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How Astronomers Measure the Cosmos

How Astronomers Measure the Cosmos

The big picture

Sound waves at the beginning of time may help scientists How find the ’s expansion rate today. by C. Renée James measure the ost astronomical distance determinations rely on cosmic standard candles — objects researchers have established maintain a known intrinsic brightness. But if you want a shot at understand- ing the of and how this mys- terious force is accelerating the expansion of our universe, you Mmust measure distances like a BOSS. To carry out the Oscillation Spectroscopic Survey (BOSS), you need dozens of observational and theoretical astrono- mers, programmers, and engineers. You need a dedicated telescope that allows you to obtain vast amounts of data spanning at least a quarter of the sky — and more is always better. You also need a large scope, one with the ability to peer deep into the universe to get the picture in three dimensions, pulling information about both distance and motion from the data. Only then will a standard ruler come into focus. It’s a ruler that formed with the universe more than 13 billion years ago and has been expanding with it ever since. This ruler can tell astronomers precisely what the cosmos was doing at every stage in its life since then. If you can figure that out, you might finally understand the mysterious forces driving the behavior of everything we see. And that understanding could very well hold the key to the next life- changing breakthrough in physics. Catching waves Drop a single pebble into a pond, and watch the ripples radiate out- ward. Then simultaneously drop a handful of pebbles into the pond, and take a snapshot of the ripples after a few seconds. Imag- ine them radiating outward not just on the pond’s surface, but as the surfaces of moving in every direction. Further imagine a series of ever-growing enlargements of this extra-dimensional snapshot, erasing the ripples and replacing them with dots that congregate on their edges. Finally, try imagining giving that image to a complete stranger, asking only that they divine the conditions of the original pond and a precise history of the subsequent enlargements. Welcome to the study of baryon acoustic oscillations (BAOs). “Baryon” is a catchall term for the majority of normal, everyday Waves — similar to those made by rocks falling into a pond — undulated through the early universe at nearly 60 percent the speed of . To find C. Renée James is a professor of at Sam Houston State evidence for those waves today, astronomers search for telltale features 490 million light-years in diameter that formed in the cosmos’ first 100 seconds, University in Huntsville, Texas, and author of Unshackled (Johns expanded, and interacted with other such ripples. ASTRONOMY: ROEN KELLY Hopkins University Press, 2014).

© 2015 Kalmbach Publishing Co. This material may not be reproduced in any WWW.ASTRONOMY.COM 45 form without permission from the publisher. www.Astronomy.com This pushback caused an acoustic (compression) wave that traveled What’s more, the scale imprinted on the CMB provides astron- EVOLUTION OF A BAO through the -baryon fluid at the speed of sound — but not omers with a long-awaited standard ruler. In today’s universe, that How a BAO moves through time the speed of sound as we measure it today on . Unlike every- last point of photon-baryon contact translates to a huge of 3.8 billion years ago 1 2 day sound waves that crawl through comparatively cold, empty air, material with a radius of some 150 megaparsecs (Mpc), or about early acoustic waves raced through the cosmos at nearly 60 percent 490 million light-years. the . 5.5 billion years ago Complicating the situation was the presence of dark — Setting the standard 13.8 billion the universe’s hypothetical invisible mass that, according to theory, “The early universe is a remote place to calibrate one’s ruler,” says years ago accounts for some phenomena astronomers observe. Immune to Eisenstein. But having such a standard can circumvent some of light’s push, was largely a spectator to the fight the problems that plague modern distance determinations. between the and . Slight variations in the universe Astronomers realized more than a century ago that some directed the dark matter into clumps. objects act as standard candles — phenomena of known Outweighing normal matter by a factor of six, dark matter’s intrinsic brightnesses like certain variable or particular gravity tried to coax the massive baryons toward the denser regions stellar explosions called type Ia supernovae. If scientists 3 4 of the cosmos, but the radiation pressure fought back. Light pushed observe the apparent brightness of a standard candle and the baryons outward from the central matter concentration like a know its absolute brightness, then it’s fairly straightfor- multidimensional snowplow. Gravity continued to pull the mate- ward to figure out how far away the object is. rial back to the central dark matter, and the process repeated itself, The problem with standard candles is that the far- wave after wave. ther away they are, the more likely it is that the inter- Then, around 380,000 years after the , something new vening universe has tainted their light. Maybe a type happened. The conditions in the rapidly expanding universe Ia supernova appears fainter because it’s more dis- allowed the first atoms to form as electrons and nuclei (composed tant. On the other hand, perhaps there’s simply more dust of and neutrons) came together in an event known as obscuring it, or perhaps there was something different about type The record of baryon acoustic oscillations in maps helps astrono- recombination, although most astronomers agree that it should Ia supernovae in the past. Uncertainties can add up. mers retrace the universe’s history. This illustration presents the cosmos simply be “combination.” A standard ruler, on the other hand, provides a robust way of at three different times. The false-color image on the right shows the This series of four illustrations shows the progression of a single Because neutral atoms are largely immune to light’s push and measuring things because it has a known length. If, for instance, cosmic microwave background, a record of what the universe looked baryon acoustic oscillation by studying just one such event over time. like just 380,000 years after the Big Bang, including an original baryon because there were fewer things to slam into, the photons were we knew that every galaxy had a diameter of 100,000 light-years, acoustic oscillation (white ring). As the universe expanded, evidence of 1) In all images, the blue center represents dark matter while the dark finally