is thought to be a roughly spherical ½laments and voids apparent in the ½rst distribution around the center of our redshift surveys of the early 1980s appear galaxy, termed the halo of the Milky Way, in all their glory here. The largest struc- extending to tens of thousands of light ture in the map, dubbed “The Sloan Great years. It was long thought that the halo Wall,” is 1.4 billion light years across. was smooth, with stellar density falling Interestingly, there appears to be an “end steadily as one moves from the galactic to greatness,” to use astrophysicist Bob center. But the maps of the stars from the

143 (4) Fall 2014 97 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 98

Surveys of Figure 2 F the Sky as A Slice through the Three-Dimensional Distribution of Galaxies Mapped T Discovery by the Sloan Digital Sky Survey Engines in Astronomy

T b h

Each of the more than ½fty thousand dots in this ½gure represents a galaxy as large as the Milky Way. The Milky Way itself sits in the center of the ½gure; the outer circle represents a distance of two billion light years from the sun. The ½lamentary nature of the galaxy distribution is readily apparent in this ½gure. The segments devoid of data are regions of the sky that the sdss did not survey. Source: The Sloan Digital Sky Survey, http://www.sdss.org/.

sdss and from earlier photographic sur- than ten thousand light years long. It is veys show that things are more interesting now un derstood that as the globular clus- than that. A taste of the results is shown in ter orbits our Milky Way, gravitational tidal Figure 3, which maps the distribution of forces (the difference in the gravitational stars in the vicinity of a so-called globular acceleration between the near and far sides cluster (named Palomar 5), a conglomer- of the globular cluster) are tearing the clus- ation of roughly one hundred thousand ter apart, pulling streams of stars from it. stars 150 light years across. Palomar 5 is Indeed, the star maps from the sdss have accompanied by a stream of stars more shown that such star streams are com-

98 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 99

Figure 3 Michael A. The Distribution of Stars in the Vicinity of the Globular Cluster Palomar 5 Strauss

0 Decl. [deg. J2000]

–1

–2

–3 236 235 234 233 232 231 230 229 228 227 226 225 224

R.A. [deg. J2000]

The globular cluster itself is the dark region to the lower-right of the center of the ½gure. It is accompanied by both leading and trailing streams of stars, stretching over ten thousand light years. This stream is believed to have been pulled out from the globular cluster by tidal forces from the Milky Way. Source: Michael Odenkirchen et al., “The Extended Tails of Palomar 5: A 10° Arc of Globular Cluster Tidal Debris,” The Astro- nomical Journal 126 (2003): 2385. Reprinted with permission.

mon: in a process predicted by some the- history of the universe. Of course, very dis- orists and now con½rmed by these obser- tant galaxies will be tremendously faint vations, the halo appears to be made as seen from Earth, so, all else being equal, y largely of the debris of small galaxies and the easiest distant galaxies to see will be e e globular clusters that have fallen into, those with the highest intrinsic luminosi- and been torn apart by, the gravity of our ties. These include quasars: galaxies with Milky Way.3 a supermassive black hole (a black hole with a mass up to several billion times that Surveying large swaths of sky to very of the sun) in their center; as described in faint levels makes one sensitive to very rare Scott Tremaine’s article in this volume, gas (and therefore interesting) objects, such orbiting close to the black hole heats up as quasars at very great distances. Because tremendously and glows enough to out- of the ½nite speed of light, we see a distant shine the hundred billion stars of the gal - galaxy not as it is today, but as it was when axy by a factor of one hundred or more. the universe was signi½cantly younger. Because the light from a quasar is dom- Astronomers thus can use their telescopes inated by the tiny region in the vicinity of as time machines to directly observe the the black hole, they are usually unresolved

143 (4) Fall 2014 99 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 100

Surveys of in sdss images, and thus look like stars. we found ourselves stumbling over some the Sky as The sdss was designed to identify quasars of the nearest stars to our own! Discovery Engines in by their distinctive colors in the images. Despite these problems (and the seren - Astronomy In particular, due to absorption by neutral dipity of ½nding brown dwarfs was excit- hydrogen in their spectra, the highest- ing and scienti½cally important in its own red shift quasars are extremely red, ap pear - right), we broke the record multiple times ing only in the longest-wavelength ½lter for the most distant quasar discovered, in - that the sdss measures (the “z” band). cluding one whose light we observe today Soon after the sdss had garnered its ½rst was emitted less than a billion years after images of the sky, my student Xiaohui Fan the Big Bang. This is remarkable: the cos- (now a professor at the University of Ari- mic microwave background, emitted by zona) and I started searching for record- gas in the universe four hundred thousand breaking quasars. years after the Big Bang is smooth to one Finding these most distant quasars is part in one hundred thousand, and yet a straightforward in principle: simply iden- billion years later, a supermassive black tify those objects that appear only in the z hole, the densest conceivable object with band, and con½rm that they are quasars a mass billions of times that of the sun, by taking a spectrum. Making this work had managed to form. It is still poorly un - in practice, however, required a very de - derstood how this happened, and discov- tailed understanding of every way the data eries of yet more distant quasars may shed could fool us; even extremely rare glitches some light into this process. Our record that affect one in a million stars would has since been broken in spectacular fash- swamp our search for these objects. We ion with a quasar seen one hundred mil- learned that doing science from surveys re - lion years earlier than our own, found in quires exquisite quality control and a deep data from the United Kingdom Infrared understanding of the nature of the data! Telescope Deep Sky Survey. But we were stymied in our search by One of the most important lessons from another problem, this one astrophysical. the sdss and other surveys is that if the Extremely cool stars also appear quite red. data are of high quality and are made avail- About the time that the sdss was taking able to the world, they enable science far its ½rst data, astronomers were using near- beyond the initial goals of the survey. The infrared surveys of the sky to discover sdss, which is now in its ½fteenth year of new classes of very cool red stars, called full operation, has plans to continue gath - brown dwarfs. These have very low mass es, ering data through the year 2020. Almost so low that their gravity is inadequate to six thousand refereed articles have been ignite thermonuclear fusion in their cores. written to date by scientists all over the Indeed, these are only a bit more massive world using the sdss data, a level of pro- than many of the planets described in ductivity that rivals that of much larger Gáspár Bakos’s article in this vol ume on telescopes, such as the ten-meter Keck exoplanets. Brown dwarfs have surface Tele scopes in Hawaii. Thus astronomers temperatures below 2000 K (for com par - are motivated to consider the next gener- ison, the sun has a surface temperature of ation of surveys beyond the sdss. The 6000 K), making them dim (and thus only sdss telescope has a primary mirror with visible at very small distances from Earth, a diameter of only two-and-a-half meters, typically thirty light years or less) and very which is small compared to the largest op- red, just like the qua sars. In our search for tical telescopes in the world today: ten the most distant quasars in the universe, meters across (and three signi½cantly larg-

100 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 101

er telescopes are being planned for the next of the sky: “celestial cinematography,” as Michael A. decade, with diameters of twenty-two, lsst Chief Scientist Tony Tyson likes to Strauss thirty, and thirty-nine meters). Larger tele- phrase it. Indeed, surveys on much smaller scopes are capable of seeing much fainter, telescopes that repeatedly image the sky and therefore more distant, galaxies. As have discovered a wide range of variable we have already seen, this means that they phenomena, including the motions of as - can probe back to times when the galax- teroids in our own solar system, pulsating ies, and the universe as a whole, were much and exploding stars of all sorts, the subtle younger than they are today. motions of distant stars that reflect the gravitational potential of the Milky Way, As Pieter van Dokkum’s article in this and the flickering of quasars as parcels of volume describes, the early universe was gas swirl around the black holes that power very different from the universe today. The them. The lsst will extend such studies to ½rst galaxies are thought to have formed much fainter astronomical objects, and is several hundred million years after the bound to ½nd new kinds of variable and Big Bang, and one of the most important transient phenomena that have not been areas of research today is to determine how anticipated to date. they grew and evolved from their initial Telescope observations from the surface formation. The next generation of planned of the Earth are affected by the atmo- surveys is designed to address this ques- sphere, which both blurs images and adds tion. The 8.2-meter Subaru Telescope op - a substantial amount of background light, erated by the National Astronomical Ob- especially at near-infrared wavelength. servatory of Japan has the largest ½eld of As the Hubble Space Telescope has dra- view of any existing telescope of its size, matically demonstrated, placing an obser- and is thus particularly well-suited for car - vatory in space allows much sharper im - rying out surveys. It has a wide-½eld imag- ages and the ability to observe much ing camera that has just started a major fainter objects. The Hubble Telescope it- survey covering over one thousand square self has a tiny ½eld of view, and is thus not degrees of sky, sensitive to objects twenty- well-suited for survey work, but the U.S. ½ve times fainter than the sdss was able National Reconnaissance Of½ce has re- to reach. In 2018, this will be followed up cently given nasa two telescopes that have with a spectroscopic survey of hundreds mirrors as large as Hubble’s (2.4 meters of thousands of galaxies from the ½rst few in diameter, similar to that of the sdss) bil lion years after the Big Bang. but are designed with a much larger ½eld The era of surveys will continue into the of view. There are active plans to use one next decade. The Large Synoptic Survey of them to map the large-scale distribu- Telescope (lsst) will spend ten years in a tion of dark matter by measuring the dis- dedicated wide-½eld imaging survey of the tortions it causes on the shapes of galax- sky.4 Polishing of its 8.4-meter primary ies. The combination of this space-based mirror is nearing completion; the tele- telescope with the lsst will be particu- scope will be constructed in the Chilean larly powerful; the coarser images of the Andes (famed for their clear and steady lsst will be balanced by its much larger skies) and will start a ten-year survey of sky coverage and measurements over a the sky in 2022, covering half the celestial wide range of wavelengths. The quantity sphere four times deeper than the Subaru of data these sur veys will produce will be survey will go. In doing so, it will map the measured in petabytes (one petabyte is a heavens multiple times, making a movie million gigabytes); analyzing and inter-

143 (4) Fall 2014 101 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 102

Surveys of preting these data will drive new technol - omers. With these surveys well underway the Sky as ogies in computer processing and analysis. a decade from now, astronomical discov- Discovery Engines in These data will be made public, allowing ery will continue to be driven by the Astronomy schoolchil dren to search for supernovae amazing datasets that they produce. at the same time as professional astron -

endnotes 1 The Sloan Digital Sky Survey is described in detail at http://www.sdss.org. See also the popular book describing the building of the survey: Ann Finkbeiner, A Grand and Bold Thing: An Extraordinary New Map of the Universe Ushering in a New Era of Discovery (New York: Free Press, 2010). 2 The discovery of distant quasars in the sdss is described in Xiaohui Fan et al., “A Survey of z > 5.7 Quasars in the Sloan Digital Sky Survey,” The Astronomical Journal 125 (2003): 1649. 3 The structure of the halo of the Milky Way as seen in the sdss is described in V. Belokurov et al., “The Field of Streams: Sagittarius and Its Siblings,” The Astrophysical Journal 642 (2006): L147. 4 The Large Synoptic Survey Telescope is described at http://www.lsst.org. A comprehensive description of its science opportunities may be found in the LSST Science Book v. 2.0 (2009) at http://www.lsst.org/lsst/scibook.

102 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 103

The Odd Couple: Quasars & Black Holes

Scott Tremaine

Abstract: Quasars emit more energy than any other object in the universe, yet are not much bigger than our solar system. Quasars are powered by giant black holes of up to ten billion (1010) times the mass of the sun. Their enormous luminosities are the result of frictional forces acting upon matter as it spirals toward the black hole, heating the gas until it glows. We also believe that black holes of one million to ten billion solar masses–dead quasars–are present at the centers of most galaxies, includ ing our own. The mass of the central black hole appears to be closely related to other properties of its host galaxy, such as the total mass in stars, but the origin of this relation and the role that black holes play in the formation of galaxies are still mysteries.

Black holes are among the most alien predictions of Einstein’s general theory of relativity: regions of space-time in which gravity is so strong that nothing –not even light–can escape. More precisely, a black hole is a singularity in space-time surrounded by an SCOTT TREMAINE, a Foreign Hon - event horizon, a surface that acts as a perfect one-way orary Member of the American membrane: matter and radiation can enter the event Academy since 1992, is the Richard horizon, but, once inside, can never escape. Although Black Professor in the School of black holes are an inevitable consequence of Ein- Nat ural Sciences at the Institute for stein’s theory, their main properties were only under- Advanced Study and the Charles A. stood–indeed, the name was only coined–a half- Young Professor of Astronomy on century after Einstein’s work.1 Remarkably, an iso- the Class of 1897 Foundation, Emer- itus at Princeton University. His related, uncharged black hole is completely charac- search interests are centered on as - terized by only two parameters: its mass and its spin trophysical dynamics, including (or angular momentum). An eloquent tribute to the plan ets, small bodies in the so lar austere mathematical beauty of these objects is given system, galaxies, black holes, and by the astrophysicist Subrahmanyan Chandrasekhar galactic nuclei. His work is pub- in the prologue to his monograph The Mathematical lished in such journals as The Astro- Theory of Black Holes: “The black holes of nature are phys ical Journal and Monthly Notices of the Royal Astronomical Society. the most perfect macroscopic objects there are in the With James Binney, he is the author universe: the only elements in their construction are of the graduate textbook Galactic our concepts of space and time. And since the gen- Dynamics. eral theory of relativity provides only a single unique

103 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 104

The Odd family of solutions for their descriptions, M and the speed of light c to an energy E Couple: they are the simplest objects as well.”2 called the rest-mass energy. Using this re- Quasars & Black Holes (Any one who scans the six hundred pag - lation, there is a natural measure of the ef½ - es that follow, however, is unlikely to ciency of this or any other furnace: the ra - agree that they are as simple as claimed.) tio of the energy it produces to the rest- Simple or complex, we are now almost mass energy of the fuel that it consumes. cer tain, for reasons out lined in this essay, For furnaces that burn fossil fuels, the that black holes do exist and that a giant ef½ ciency is extraordinarily small (about black hole of several million times the 5 x 10−10), and all combustion processes mass of the sun is present at the center of based on chemical reactions have similarly our galaxy. low ef½ciencies. For ½ssion reactors using Laboratory study of a black hole is im - uranium fuel, the ef½ciency is much bet- pos sible with current or foreseeable tech - ter, around 0.1 percent; and for the fusion no logy, so the only way to test these pre - reactions that power the sun and stars, dictions of Einstein’s theory is to ½nd black the ef½ciency can reach 0.3 percent. holes in the heavens. Not surprisingly, iso- Black-hole furnaces can have far higher lat ed black holes are dif½cult to see. Not on - ef½ciency than any of these: between 10 ly are they black, they are also very small: a and 40 percent for accretion of gas from a black hole with the mass of the sun is only thin accretion disk. In the unlikely event a few kilometers in diameter (this state - that we could ever domesticate black holes, ment is deliberately vague: because black the entire electrical energy consumption holes bend space to such extremes, our no- in the United States could be provided by tions of “distance” close to a black hole a black-hole furnace consuming only a few cease to be unique). However, the pros - kilograms of fuel per year. pects for detecting black holes in gas-rich environments are much better. The gas Despite the relatively low ef½ciency of fu - close to the black hole normally takes the sion reactions, most of the light in the uniform of a rotating disk, called an accretion verse comes from stars. Most of the stars disk: rather than falling directly into the in the universe are organized in gal axies: black hole, the orbiting gas gradually spi- assemblies of up to 1011 stars orbiting in a rals in toward the event horizon as it loses complex dance determined by their mutu- orbital energy, most likely transferred to al gravitational attraction. Our own gal - turbulence and magnetic ½elds in the disk.3 axy contains a few tens of billions of stars The energy is eventually transform ed into arranged in a disk; the nearest of these is thermal energy, which heats the gas until about 1 parsec (3.26 light years) from us, it begins to glow, mostly at ultraviolet and and the distance to the center of our gal - X-ray wavelengths. By the time the inward- axy is about 8 kiloparsecs (or about 26,000 spiraling gas disappears behind the event light years). The diffuse light from distant horizon, deep within the gravitation al well stars in the galactic disk is what we ob - of the black hole, a vast amount of radia- serve as the Milky Way.4 tion has been emitted from every kilogram A small fraction of galaxies contain mys- of accreted gas. terious bright and compact sources of ra - In this process, the black hole can be diation at or near their centers, called ac - thought of as a furnace: when provided tive galactic nuclei.5 The brightest of these with fuel (the inward-spiraling gas) it pro- are the quasars; remarkably, they can emit duces energy (the outgoing radiation). Ein- up to 1013 times more light than stars like stein’s iconic formula E = Mc2 relates mass the sun, thereby outshining the entire gal -

104 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 105

axy that hosts them. Even though quasars How can quasars emit so much energy? Scott are much rarer than galaxies, they are so The suggestion that they are black-hole Tremaine bright that they contribute almost 10 per- furnaces was made independently by cent of the light emitted in the universe. Edwin Salpeter in the United States and Ironically, the extraordinary luminosity Yakov Zel’dovich in the Soviet Union, soon of quasars is what made them hard to dis- after quasars were ½rst discovered. But in cover. Except in a few cases, they are so the 1960s the black hole was a novel and bright that the host galaxy cannot be seen exotic concept, and staggeringly massive in the glare from the quasar, and so small black holes (roughly one hundred million that they look like stars, even at the reso- solar masses) were required in order to ex - lution of the Hubble Telescope (in fact, plain quasar properties. Thus, most astron - “qua sar” is a contraction of “quasi-stellar omers quite properly focused on more ob ject”). Thus, even the brightest quasars conservative models for quasars, such as are usually indistinguishable from millions supermassive stars, dense clusters of ordi - of stars of similar brightness. Fortunately, nary stars or neutron stars, and collaps- some quasars are also strong sources of ra- ing gas clouds. Over the next two decades, dio emission, and in 1963 this clue enabled however, all of these models were subjected astronomer Maarten Schmidt at Caltech to to intense scrutiny that increasingly sug- identify a radio source called 3C 273 with a gested that they were inadequate to ex - faint optical source that otherwise looked plain the growing body of observations like an undistinguished star.6 With this of qua sars. Furthermore, other studies identi½cation in hand, Schmidt was able to showed that the alternative models gener - show that 3C 273’s spectral lines were red- ically evolve into a black hole containing shifted–Doppler shifted by the cosmolog- most of the mass of the original system, ical expansion of the universe–to wave- thereby sug gesting that the formation of lengths 16 percent longer than laboratory massive black holes is natural and perhaps spectra, and thus that 3C 273 was at a dis- even inevitable. tance of eight hundred megaparsecs, ten A number of indirect but compelling ar- million times farther away than it would gu ments also support the black-hole fur- have been if it were a normal star. nace hypothesis. The ½rst of these relates to Now almost one hundred thousand qua- ef½ciency. The luminous output of a bright sars have been identi½ed. The most distant quasar over its lifetime corresponds to a of them is almost one hundred times far- rest-mass energy of about one hundred ther away from Earth than 3C 273, and its mil lion times the mass of the sun. If this light was emitted when the universe was were produced by the fusion reactions that only 6 percent of its present age. However, power stars with the ef½ciency of 0.3 per- the formation of quasars that early in the cent given earlier, the mass of fuel re - history of the universe was a very rare quired would be almost the total of all the event. Most were formed when the uni- stars in our galaxy. There is no plausible verse was 20–30 percent of its current age, way to funnel this much mass into the tiny and quasars today are a threatened species: central region that the quasar occupies, nor the population has declined from its peak is there evidence that so much mass resides by almost two orders of magnitude, pre - there. On the other hand, for a black-hole sum ably because the fuel supply for qua - furnace the ef½ciency is 10 percent or more, sars dri ed up as the universe expanded at an so the required mass is less than 109 solar accelerating rate. masses, and this much gas is not hard to

143 (4) Fall 2014 105 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 106

The Odd ½nd close to the center of many galaxies. In a few cases space-based observatories Couple: Thus, the black-hole furnace is the only can measure X-ray spectral lines emitted by Quasars & Black Holes model that does not bankrupt the host quasars. These are not the narrow lines galaxy’s fuel budget. seen in spectra of the sun, stars, or inter- A second argument is based on the stellar gas; instead they are grossly mis- small size of quasars. We have known since shapen, with broad tails extending to much their discovery that quasars appear as un - longer wavelengths than such lines would resolved point sources of light even in the have in weak gravitational ½elds. The only best optical telescopes; this observation plausible explanation for these distortions alone implies that they must be less than is that they arise from gravitational redshift about a kiloparsec across, and long-base- –the loss of energy as the X-rays climb out line radio interferometry shows that qua - of a deep gravitational well on their way to sars must be far smaller, less than one par - us–and/or from extreme Doppler shifts sec across. Even stronger limits on the size caused by relativistic motions, most prob- come from indirect measurements. Qua- ably in a rotating accretion disk. Either sars vary irregularly in brightness on a wide explanation requires that the X-rays were range of timescales, from weeks to de- emitted from a region only a few times cades and probably even longer. It proves larger than the event horizon of a black quite dif½cult to construct any plausible hole, as no other known astrophysical sys - model of a luminous astrophysical object tem has such high velocities and deep po - that varies strongly on a timescale smaller tential wells.8 than the time it takes light to travel across Some quasars emit powerful jets of plas- the object: the separate parts of the object ma that extend for up to a megaparsec (see are not causally connected on this time- Figure 1),9 probably collimated and accel- scale, so they vary independently and their erated by magnetic ½elds near the black contributions tend to average out. This ar - hole that are twisted up by the rotation of gument suggests that the size of the most the surrounding accretion disk. The pro- rapidly varying quasars must be less than duction of these jets is not so remarkable: the distance light travels in a few weeks for example, various kinds of star also pro- (which is around a few hundredths of a par- duce jets, though on a much smaller scale. sec or a few thousand times the Earth-sun What is more striking is that quasar jets distance). This upper limit is consistent typically travel at close to the speed of light. with size estimates from a number of other Once again, there is no plausible way to methods, such as reverberation mapping, produce such high velocities except close photoionization models, and gravitational to the event horizon of a black hole. More- lensing.7 over, in most cases the jets are accurately A few hundredths of a parsec is large by straight, even though the innermost plas- our standards but extremely small on ga - ma in the jet was emitted a million years lac tic scales: a millionth of the size of the after the material at the far end. Thus, what- galaxy as a whole. A black hole of one hun - ever mechanism collimated the jet must dred million solar masses and its surround- maintain its alignment over several mil- ing accretion disk would ½t comfortably lion years; this is easy to do if the jets are in side this volume–its relativistic event squirted out along the polar axis of a spin- hor izon has a radius of about the Earth-sun ning black hole, but dif½cult or im pos sible distance–but almost all of the alternative in other quasar models. models that might explain quasars fail to Finally, there is strong evidence that a do so. handful of systems that emit strong X-ray

106 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/18/2014 2:48 PM Page 107

Figure 1 Scott An Image at Radio Wavelengths of Jets from the Quasar Cygnus A Tremaine

The bright spot at the center of the image is the quasar, which is located in a galaxy 240 megaparsecs away. The long, thin jets emanating from the quasar terminate in bright “hotspots” when they impact the intergalactic gas that surrounds the galaxy. The hotspots are roughly 70 kiloparsecs (or 228,000 light years) from the quasar. The brighter of the two jets is traveling toward us; its brightness has been boosted by relativistic effects. Source: National Radio Astronomy Observatory/Associated Universities Inc.; reproduced by permission of the American Astronomical Society. From R. A. Perley et al., “The Jet and Filaments in Cygnus A,” The Astrophysical Journal 285 (1984): L35–L38.

radiation consist of a normal star and a eral decades: if quasars are found in galax - black hole. The black holes are much less ies, and the number of quasars shining massive than those in quasars, only a few now is far smaller than when the universe times the mass of the sun.10 The black hole was young, and quasars are black-hole fur - and the normal star orbit one another at a naces, then many “normal” galaxies should small enough distance–a few stellar radii– still contain the massive black holes that that material lost from the normal star fu - used to power quasars at their centers, but els a miniature black-hole furnace. These are now dark. Can we therefore ½nd “dead sys tems reinforce our con½dence in the ex - quasars” in nearby galaxies? istence of black holes, and allow us to re - There are two important guideposts in ½ne our understanding of the complex phy- the search for dead quasars. The ½rst comes sics of a black-hole furnace. from a simple argument by the Polish astro nomer Andrzej Soltan.11 We know that Based on these and other arguments, the universe is homogeneous on large there is near-complete agreement among scales, and therefore on average the energy astrophysicists that the power source for density in quasar light must be the same quasars is the accretion of gas onto black everywhere in the universe (here average holes of one hundred million solar masses means averaged over scales greater than or more. Accepting this position leads to a about ten to twenty megaparsecs, which is simple syllogism that has driven much of still small compared to the overall “size” the research on this subject for the past sev - of the universe, at a few thousand mega-

143 (4) Fall 2014 107 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 108

The Odd parsecs). We can measure this energy den- ½nally, like the drunkard looking for his Couple: sity by adding up the contributions from all keys under the lamppost, we look for dead Quasars & Black Holes the quasars found in surveys (after straight - quasars at the centers of galaxies because forward corrections for incompleteness). that is where it is easiest to ½nd them: the If this energy were produced by black-hole search area is small and the density of stars furnaces with an ef½ciency of 10 percent, that are affected by the black hole’s grav- for example, then a mass M of material itational ½eld is high. accreted by black holes would produce Stars that come under the influence of 0.1Mc2 in quasar light. Similarly, if the av - the black hole’s gravitational ½eld–typi- erage mass density of dead quasars is ρ, cally those within a distance of a few tenths then the energy density of quasar light of a parsec to a few tens of parsecs, depend- must be 0.1ρc2. ing on the black hole’s mass–are acceler- Since we know the latter ½gure, we can ated to higher velocities; although indi- invert the calculation to determine the vidual stars cannot be detected in galaxies mass density of dead quasars. The power other than our own, this acceleration leads of this argument is that it requires no as - to increased Doppler shifts, which broaden sumptions about the masses or numbers the spectral lines from the collective stellar of black holes; no knowledge of when, population. This broadening can be detect- where, or how quasars formed; and no un- ed by spectroscopic observations with suf - derstanding of the physics of the quasar ½ciently high spatial resolution and signal- furnace except its ef½ciency. Soltan’s argu- to-noise ratios. The search for this effect ment tells us that the density of dead qua - in the centers of nearby galaxies began sars should be a few hundred thousand around 1980 and yielded evidence for black solar masses per cubic megaparsec, com- holes in a handful of cases. Strictly, the ev - pared to a density of large galaxies of about idence was for massive dark objects with one per hundred cubic megaparsecs. What masses of millions to billions of solar mas- it does not tell us is how common dead ses, since the angular resolution (the smal- qua sars are: on average there could be, for lest size of distinct object that the telescope example, one dead quasar of ten million can clearly image) of these observations so lar masses in every galaxy, or one of one was still far larger than the size of the event billion solar masses in 1 percent of galaxies. horizon of the putative black hole. These The second guidepost is that the centers results were tantalizing, but incomplete: of galaxies are the best places to prospect the problem was that the angular resolu- for dead quasars. There are several reasons tion of ground-based telescopes is limited for this. First, live quasars seem to be found by blurring caused by the atmosphere, so near the centers of their host galaxies (al - the effects of a black hole could be detected though this is dif½cult to tell with precision only in the closest galaxies, and then only because the glare from the quasar obscures over a limited range of distances from the the structure of the host). Second, the fuel center. Precisely this problem was one of supply for a black hole sitting at rest in the motivations for constructing the Hub - the center of the galaxy is likely to be much ble Space Telescope, which at the time of larger than the fuel supply for one orbiting its launch in 1990 had roughly ten times in the outskirts of the galaxy. Third, mas- the angular resolution of the best ground- sive black holes orbiting in a galaxy tend based telescopes. Since then the Hubble to lose orbital energy through gravitational Telescope has devoted many hundreds of interactions with passing stars, so they hours to the hunt for black holes at the spiral into the center of the galaxy.12 And centers of galaxies, and by now Hubble has

108 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 109

con½rmed and strengthened the ground- of the disk, and that its mass is 37.8 million Scott based detections in nearby galaxies and solar masses with a measurement uncer- Tremaine produced ½rm evidence for black holes in tainty of only 0.3 percent (possible system - over two dozen more distant ones.13 Even atic errors due to the choice of model are in the best cases, this method can only larger, with about a 1 percent margin of er - probe to a few tenths of a parsec from the ror). This is by far the best case we have for galaxy center, but we are persuaded that a massive dark object at the center of any the massive dark objects observed by Hub - distant galaxy.14 ble must be black holes because the alter- Finally, our own galaxy offers unique ev - natives (for example, a cluster of low-lumi- idence for a black hole.15 Very close to the nosity stars) are far less plausible. By now geometric center of the distribution of stars the Hubble Telescope has turned to other in the Milky Way is a compact source of tasks, but the search for dead quasars has strong radio emission known as Sagittarius been resumed by ground-based telescopes, A*. This region is dif½cult to study be cause now using adaptive optics systems that can small solid particles in interstellar space correct for atmospheric blurring in real (commonly called “dust” but really more time. Adaptive optics is beginning to pro- like haze or smoke) obscure visible light vide angular resolutions that equal or ex - coming from stars near the center. The ceed Hubble’s: these new observations smoke can be penetrated by infrared radi - also have far higher signal-to-noise ratios, a tion, and high-resolution observations at since the collecting area of the biggest these wavelengths reveal a handful of ground telescopes is ten times that of bright stars within a few hundredths of a Hubble. parsec from Sagittarius A*. The positions The painstaking measurement of stellar and velocities of these stars have been motions near the centers of galaxies has track ed, some for as long as two decades; been supplemented by an unexpected gift in particular, the star S2 has an orbital pe - from the heavens: the otherwise unre- riod of only 15.8 years and now has been mark able galaxy ngc 4258 contains at its tracked through more than one complete center a thin, nearly flat, rotating disk of orbit. Some four centuries ago, Johannes gas, about a tenth of a parsec in radius. The Kepler showed that the orbits of the plan- gas includes water vapor, and the temper- ets around the sun were ellipses; here the ature and density in the disk are right for orbit of S2 is also an ellipse (Figure 2). Us- the production of maser (microwave la - ing ½rst-year mechanics, we can deduce ser) emission in the water, stimulated by from this orbit that the star is orbiting a a weak active galactic nucleus at the center body that is located at the radio source of the disk. The maser emission consists of Sagittarius A*, that this body has a mass tiny, intensely bright sources of radiation of 4.3 million solar masses, with an un - concentrated in wavelength at the spectral certainty of less than 10 percent, and that line of water, and by measuring the Dopp- the size of this body is less than only one ler shift of these sources and their mo tion hundred times the Earth-sun distance, or a across the sky using an intercontinental few thousand times the radius of the event array of radio telescopes, we can map out horizon for a black hole of this mass. This the rotation of the disk with exquisite extreme concentration of mass is incom- precision. The disk is found to rotate patible with any known long-lived astro- around the active nucleus; from the disk physical system other than a black hole. kinematics, we can deduce that the nucle - The center of our galaxy thus offers the us is much smaller than the inner radius single best case for the existence of black

143 (4) Fall 2014 109 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 110

The Odd Figure 2 Couple: Orbits of Stars Near the Center of Our Galaxy Quasars & Black Holes

The radio source Sagittarius A*, believed to coincide with the black hole at the galaxy center, is at the zero point of the coordinates. The width of the frame is 0.03 parsecs or 6,700 times the Earth-sun distance. Each orbit is well- ½t by an ellipse with one focus at Sagittarius A* (the focus is not on the symmetry axis of the orbit because the normal to the orbit is inclined to the line of sight). The smallest orbit, called S2, has a period of 15.8 years, and its point of closest approach to Sagittarius A* is 120 times the Earth-sun distance. These parameters imply that Sagittarius A* is associated with a mass of 4.3 million solar masses contained within about 100 times the Earth-sun distance. Source: Reinhard Genzel, Frank Eisenhauer, and Stefan Gillessen, “The Galactic Center Massive Black Hole and Nuclear Star Cluster,” Reviews of Modern Physics 82 (4) (2010): 3121–3195.

holes and strongly suggests that the mas- about 0.2 percent of the mass of the stars sive dark objects found in the centers of in the galaxy. But are the black holes we are other galaxies are also black holes. ½nding in nearby galaxies really dead qua- What else have we learned from these sars? From galaxy surveys we can deter- dis coveries? First, black holes seem to be mine the average mass density in stars in present in most galaxies, except perhaps the local universe, and since black-hole for a class known as late-type galaxies. Sec- masses are typically 0.2 percent of the stel- ond, the mass of the black hole is strongly lar mass in a galaxy, we can estimate the correlated with the mass or luminosity of mass density of black holes in the local uni- the galaxy; roughly, the black-hole mass is verse. Soltan’s argument, described earlier,

110 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 111

gives the mass density of dead quasars in have a dramatic influence on the galaxy Scott the local universe from completely differ- for mation process. In an extreme case, the Tremaine ent data (surveys of distant quasars as op- quasar feedback could blow the gas out of posed to surveys of nearby galaxies). The the galaxy and thereby quench the forma - two estimates agree to within a factor of tion of new stars. Are black holes and qua- about two–well within the uncertainties– sars an interesting by-product of gal axy for - so there is little doubt that the black holes mation that has no influence on the for- we have found are indeed the ash from mation process, or do they play a central quasars or other active galactic nuclei. role in regulating it? More succinctly, do galaxies determine the properties of qua - This essay has described briefly what we sars or vice versa? have learned about the intimate relation The second profound question is one of be tween quasars, one of the most remark - physics rather than astronomy. All of the able components of the extragalactic uni- tests of Einstein’s theory so far–which it verse, and black holes, one of the most ex- has passed with flying colors–have been otic predictions of twentieth-century theo- con ducted in weak gravitational ½elds, retical physics. Many aspects of this rela- such as those on Earth or in the solar sys- tion remain poorly understood; to close, I tem. In contrast, we have no direct evi- will mention two of the most profound un - dence that the theory works in strong grav- answered questions. itational ½elds. Many naturally occurring The ½rst of these is the relation between processes near black holes in galaxy cen- black holes and galaxy formation. Although ters–tidal disruption of stars, swallowing black holes make up only a fraction of a of stars, accretion disks, and even black- percent of the mass of the stars in galaxies, hole mergers–may potentially be mea - the energy released in forming them is sured with the next generation of astro - hundreds of times larger than the energy nomical observatories. Can we understand released in forming the rest of the galaxy. these processes well enough to test the If even a small fraction of the energy emit - unique predictions of general relativity ted by the black-hole furnace is fed back for physics in strong gravitational ½elds, to the surrounding gas and stars, it would and will Einstein turn out to be right?

endnotes 1 Textbooks on general relativity include Bernard Schutz, A First Course in General Relativity, 2nd ed. (Cambridge: Cambridge University Press, 2009); James B. Hartle, Gravity: An Introduction to Einstein’s General Relativity (San Francisco: Addison-Wesley, 2003); Sean Carroll, Spacetime and Geometry (San Francisco: Addison-Wesley, 2004); and Robert M. Wald, General Relativity (Chi - cago: University of Chicago Press, 1984). Monographs on black holes include Subrahman yan Chandrasekhar, The Mathematical Theory of Black Holes (Oxford: Clarendon Press, 1992); and Valeri P. Frolov and Andrei Zelnikov, Introduction to Black Hole Physics (Oxford: Clarendon Press, 2011). A popular account closely related to the subject of this essay is Mitchell Begelman and Martin J. Rees, Gravity’s Fatal Attraction, 2nd ed. (Cambridge: Cambridge University Press, 2009). 2 Subrahmanyan Chandrasekhar, The Mathematical Theory of Black Holes (Oxford: Clarendon Press, 1992), prologue. 3 The physics of accretion disks is described in Marek A. Abramowicz and P. Chris Fragile, “Foun dations of Black Hole Accretion Disk Theory,” Living Reviews in Relativity 16 (2013): 1, arXiv:1104.5499; H. C. Spruit, “Accretion Disks” (May 2010), arXiv:1005.5279 (the essay is an expanded and revised version of “Accretion Theory,” a lecture Spruit delivered at the XXI

143 (4) Fall 2014 111 Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 112

The Odd Canary Islands Winter School of Astrophysics, Puerto de la Cruz, Tenerife, Spain, November Couple: 2–13, 2009); and Juhan Frank, Andrew King, and Derek Raine, Accretion Power in Astrophysics, Quasars & 3rd ed. (Cambridge: Cambridge University Press, 2002). For a review of magnetohydrody- Black Holes namic turbulence, energy dissipation, and angular momentum transport in accretion disks, see Steven A. Balbus and John F. Hawley, “Instability, Turbulence, and Enhanced Transport in Accretion Disks,” Reviews of Modern Physics 70 (1998): 1–53. 4 Introductory texts on galaxies include L. S. Sparke and J. S. Gallagher III, Galaxies in the Uni- verse: An Introduction, 2nd ed. (Cambridge: Cambridge University Press, 2007); and Peter Schneider, Extragalactic Astronomy and Cosmology (Berlin: Springer, 2006). At a more ad vanced level there is James Binney and Michael Merri½eld, Galactic Astronomy (Princeton, N.J.: Prince- ton University Press, 1998); James Binney and Scott Tremaine, Galactic Dynamics, 2nd ed. (Princeton, N.J.: Princeton University Press, 2008); and Houjun Mo, Frank van den Bosch, and Simon White, Galaxy Formation and Evolution (Cambridge: Cambridge University Press, 2010). 5 Julian H. Krolik, Active Galactic Nuclei: From the Central Black Hole to the Galactic Environment (Princeton, N.J.: Princeton University Press, 1999). 6 For the history of this discovery, see K. I. Kellermann, “The Discovery of Quasars,” Bulletin of the Astronomical Society of India 41 (2013): 1–17. An interview with Maarten Schmidt is available at http://oralhistories.library.caltech.edu/118/. A snapshot of the intense early debates about the nature of quasars can be found in George B. Field, Halton Arp, and John N. Bahcall, The Redshift Controversy (Reading, Mass.: W. A. Benjamin, 1973). 7 Most measurements constrain the size of the so-called broad-line region, in which the broad op tical emission lines of the quasar are produced. In black-hole models of quasars the broad- line region is much larger than the event horizon or accretion disk. Reverberation mapping is based on the average time delay between variations in the continuum luminosity, believed to arise from the accretion disk, and variations in the broad lines, which measure the light-travel time to the broad-line region. Photoionization models are based on an empirical correlation R ∝ L0.5 between the size of the broad-line region R revealed by reverberation mapping and the continuum luminosity L; this correlation is natural if the broad lines are in ionization equi - librium and brighter active galactic nuclei are simply scaled-up versions of fainter ones. For a review and references, see Yue Shen, “The Mass of Quasars,” Bulletin of the Astronomical So- ciety of India 41 (2013): 61–115. An alternative is to study quasars that are gravitationally lensed by an intervening galaxy; lensing by individual stars in the lens galaxy then leads to fluctuations in the brightness of the quasar image that depend on the ratio of the size of the broad- line region to the Einstein radius of the star. See E. Guerras et al., “Microlensing of Quasar Broad Emission Lines: Constraints on Broad Line Region Size,” The Astrophysical Journal 764 (2013): 160. 8 J. C. Miller, “Relativistic X-Ray Lines from the Inner Accretion Disks Around Black Holes,” An- nual Review of Astronomy and Astrophysics 45 (2007): 441–479. 9 M. Boettcher, D. E. Harris, and H. Krawczynski, eds., Relativistic Jets from Active Galactic Nuclei (Weinheim, Germany: Wiley-vch, 2012). 10 Ronald A. Remillard and Jeffrey E. McClintock, “X-Ray Properties of Black-Hole Binaries,” An- nual Review of Astronomy and Astrophysics 44 (2006): 49–92. 11 Andrzej Soltan, “Masses of Quasars,” Monthly Notices of the Royal Astronomical Society 200 (1982): 115–122. 12 This process, known as dynamical friction, is a manifestation of energy equipartition familiar from statistical mechanics. See James Binney and Scott Tremaine, Galactic Dynamics (Prince- ton, N.J.: Princeton University Press, 2008). 13 John Kormendy and Luis C. Ho, “Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies,” Annual Review of Astronomy and Astrophysics 51 (2013): 511–653; Kayhan Gültekin et al., “The M–σ and M–L Relations in Galactic Bulges, and Determinations of Their In trin sic Scatter,” The Astrophysical Journal 698 (2008): 198–221; and Nicholas J. McConnell and

112 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:06 AM Page 113

Chung-Pei Ma, “Revisiting the Scaling Relations of Black Hole Masses and Host Galaxy Scott Properties,” The Astrophysical Journal 764 (2013): 184. Tremaine 14 J. M. Moran, “The Black-Hole Accretion Disk in ngc 4258: One of Nature’s Most Beautiful Dynamical Systems,” in Frontiers of Astrophysics: A Celebration of NRAO’s 50th Anniversary, ed. A. H. Bridle, J. J. Condon, and G. C. Hunt (San Francisco: Astronomical Society of the Paci½c Conference Series, 2008): 395, 87; R. Herrnstein et al., “The Geometry of and Mass Accretion Rate Through the Maser Accretion Disk in ngc 4258,” The Astrophysical Journal 629 (2005): 719–738; and Jenny E. Greene et al., “Precise Black Hole Masses from Megamaser Disks: Black Hole-Bulge Relations at Low Mass,” The Astrophysical Journal 721 (2010): 26–45. 15 For a comprehensive review of the central parsec of our galaxy, see Reinhard Genzel, Frank Eisenhauer, and Stefan Gillessen, “The Galactic Center Massive Black Hole and Nuclear Star Cluster,” Reviews of Modern Physics 82 (4) (2010): 3121–3195.

143 (4) Fall 2014 113 Book_Fall 2014_Shinner.qxd 9/18/2014 2:49 PM Page 114

F The Formation & Evolution of Galaxies T

Pieter van Dokkum

Abstract: Weighing in at 1042 kilograms and measuring 1021 meters across, galaxies are perhaps the most awe-inspiring objects known to mankind. They are also the only places in an otherwise dark and unforgiving universe where stars and planets are able to form. In the past ½ve to ten years we have made enormous progress in understanding when galaxies came into being and how they changed and evolved over the course of cosmic time. For the ½rst time, we have a rudimentary idea of what our own Milky Way looked like in the distant past, and we can now simulate Milky Way–like galaxies inside powerful computers. As we are starting to understand what happened in our galaxy’s past, we are now turning to the question of why it happened. Untangling the complex physical processes that shape galaxies is extremely dif½cult, and will require continued advances in computers and information from powerful new telescopes coming online in the next decade.

If a forest is a collection of trees, and a city a collection of buildings, then the universe is a collection of galaxies. This is apparent when we look at the Ultra Deep Field, a remarkable image obtained with the Hubble Space Telescope that shows the faintest light humanity has yet detected (see Figure 1).1 The blackness of space is punctuated by little blobs of light, each comprising a gravitationally bound system of tens of billions of stars. Galaxies contain nearly all the stars and planets in the universe, play host to the supermassive black holes in their centers, and serve as signposts delineating the large-scale cosmic web of dark matter structure. How galaxies were formed is a central question in astronomy. And PIETER VAN DOKKUM is the because galaxies live at the intersection of the study Chair of the Department of Astron- of the structure of the universe as a whole and of omy at Yale University, and the Sol the properties of the dark matter, gas, stars, and plan- Goldman Professor of Astronomy ets within them, the question is interwoven with and of Physics. His research focus- many other ½elds of astronomy. Furthermore, un - es on observational studies of the derstanding galaxy formation also means under- formation and evolution of galaxies. His many publications include standing our own galaxy, the Milky Way, and there- articles in such journals as Nature, fore our own cosmic history. The Astrophysical Journal, and The As - This is a young ½eld of research. One hundred years tronomical Journal. ago, astronomers were trying to measure the extent

Figure 1 Pieter van The Hubble Ultra Deep Field Dokkum

t g d

The Hubble Ultra Deep Field is a patch of sky observed for many days with the Hubble Space Telescope. Every little white blob is a distant galaxy, typically containing tens of billions of stars. Although the image covers only a tiny part of the sky (more than ten million of such patches would be needed to cover the entire sky!) it contains thousands of distant galaxies. Source: National Aeronautics and Space Administration and esa/Hubble, http:// www.spacetelescope.org/.

of the Milky Way, and the question wheth- has been tremendous progress over the er the Milky Way makes up the entire uni- past decade, with some of the major ques- verse or other galaxies exist beyond its tions that astronomers were struggling boundaries was hotly debated. The cos- with now settled. We now have an idea of mological framework for understanding how galaxies came into being and how galaxy formation and evolution was de - they changed over time. It is an incom- veloped in the 1980s and 1990s,2 and the plete story, with glaring omissions, incon- ½rst large surveys of the distant universe sistencies, and unanswered questions; but were undertaken in the early 2000s. There a story nonetheless.

143 (4) Fall 2014 115 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 116

The Largely thanks to surveys such as the mass of a galaxy’s dark matter structure is F Formation Sloan Digital Sky Survey (see Michael typically ½ve to ten times greater than the A & Evolution of Galaxies Strauss’s article in this volume for more on mass of the rest of the galaxy, which means the Sloan survey) we now have a fairly com- that it is the dark matter that dictates the plete census of galaxies in the “nearby uni- processes driven by gravity. In many ways, verse”: a loosely de½ned sphere with a vol- dark matter controls galaxy evolution and ume of about a billion cubic light-years cen - the stars are just along for the ride. The tered on our own galaxy. The Sloan sur- nature of dark matter is still elusive be- vey has mapped about a million galaxies in cause we have not yet identi½ed a dark this sphere, and we can study their lumi- matter particle (if one even exists). Nev- nosities, colors, morphologies, star forma - ertheless, although we do not yet know tion rates, as well as other properties. what dark matter is, we have a very good Luminous galaxies show a remarkable idea of where it is. In fact, again using the regularity in the nearby universe and can Sloan survey, we can now statistically tie usefully be divided into two basic types. galaxies to their dark matter halos and of - Most stars live in large spiral galaxies (as fer a full description of the relation be- shown in Figure 2), which are character- tween dark matter and normal matter.4 ized by majestic rotating disks of young This description works remarkably well, stars with a dense central bulge of old stars. and it has been tested by studying the grav- Our sun resides in such a galaxy: a piece itational lensing of faint background gal - of knowledge that reinforces the notion axies by the dark matter. It turns out that that “we are not in a special location,” as galaxies are also very regular in terms of Copernicus ½rst put forth in 1543. Spiral the relationship between dark and conven- galaxies continuously form new stars in tional matter: when a galaxy’s mass in their arms. The hot, short-lived massive normal matter is known, the mass in dark stars that are formed in this process give matter can be predicted with an accuracy the disks their characteristic blue color. of about 40 percent. The other basic type of galaxy lacks spiral This regularity in galactic properties arms, is red, and has stopped forming new makes our task easier: we do not have to stars long ago (see Figure 3). Historically, decipher the formation history of every these galaxies are called early-type galaxies individual galaxy. But we do have to un- since it was once thought that they repre- derstand how the two basic galaxy types sent an early stage of galactic evolution; came into being and why they are so dis- and as often is the case, the name stuck tinct from one another. And once we un - though the interpretation evolved. One of derstand how typical spiral galaxies were the ma jor results of the Sloan Digital Sky formed, we can apply this lesson to the Survey is that these two basic galaxy types Milky Way and learn about our own cos- are well separated in many projections of mic past. the parameters of galaxy mass, kinematics, stellar age, color, luminosity, and galactic Our galaxy and the other galaxies in the environment.3 A galaxy is usually clearly local volume of the universe hold impor- either a spiral galaxy or an early-type gal- tant clues to their history. The great ma - axy: intermediate types are rare. jority of stars that have formed in the his- Signi½cantly, it has also become clear tory of the universe are still around today, that all galaxies are embedded in large and the present-day appearance of a gal - structures composed of dark matter, some- axy is the accumulation of all the things what confusingly termed “halos.” The that happened to it over the course of cos-

116 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 117

Figure 2 Pieter van A Spiral Galaxy Dokkum

Galaxies come in two basic types. This is a spiral galaxy, characterized by a spinning disk with its eponymous spiral arms. In these spiral arms hydrogen gas is continuously processed into new stars. The Milky Way is a spiral galaxy; the Orion nebula is a region of intense star formation in a spiral arm relatively close to us. Source: The Dragonfly Telephoto Array, http://dunlap.utoronto.ca/instrumentation/dragonfly/; Roberto Abraham and Pieter van Dokkum.

mic time. Just like a paleontologist recon- higher in the past. Studies of the ages of structs a dinosaur from bone fragments, the various components of the Milky Way we can use this “fossil evidence” of earlier and its neighbors suggest that the distinc- epochs to reconstruct what galaxies looked tion between spiral galaxies and early-type like in the past. galaxies, which is so characteristic of the From studies of the Milky Way and other distribution of galaxies, may be a transient nearby galaxies, it appears that the uni- phenomenon in the history of the universe is currently in a much more sedate verse. Spiral disks are relatively young and state than it was in the past. Most galax- may have arrived on the scene in their pres- ies form new stars at a relatively low pace, ent form only in the past ½ve to eight bil- and so to have built up the vast reservoirs lion years.5 of existing stars that we observe today Perhaps the most spectacular result of their formation rates must have been much studies of nearby galaxies lies hidden in

143 (4) Fall 2014 117 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 118

The Figure 3 Formation Early-Type Galaxies & Evolution of Galaxies

Basically smooth big balls of stars, early-type galaxies constitute the other basic type of galaxy. In these galaxies all the stars are old, and new star formation has not occurred for many billions of years. Source: National Aero- nautics and Space Administration and esa/Hubble, http://www.spacetelescope.org/.

their outskirts. Sensitive stellar mapping Galactic paleontology has fundamental programs of the Milky Way and its neigh- limitations, just as looking at dinosaur bor M31 (the Andromeda galaxy) have bones only gives us incomplete and frag- dem onstrated that they are embedded in mented information on living, breathing a vast network of stellar streams, the debris dinosaurs. Galactic collisions and mergers of previous encounters with other galax- erase much of the past history of a galaxy, ies.6 Such features had ½rst been seen making it dif½cult to discern how it was around some galaxies many decades ago, built up. Furthermore, other processes but it is now thought that all galaxies may such as bar instabilities and stellar migra- have vast debris ½elds around them, al- tion gradually change the appearance of though often just below the detection galaxies over time even if they do not ex - thresh old of present-day instrumentation. perience collisions and are instead left to These streams point toward a violent past their own devices. As a result, the key when interactions and collisions among build ing phases of today’s galaxies cannot gal axies were much more common than be deciphered from their present-day ap - they are today. pearance alone. Luckily, we are not limited

118 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 119

to our own neighborhood, and we can do ing time, the candels, 3d-hst, and Pieter van something that dinosaur-hunting pale- hudf09+12 projects are providing us with Dokkum ontologists can only dream of. samples of hundreds of thousands of gal - axies at cosmological distances, with well- Owing to the ½nite speed of light, we measured sizes, star formation rates, mass- can directly observe the past. Looking out es, and colors. These data allow us to create into space, we see the moon as it was a sec- snapshots of the universe at different ep - ond ago, the sun as it was eight minutes ochs and to study how the distribution of ago, and the Andromeda galaxy (the most galaxy properties has changed over time. distant object visible to the unaided eye) This measurement is essentially statistical as it was 2.5 million years ago. Two and a in nature: we cannot see the whole of the half million years is only 0.17 percent of Milky Way when we look out into space, the 14.8-billion-year-old universe, which but we can see galaxies that are like the is why we (again, somewhat loosely) con- Milky Way. We are also learning how to sider the local volume representative of the “connect the dots”; that is, to ½nd which present-day universe. galaxy populations at early epochs were an- Using large telescopes on Earth and in cestors of which galaxy populations today. space we are able to detect and study gal - axies well beyond the local volume, at dis- The Hubble observations, aided by stud - tances where the look-back time is a sig - ies at other wavelengths and with large ni½cant fraction of the age of the universe. telescopes on Earth, paint a picture of dra- Until a few decades ago, we could identi- matic change. Ten billion years ago gal - fy samples of galaxies at distances of about axies were two to four times smaller than seven billion light-years, allowing us to they are today, and yet the rate of star for- look back in time about half the age of the mation was ten times greater. Combining universe. In the mid-1990s, with the com- these results, the density of star formation bination of Hubble in space and the Keck (how many new stars are formed in a ½xed telescopes on Earth, we learned to take region of space within a galaxy) was up to sharp images of galaxies over 95 percent of one hundred times greater in these early- cosmic time, lifting a veil from the early universe galaxies than it is in galaxies to - uni verse. This frontier is continuously day.7 Not surprisingly, the early-universe pushed farther into the past, as new de - galaxies also look very different from gal- tector technology greatly expands the ca - axies today: they are bluer and have a more pabilities of existing telescopes. The ½nal irregular appearance. The grand spiral gal- space shuttle servicing mission of the axies with gently spinning thin disks that Hubble Space Telescope was of particular are now so ubiquitous were rare in the early importance, as the new instruments im- universe. proved the sensitivity of Hubble by fac- The high star formation rates of early- tors of ½ve to twenty. universe galaxies tell us that they built up One of the new cameras installed on rapidly–so rapidly that many doubled the Hubble during that 2009 servicing mis- their mass in less than a billion years. Elev - sion has been used over the past three years en billion years ago, the Milky Way was a to capture images for one of the largest- faint little blob with only 10 percent of its ever projects undertaken by the telescope, present-day stellar content but a very large which aims to determine how galaxies amount of gas: the fuel for star formation. were assembled. Using more than one Over the next three to four billion years it thousand two hundred hours of observ- proceeded to convert this gas into stars at

143 (4) Fall 2014 119 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 120

The a ferocious rate, adding about ten times the universe seemed to be experimenting Formation the mass of the sun every year. It then with galaxy shapes and sizes. & Evolution of Galaxies gradually quieted down, became red der as Our views of even earlier epochs are its stellar population aged, and settled in necessarily less complete, since the feeble its current spiral galaxy morphology about light of the galaxies comes from even ½ve billion years ago. Over this en tire pe - greater distances. Nevertheless, using the riod, Milky Way–like galaxies in creased deepest images of the night sky ever their mass by a factor of ten and grew in obtained (see again Figure 1) astronomers size by a factor of two.8 have identi½ed galaxies to within a few Interestingly, some galaxies did not par - hundred million years of the Big Bang and ticipate in the overall gas feeding frenzy in characterized their star formation rates, the young universe. Despite the availabil- sizes, and other properties.10 It now ap - ity of large amounts of fuel, about 50 per- pears that the average star formation rate cent of the most massive galaxies stopped in the universe increased rapidly in the forming new stars as early as ten billion ½rst billion years after the Big Bang, after years ago. This strange reluctance to form which it had a broad peak and then de- new stars was already suggested by the old clined. Interestingly, we can now study gal- ages of stars in present-day massive galax- axies at epochs when the hydrogen that ies, and it has now been con ½rmed by di - exists in the vast spaces between them was rect observations of massive “dead” an - still partially neutral; it is an open question cestor galaxies in the young universe made whether the ultraviolet radiation of the with the Hubble, Keck, and Gemini tele- hot, massive stars in these early galaxies scopes.9 The structure of these ancestors, were responsible for ionizing the universe. as revealed by the Hubble Space Telescope, did yield a surprise. It turns out these gal - As observations of the universe are pro - axies were extremely compact in the past, viding an increasingly detailed description much smaller than they are today. Re - of the properties of galaxies, the research markably, their sizes increased by a factor focus is not only on what happened but of four over the past ten billion years, also on why it happened. There is broad whereas their masses increased by only a consensus on the general outline of the factor of two. process of galaxy formation: gravity and Overall, the epoch around eight to ten the expansion of the universe rule the be - billion years ago was characterized by a havior of dark matter, and give rise to dark high degree of diversity. We see very com- matter objects with a distribution of pact “dead” galaxies, large and thick star- masses that roughly follows a power law forming disks, dust-enshrouded collisions, (wherein the mass of dark matter can be and many other galaxy types. This was also predicted approximately as a power of the the era when massive black holes at the mass of conventional matter). Gas initially cen ters of galaxies were growing rapidly: follows the distribution of dark matter but many galaxies show activity in their nuclei then cools and forms stars. The ef½ciency that cannot be explained by star formation of this process depends on the dark mat- and instead reveals the en ergetic processes ter mass, such that the ½nal distribution associated with black hole growth. This pe - of the stellar masses of galaxies is not a riod has been de scribed as “high noon,” power law, but has a preferred scale around the “heyday of galaxy formation,” or– the mass of the Milky Way. with a nod to our colleagues in the biolog- The details of these processes are ½end - ical sciences–the “cosmic Cambrian,” as ishly complex because the dark matter, gas,

120 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 121

and stars are all intertwined and continu- tle star-forming fragments assemble from Pieter van ously changing as structures grow. Futher - gas clouds in the ½rst billion years after the Dokkum more, the relevant physical processes hap- Big Bang, how these fragments grow and pen on an enormous range of scales, from merge with one another, how spinning the distances between galaxies to the cen- disks form from gas that either condens- tral regions of the birth clouds of individual es gradually out of the dark matter halo stars: a range that spans thirteen orders of or is injected by cold streams, how these magnitude. To put this challenge into con- disks are destroyed or puffed up in subse- text, it is equivalent to simultaneously un- quent collisions with other galaxies, and derstanding the processes that operate on how massive galaxies continue to grow by the scale of the Earth-moon system and consuming their little neighbors. processes that operate on the scale of the Several results stand out. Perhaps the width of a human hair. As if this were not most impressive is that we now have ar- dif½cult enough, the relevant time scales ti½cial galaxies living in computers that range from the billions of years of galaxy look a lot like normal spiral galaxies and interactions to the ten-thousand-year free- early-type galaxies in the actual universe fall timescale of protostellar clouds. (Figure 4).11 This is an outstanding achieve- Despite these seemingly insurmount- ment: until recently it was not possible to able challenges, in the past ½ve years there start a simulation shortly after the Big Bang have been some remarkable successes in and end up with anything that re motely modeling the process of galaxy formation. resembled the Milky Way. In terms of the These have mostly come from advanced relevant physical processes, a critical algorithms running on the world’s fastest breakthrough was the discovery that gas computers, a method that has its roots in can get into the central regions of dark the ½rst galaxy formation simulations done matter halos via two paths: it can be shock- in the late 1960s. The simulations treat the heated to the virial temperature of the halo dark matter and stars as collisionless par- followed by gradual cooling, and it can ticles, where a single “particle” usually flow directly to the center along a ½lament stands for some ten thousand actual stars. or stream.12 The simulations have also Gas is treated with hydro-dynamical tech- demonstrated the importance of mergers niques, which simulate the flow of gas and with small galaxies for the growth of mas - incorporate cooling and heating. The prob- sive early-type galaxies: the cores of mas- lem of the vast range of scales is amelio- sive galaxies form ½rst, and then their outer rated by adaptively changing the physical envelopes are added gradually by accre- scale in the simulation, using a coarse grid tion.13 Finally, the simulations consistently for the space in between galaxies and a very demonstrate the overwhelming impor- ½ne grid inside the star-forming complexes tance of feedback processes; that is, how of spiral galaxies. As even this ½ne grid can- much energy is returned to the interstel- not capture the formation of individual lar medium by newly formed stars and stars, analytical prescriptions are used for black holes. the “subgrid physics”; that is, the process- These accomplishments come with sev- es that happen on scales that are not re - eral crucial asterisks. Many of the key pro- solved by the simulation. cesses, in particular those relating to stel- The simulations provide us with movie lar and black hole feedback, take place on clips showing the formation of galaxies unresolved scales (the subgrid), which over the entire history of the universe, sped means they are essentially free parameters up by a factor of 1016. They show how lit- in the simulations. Furthermore, galaxies

143 (4) Fall 2014 121 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 122

The Figure 4 Formation Computer Simulation of the Milky Way Compared to the Actual Milky Way & Evolution of Galaxies

In the past few years, astronomers have managed to create arti½cial galaxies that look very much like real galaxies. Here, the computer simulated Milky Way sits atop a photographic image of the Milky Way. Source: Javiera Guedes, Simone Callegari, Piero Madau, and Lucio Mayer, “Forming Realistic Late-Type Spirals in a Λcdm Universe: The Eris Simulation,” The Astrophysical Journal 742 (76) (2011), arXiv:1103.6030.

in our computer simulations simply love axies. Finally, the simulations are typically forming stars: they are much more ef- tuned to match the appearance of galaxies ½cient at it than the real ones appear to be. in the present-day universe, and they may We can arti½cially lower the star formation not fare as well when compared to the new- ef½ciency by tuning the subgrid parame- ly available observational data on galaxies ters, but this is a far cry from understand- at earlier times. ing the physical processes involved. A possibly related issue is that it has proven dif- Over the past decade–and even in the ½cult to match all observations at once. For last ½ve years–we have made dramatic example, the simulations that successfully progress in our understanding of galaxy produce early-type galaxies have dif½culty formation and evolution. We now have a producing realistic Milky Way–like gal - broad idea of the processes that governed

122 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 123

the fourteen-billion-year-long home im - and as we gain a better understanding of Pieter van provement project that was the making the physical processes driving galaxy for- Dokkum of the Milky Way. At the same time, we mation through advances in other sub - are only at the beginning: we now have a ½elds of astronomy, we can expect fur- better idea of the problems we need to ther progress. The alma facility in Chile solve (such as the too-high star formation will provide us with detailed images of ef½ciency in model galaxies); but that is distant galaxies in the light of molecular quite different from actually having the so- gas, allowing us to directly connect the ex- lutions. Additionally, our information on isting stars to the fuel for new star forma- distant galaxies–while greatly improved tion. The James Web Space Telescope, the over the course of the past decade–is still successor to the Hubble Space Telescope, crude by most standards, since we typi- will open up the earliest epochs of galaxy cally only have a few characteristic num- formation for study and provide vastly bers to work with: galaxies’ luminosities, deeper and higher resolution views of dis- colors, sizes, and a rough measure of their tant galaxies than we have access to now. star formation rates. Perhaps most fun- New ground-based telescopes will explore damentally, this review did not touch on both the large scale distribution of galax- the biggest questions of all: until we pin ies and provide high resolution images and down the nature of dark energy and dark spectroscopy of small samples. Finally, the matter, we can hardly claim to understand gaia mission will provide a three-dimen- the formation of structure in the universe. sional map of about a billion stars in our As new observing facilities come online own Milky Way, allowing us to piece to - and computers and computer algorithms gether our own history via “galactic pale- continue to improve in the next decade, ontology” on a vast scale.

endnotes 1 Steven V. W. Beckwith et al., “The Hubble Ultra Deep Field,” The Astronomical Journal 132 (2006): 1729. 2 Simon D. M. White and Carlos S. Frenk, “Galaxy Formation through Hierarchical Clustering,” The Astrophysical Journal 379 (1991): 52. 3 Guinevere Kauffmann et al., “Stellar Masses and Star Formation Histories for 105 Galaxies from the Sloan Digital Sky Survey,” Monthly Notices of the Royal Astronomical Society 341 (2003): 33. 4 Charlie Conroy and Risa H. Wechsler, “Connecting Galaxies, Halos, and Star Formation Rates across Cosmic Time,” The Astrophysical Journal 696 (2009): 620. 5 Hans-Walter Rix and Jo Bovy, “The Milky Way’s Stellar Disk,” The Astronomy and Astrophysics Review 21 (1) (2013), arXiv:1301.3168. 6 Alan W. McConnachie et al., “The Remnants of Galaxy Formation from a Panoramic Survey of the Region around M31,” Nature 461 (2009): 66. 7 L. J. Tacconi et al., “High Molecular Gas Fractions in Normal Massive Star-Forming Galaxies in the Young Universe,” Nature 463 (2010): 781. 8 Pieter G. van Dokkum et al., “The Assembly of Milky Way–Like Galaxies since z~2.5,” The Astrophysical Journal Letters 771 (2013), arXiv:1304.2391. 9 Mariska Kriek et al., “Spectroscopic Identi½cation of Massive Galaxies at z~2.3 with Strongly Suppressed Star Formation,” The Astrophysical Journal 649 (2006): L71.

143 (4) Fall 2014 123 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 124

The 10 R. J. Bouwens et al., “Ultraviolet Luminosity Functions from 132 z~7 and z~8 Lyman-Break Formation Galaxies in the Ultra-Deep hudf-09 and Wide-Area Early Release Science wfc3/ir Obser- & Evolution vations,” The Astrophysical Journal 737 (2011): 90. of Galaxies 11 Javiera Guedes, Simone Callegari, Piero Madau, and Lucio Mayer, “Forming Realistic Late- Type Spirals in a Lambda-cdm Universe: The Eris Simulation,” The Astrophysical Journal 742 (2011): 76; and L. Oser, T. Naab, J. P. Ostriker, and P. H. Johansson, “The Cosmological Size and Velocity Dispersion Evolution of Massive Early-Type Galaxies,” The Astrophysical Journal 744 (2012): 63. 12 D. Keres, N. Katz, D. H. Weinberg, and R. Dave, “How Do Galaxies Get Their Gas?” Monthly Notices of the Royal Astronomical Society 363 (2005): 2. 13 Oser et al., “The Cosmological Size and Velocity Dispersion Evolution of Massive Early-Type Galaxies.”

124 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/18/2014 2:49 PM Page 125

Cosmology Today

David N. Spergel

Abstract: We seem to live in a simple but strange universe. Our basic cosmological model ½ts a host of astronomical observations with only ½ve basic parameters: the age of the universe, the density of atoms, the density of matter, the initial “lumpiness” of the universe, and a parameter that describes whether this lumpiness is more pronounced on smaller physical scales. Our observations of the cosmic microwave back - ground fluctuations determine these parameters with uncertainties of only 1 to 2 percent. The same model also provides an excellent ½t to the large-scale clustering of galaxies and gas, the properties of galaxy clusters, observations of gravitational lensing, and supernova-based measurements of the Hubble relation. This model implies that we live in a strange universe: atoms make up only 4 percent of the visible universe, dark matter makes up 24 percent, and dark energy–energy associated with empty space–makes up 72 percent.

Cosmology is a historical science. Because light travels at a ½nite speed, when we look out in space, we look back in time. We see the sun as it was eight minutes ago, and we see nearby stars as they were ½ve, ten, or a hundred years ago. It takes light ap - proximately 2.5 million years to travel from the An - dromeda galaxy to our eyes, so when we stare at our nearest major neighbor with a telescope, we ob - serve Andromeda as it was back before the dawn of man. The farther out we look, the farther back we look in time. When the Hubble Space Telescope ob - serves a distant galaxy, it sees the galaxy as it was perhaps 12 billion years ago. Our observations of the cosmic microwave background involve the oldest light, photons that formed only one year after the Big Bang DAVID N. SPERGEL, a Fellow of the and last interacted with atoms just four hundred American Academy since 2012, is thou sand years after the Big Bang. This light travels the Charles A. Young Professor of for 13.7 billion years before reaching us, and it brings Astronomy on the Class of 1897 us our universe’s baby picture. Foundation and Professor of As - Our basic model of cosmology rests on Einstein’s trophysical Sciences at Princeton nearly century-old theory of general relativity. As my University. He is a theoretical astrophysicist with interests ranging late academic great grandfather Johnny Wheeler from the search for planets around used to teach, “General Relativity consists of two nearby stars to the shape of the simple ideas: matter tells space how to curve and universe. space tells matter how to move.” On the scale of our

Cosmology solar system, the mass of the sun curves fant galaxy 13.3 billion years ago, it was Today space around it, and our Earth moves on a only 500 million years after the Big Bang. nearly circular orbit in this curved space. Today, we see this light as infrared radia- On the cosmological scales, the distribution and use our observations to study the tion of matter is nearly uniform. General properties of early galaxy formation. relativity implies that a nearly uniform uni - verse must be either expanding or con- General relativity relates the expansion tracting. Since Edwin Hubble’s observa- rate of the universe to the density and ge - tions in the 1920s, we have known that our ometry of the universe. If the energy in universe is expanding. expansion exceeds the self-gravity of the As the universe expands, the distance be- matter in the universe, then the universe tween galaxies grows. Today, it takes light is negatively curved and will expand for- roughly ½fty million years to travel to the ever, growing increasingly cold and emp - Virgo cluster. Eight billion years ago, the ty. On the other hand, if the energy in ex- distance between objects was a factor of pansion is less than the self-gravity of the two smaller, so light would have taken only universe’s matter, the expansion will slow twenty-½ve million years to travel from our down and reverse, and the universe will galaxy to the Virgo cluster. As we go farther collapse in a future big crunch. As Rob ert back in time, objects get closer and closer Frost prophesized, the universe will end in together. Thus, the early universe was either ½re or ice. much denser than today’s universe. Until recently, cosmologists thought that This expansion of the universe not only the expansion rate of the universe would increases the distance between galaxies, slowly decelerate. The expansion rate of but also stretches light. Light emitted from the universe is proportional to the square a distant galaxy is “redshifted.” If a gal - root of the density of the universe. Since axy eight billion years ago emits blue light, the density of matter decreases as the uni- the radiation’s wavelength is stretched as verse expands, astronomers assumed that it travels toward us, and we observe the the expansion rate of the universe has been light as red. Because astronomers can slowing with time. easily measure the wavelength of light de - But over the past thirty years, there has tected from a distant galaxy, we can de- been growing evidence that the expan- termine its redshift and then use general sion rate of the universe has been increas- relativity (and our cosmological model) ing with time.1 This result has shocked to relate the redshift of a galaxy to its age. physics: the equivalent of throwing a ball Today, our universe is 13.8 billion years upward and ½nding that gravity makes it old. When we observe a galaxy at redshift accelerate away from the point of release. 1, the wavelength of the light has been If general relativity is correct, this cosmic stretched by a factor of two due to the acceleration implies that most of the en- expansion of the universe. It took light 8 ergy in the universe is in the form of dark billion years to travel from the galaxy to us, energy: energy associated with empty so we observe this distant galaxy as it was space. In the late 1990s, measurements of 5.7 billion years after the Big Bang. Yellow the relationship between the distance and light emitted by the galaxy at redshift 1 at the redshift to supernova–powerful ex - a wavelength of 550 nanometers will ap- plosions of nearly uniform brightness pear to us in the infrared at 1100 nanome- that can be seen at very large distances– ters. The most distant known galaxy is at provided the strongest evidence for this redshift 10. When optical light left this in- strange phenomenon.2 Soon afterward,

126 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 127

measurements of the cosmic microwave During this period in the universe’s evo- David N. background fluctuations con½rmed this lution, most of the deuterium and helium Spergel surprising cosmology.3 in the universe was synthesized from neu - Dark energy is different from “dark mat - trons and protons. Our measurements of ter.” Ever since Fritz Zwicky’s work in the the abundance of these two cosmic fos- 1930s, astronomers have suspected that sils are a direct determination of the den- stars are not the dominant form of matter sity of the atoms at this early epoch. in galaxies. By the 1970s, astronomers had During the ½rst three hundred thou- assembled several independent lines of ar - sand years of cosmic history, almost all of gument all implying that dark matter was the atoms in the universe were ionized into neither gas nor stars. Dark matter appears a plasma of electrons, protons, and helium to be some new type of particle that has not ions. The cosmic background photons yet been found in our particle accelera- were frequently colliding with the elec- tors. Dark energy is even stranger: it does trons in this primordial plasma, so both not cluster in galaxies, nor does it seem to atomic matter and photons were coupled respond to any of the natural forces. Dark together in a single fluid. As the universe energy affects the universe only through cooled, the protons and helium ions were changing its expansion rate. able to combine with electrons and form neutral hydrogen and helium atoms. By The cosmic microwave background ra - four hundred thousand years after the Big diation is the oldest light in the universe, Bang, most of the electrons had combined the leftover heat from the Big Bang. This with ions, and the universe was mostly radiation ½lls all space and was once the neutral. Since the cosmic background pho- dominant form of energy in the universe. tons do not interact with these neutral gas- The expansion of the universe cools the es, they were able to propagate freely. The cosmic background radiation. Today, the photons that we observe when we look at temperature of the radiation is 2.73 de - the cosmic background radiation last in- grees K. When the distance between gal- teracted with atoms at this very early time. axies was half its present value, the tem- Thus, when we observe the background perature of the cosmic background radia- radiation, we are directly measuring phys- tion was twice its present value. When the ical conditions at this early moment in the distance between galaxies was a tenth its history of the universe. present value, the temperature of the cosmic background radiation was ten times In 1964, astronomers Arno Penzias and its present value. Robert Wilson detected the cosmic back- As we go farther back in time and closer ground radiation with their horn antenna to the moment of the start of the Big Bang at Bell Laboratories. Twenty-½ve years expansion, the universe is ever hotter. One later, the cobe satellite found that this second after the Big Bang, the tempera- nearly uniform microwave radiation had ture of the universe was 10 billion degrees exactly the spectral properties predicted C, and the universe was a nearly uniform by the hot Big Bang model. This measure- sea of electrons, protons, neutrons, dark ment of the cosmic background is one of mat ter, and radiation. At that time, most the foun dational observations for the hot of the energy density in the universe was in Big Bang model. the form of radiation. Three minutes While the cosmic microwave back- after the Big Bang, the temperature of the ground radiation is nearly uniform, there universe was about 500 million degrees C. are tiny variations in the temperature of

143 (4) Fall 2014 127 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 128

Cosmology the radiation. These variations are primar - to accurately predict the relationship be - Today ily due to the fluctuations in the density tween the statistical properties of the fluc- and temperature of the universe four hun- tuations and the conditions in the early dred thousand years after the Big Bang, universe. the period when the electrons and pro- Cosmologists quantify the properties tons combined to make hydrogen. cobe of these fluctuations by measuring their made the ½rst detection of the micro - statistical properties. These fluctuations Kelvin-level variations in the microwave have very simple statistical properties: background temperature.4 Subsequent they are spatially homogenous and can observations by ground-based and bal- be characterized almost entirely through loon-based radio antennas mapped this measurements of the point correlation back ground with ever improving tech- function of the data or, equivalently, the nologies. In 2003, nasa’s wmap released angular power spectrum. Figure 1 shows its ½rst detailed all-sky map of the fluctu- the measured angular power spectrum ations.5 In 2013, esa’s Planck satellite team from the Planck satellite. The x-axis on this provided an even more detailed map that plot shows the angular size of the fluctu- traces the same fluctuations.6 Observa- ations; the y-axis shows the amplitude of tions from wmap, Planck, and ground- the temperature fluctuations. based telescopes such as the Atacama Cos- We can use the measurements of the am- mology Telescope in Chile and the South plitude of the peaks in the temperature Pole Telescope in Antarctica give a remark - angular power spectrum to infer the basic ably consistent picture of the physical con- parameters of modern cosmology. The ra - ditions in the early universe.7 tio of the height of the ½rst peak to the sec- The few millionth-of-a-degree temper- ond peak depends on the density of atoms ature variations in the microwave back- in the early universe. The position of the ground radiation seen by these experi- peaks depends on the geometry of the uni- ments trace variations in the density and verse. The height of the third peak is sen- temperature of the early universe. Because sitive to the total density of matter. With the microwave background photons have our current observations of the cosmic been traveling to us with minimal inter- micro wave background, we can pin down actions with intervening matter since four all of the basic parameters to the precision hundred thousand years after the Big Bang, of one part in a hundred. these fluctuations reflect physical condi- We can use observations of the nearby tions at these early times. universe to infer the same parameters Four hundred thousand years after the through very different methods: Big Bang, the early universe was a simple • Measurements of the abundance of deu- place. Electron, protons, and photons were terium and helium 4 provide determina- bound together into a warm 3000 Kplasma. tions of the density of atoms accurate to Tiny variations in the density of the uni- 10 percent and consistent with the densi- verse generated sound waves in this plas- ty inferred from the height of the peaks ma. The distance that the sound waves in the microwave background angular could move in four hundred thousand power spectrum.8 years imparted a characteristic scale on the universe, and the self-gravity of the plasma • Measurements9 of the distances to near- and the dark matter determined the height by supernovae and Cepheid stars mea - of the peaks. Because these variations were sure the expansion rate of the universe small, cosmologists can use linear theory to be 74 km/s/Mpc, within 10 percent

128 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 129

Figure 1 David N. Amplitude of Temperature Fluctuations as a Function of Angular Scale Spergel

The curve is the best-½t model based on the data from the Wilkinson Microwave Anisotropy Probe and the Atacama Cosmology Telescope; the points and their error bars are computed from a combination of the publicly available Planck satellite 217 GHz and 545 GHz data. Source: David Spergel, Raphael Flauger, and Renee Hlozek, “Planck Data Reconsidered,” submitted to Journal of Cosmology and Astroparticle Physics (2013), arXiv:1312.3313.

of the value inferred from the cosmic mi- rameters consistent with the cosmic microwave background observations. Ce- crowave background observations and pheids are variable stars with a known require a universe ½lled with dark mat- relationship between their period and ter, dark energy, and atoms. In 2011, the their luminosity. Because the intrinsic lu- Nobel Prize for Physics committee rec- minosity of supernovae and Cepheids ognized the leaders of these observations are known, they are used as standard for their discovery of cosmic accelera- candles to measure distance. A mild dis - tion. In our simplest cosmological mod - crepancy between these two measure- els, dark energy is the driver of this cos- ments is a subject of active discussion at mic acceleration. cosmology meetings. • Measurements11 of the abundance of • Supernova observations can also be used rich clusters of galaxies provide an al - to trace the relationship between dis- ternative method of measuring the den- tance and expansion. These observa- sity of the matter and the amplitude of tions10 also yield the cosmological pa - the primordial fluctuations. These mea-

143 (4) Fall 2014 129 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 130

Cosmology surements again ½t our basic cosmologi- ½nely tuned to be nearly identical to more Today cal mod el with a consistent set of para - than twenty digits at the epoch of nucle- meters. osynthesis, the period of the universe three minutes after the Big Bang, when most of • The same sound waves that produce a the deuterium and helium in the universe characteristic scale in the microwave was synthesized. sky also produce a characteristic scale in The near, but not perfect, uniformity of galaxy clustering. Using the Sloan Dig- the early universe is another puzzle in Big ital Sky Survey, astronomers have now Bang cosmology. Different regions of space measured the positions of millions of that were never in causal contact in the Big galaxies. They can compute the statisti- Bang model have nearly identical densi- cal correlations of these galaxies and in - ties. The solution to the problem must ex - fer the density fluctuations in matter in plain this near, but not perfect, equality; the nearby universe.12 These measure- for if the early universe were perfectly uni- ments agree remarkably well with the form, it would still be uniform today. cosmic microwave background obser- The inflationary paradigm offers an an- vations. swer to these questions. It posits that the very early universe underwent an extreme- Despite the remarkable success of the ly rapid period of exponential expansion. Big Bang model in describing the evolu- Cosmologists call this very rapid period tion of the universe and the growth of fluc- of expansion “inflation.” This rapid ex - tuations, the model is incomplete. Intrigu - pansion stretched the universe to a very ingly, inflation–currently the most popu- large size. During this rapid period of ex- lar extension of the Big Bang model–not pansion, the kinetic energy of the universe only addresses these problems but also was driven to match the gravitational en - makes predictions that we can test with ergy, and this enormous stretching erased our cosmic microwave background obser - any initial fluctuations in the early uni- vations.13 verse. When the inflationary paradigm was The standard Big Bang model has a num- proposed thirty years ago, cosmologists ber of profound philosophical problems: recognized that the model not only solved it does not explain why our universe is so these Big Bang cosmology problems, but large, why the kinetic energy of our uni- also offered a mechanism to produce the verse nearly perfectly balances the gravi- fluctuations that would grow to form tational energy, or why the universe is galaxies. near ly (but not perfectly) uniform. Our During the inflationary expansion, tiny universe is more than 13.7 billion light years quantum mechanical fluctuations in den- across, yet the “characteristic” scale set sity were ampli½ed enormously. Some re - by general relativity and quantum me - gions of the universe had slightly higher chanics is the Planck length, only 10−36 densities while other regions of the uni- meters. Our nearly flat universe is at an verse had slightly lower densities. The re - un stable ½xed point. Today, the kinetic gions with slightly higher densities spent en ergy of the universe (the energy in the more time in the exponential expansion expanding galaxies) is within 1 percent of phase and exited inflation later. After the the gravitational energy of the universe. universe cooled and became dominated Since these two quantities tend to rapidly by matter, these denser regions grew and evolve away from each other in the stan- eventually collapsed to form galaxies, stars, dard cosmology, they would have to be and planets. Thus, the inflationary model

130 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 131

implies that the origin of all of the struc- the inflationary scenario: the fluctua- David N. ture in the universe was the tiny quantum tions are adiabatic, Gaussian, nearly scale- Spergel mechanical fluctuations ampli½ed during in variant, and coherent over scales that are the ½rst moments of the Big Bang. larger than the “horizon” scale (the dis- This remarkable explanation for the tance that light can travel). The statistical origin of structure is testable. The infla- properties of the millions of points in the tionary model is highly predictive about sky are described by two basic numbers, the statistical properties of these fluctua- an amplitude and a scale-dependence–a tions. This instability during the inflation- remarkable success for the inflationary ary phase led to a very speci½c prediction scenario.14 for the statistical properties of the varia- Another prediction of the inflationary tions in density: the fluctuations should be model is that the geometry of the universe “Gaussian random phase, adiabatic, nearly should be very close to flat. There should scale-invariant” fluctuations. Gaussian be nearly equal amounts of kinetic energy ran dom phase fluctuations have very sim - and gravitational energy. At the time that ple statistical properties and are described the inflationary universe was proposed, entirely by their two-point correlation most astronomers would argue that the function. Adiabatic fluctuations have the gravitational energy associated with the same ratio of photons, electrons and pro- known galaxies was too small to balance tons, and dark matter everywhere. Scale- the kinetic energy in expansion. In the invariant fluctuations have the same am - 1980s, most cosmologists would have ar- plitude on all scales. Thus, the inflation- gued that the observations implied that the ary model predicts that the statistical prop- ratio of the two, usually called Ω, was 0.2– erties of the temperature of the tens of mil- 0.3. Inflationary models predicted Ω = 1. lions of points in the Planck satellite maps While a number of theorists noted the pos- and the statistical properties of the posi- sibility that this discrepancy could be re - tions of millions of galaxies can be de - solved if the universe was ½lled with dark scribed by only two numbers: an ampli- energy, this possibility was considered ex - tude and a small deviation from scale in - otic, the last refuge of the scoundrels who variance. wanted to preserve the inflationary model. One of the predictions of the inflation- Today, the observational situation is very ary model is that there should be equal different. The wmap and Planck satellite numbers of hot and cold spots in the mi- observations imply that Ω = 1 to better crowave sky, and that the statistical prop- than 1 percent. When these cosmic micro- erties of hot spots and cold spots should wave observations are combined with ob - be identical. Analyses of both the wmap servations of large-scale structure, the cur- and Planck satellite data reveal no evi- rent best measurements imply that Ω = 1 dence for this symmetry. Quantifying to better than 0.1 percent, another remark- this through constraints on the three-point able success for the inflationary model. function, analyses show that the primor- Despite these many predictive successes, dial fluctuations are symmetric to better the inflationary model faces a number of than one part in a thousand. Any detec- theoretical challenges. The inflationary sce- tion of these features would have been a nario does not explain the origin of the uni- signi½cant challenge to the inflationary verse and requires special initial conditions. model. The statistical properties of these For inflation to match the observed large observations also show a remarkably size of the universe and the low amplitude strong agreement with the predictions of of initial fluctuations, the parameters in the

143 (4) Fall 2014 131 Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 132

Cosmology model must be ½ne-tuned. Since inflation the mechanism that drove this early pe- Today occurs in the ½rst moments of the Big Bang riod of inflation? and at energy scales far beyond those There are many different routes toward accessible in the laboratory, our ex - addressing these questions. Developments ploration of its physics stretches our basic in string theory and other attempts at uni - understanding of the underlying nature fying physics may provide new insights of matter, space, and time. into the nature of space and time. Future observations of the geometry of the uni- Over the past few decades, cosmologists verse, the statistics of the primordial fluc- have developed a remarkably successful tuations, as well as the gravitational waves cosmology model that ½ts a host of astro- predicted in the inflationary scenario will nomical observations. However, while this either con½rm this basic model or chal- model addresses many of the previously lenge its underlying tenets. Searches for unsolved questions of cosmology, it raises dark matter could reveal the nature of these a new set of questions: unknown particles. Astronomical mea- • Why is the universe accelerating? What surements of distances or the growth rate is the nature of dark energy? Are we see- of structure will test the notion that vac- ing the breakdown of gravity on cosmo- uum energy drives cosmic acceleration. logical scales? Of course, if we can address any of these questions, the answers will likely point to - • What is the nature of dark matter? ward even deeper and more profound mys- • Did the early universe also undergo a teries. period of acceleration? If so, what was

endnotes 1 P. James E. Peebles, “Tests of Cosmological Models Constrained by Inflation,” The Astrophysical Journal 284 (1984): 439–444; and Jeremiah P. Ostriker and Paul J. Steinhardt, “The Obser- vational Case for a Low-Density Universe with a Non-Zero Cosmological Constant,” Nature 377 (1995): 600–602. 2 Adam G. Riess et al., “Observational Evidence from Supernovae from an Accelerating Uni- verse and a Cosmological Constant,” The Astronomical Journal 116 (1998): 1009–1038; and Saul Perlmutter et al., “Measurements of Omega and Lambda from 42 High-Redshift Super- novae,” The Astrophysical Journal 517 (1999): 565–586. 3 David N. Spergel et al., “First-Year Wilkinson Microwave Anisotropy Probe (wmap) Obser- vations; Determination of Cosmological Parameters,” The Astrophysical Journal Supplement 148 (2003): 175–194. 4 George F. Smoot et al., “Structure in the cobe Differential Microwave Radiometer First Year Maps,” The Astrophysical Journal 396 (1992): L1–L5. 5 Charles L. Bennett et al., “First Year Wilkinson Microwave Anisotropy Probe (wmap) Observations: Preliminary Results and Basic Maps,” The Astrophysical Journal Supplement 148 (2003): 1–27. 6 Planck Collaboration; Peter A.R. Ade et al., “Planck 2013 Results, I. Overview of Products and Scienti½c Results,” accepted by Astronomy and Astrophysics, doi:10.1051/0004-6361/201321529, arXiv:1303.5062. 7 Erminia Calabrese et al., “Cosmological Parameters from Pre-Planck cmb Measurements,” submitted to Journal of Cosmology and Astroparticle Physics, arXiv:1302.1841.

132 Dædalus, the Journal ofthe American Academy of Arts & Sciences Book_Fall 2014_Shinner.qxd 9/8/2014 10:07 AM Page 133

8 Erik Aver, Keith A. Olive, and Evan D. Skillman, “An mcmc Determination of the Primordial David N. Helium Abundance,” Journal of Cosmology and Astroparticle Physics 4 (2012): 1–23; and Fabio Spergel Iocco et al., “Primordial Nucleosynthesis: From Precision Cosmology to Fundamental Physics,” Physics Reports 47 (2009): 1–76. 9 Adam G. Riess et al., “A 3% Solution: Determination of the Hubble Constant with the Hubble Space Telescope and Wide Field Camera 3,” The Astrophysical Journal 730 (119) (2011): 1–18. 10 A. Conley et al., “Supernova Constraints and Systematic Uncertainties from the First Three Years of the Supernova Legacy Survey,” The Astrophysical Journal Supplement 192 (2011): 1–29. 11 Alexei Vikhlinin et al., “Chandra Cluster Cosmology Project III: Cosmological Parameter Constraints,” The Astrophysical Journal 692 (2009): 1060–1074. 12 Beth A. Reid et al., “Cosmological Constraints from the Clustering of the Sloan Digital Sky Survey DR7 Luminous Red Galaxies,” Monthly Notices of the Royal Astronomical Society 404 (2011): 60–85; and Lauren Anderson et al., “The Clustering of Galaxies in the sdss-III Baryon Acoustic Oscillation Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Releases 10 and 11 Galaxy Samples,” Monthly Notices of the Royal Astronomical Society 441 (1) (2014): 24–62. 13 Alan H. Guth and Paul J. Steinhardt, “The Inflationary Universe,” Scienti½c American 250 (1984): 116–128; and Andrei Linde, “Particle Physics and Inflationary Cosmology,” Physics Today 40 (1987): 61–68. 14 Hiranya Peiris et al., “First-Year Wilkinson Microwave Anisotropy Probe (wmap) Obser- vations: Implications for Inflation,” The Astrophysical Journal Supplement 148 (2003): 213–231; and Planck Collaboration; Peter A.R. Ade et al., “Planck 2013 Results, XXIV. Constraints on Primordial Non-Gaussianity,” accepted by Astronomy and Astrophysics, doi:10.1051/0004-6361/ 201321554, http://planck.caltech.edu/pub/2013results/Planck_2013_results_24.pdf.

143 (4) Fall 2014 133 Book_Fall 2014_Shinner.qxd 9/8/2014 10:08 AM Page 134

Board of Directors Don M. Randel, Chair of the Board Jonathan F. Fanton, President Diane P. Wood, Chair of the Council; Vice Chair of the Board Alan M. Dachs, Chair of the Trust; Vice Chair of the Board Jerrold Meinwald, Secretary Carl H. Pforzheimer III, Treasurer Nancy C. Andrews David B. Frohnmayer Helene L. Kaplan Nannerl O. Keohane Roger B. Myerson Venkatesh Narayanamurti Pauline Yu Louis W. Cabot, Chair Emeritus

Inside back cover: Tycho’s Supernova Remnant (SN 1572, G120.1+01.4). A composite image of X-ray (yellow, green, blue), infrared (red), and optical light (white stars) displaying the remnants of a Type Ia supernova in the constellation Cassiopeia more than four centuries after explosion. The supernova remnant is named after Tycho Brahe, the Danish Renaissance astronomer who recorded the event in 1572. The image is 15.5 arcmin across. X-ray: nasa/cxc/sao; Infrared: nasa/ jpl-Caltech; Optical: mpia, Calar Alto, O.Krause et al. Composite image accessed at the Chandra X-Ray Observatory website, http:// chandra.harvard.edu/Photo/2009/Tycho. Cover_Fall 2014 9/8/2014 10:09 AM Page 2 Cover_Fall 2014 9/8/2014 10:09 AM Page 1 Dædalus coming up in Dædalus:

U.S. $13; www.amacad.org Cherishing Knowledge · Shaping the Future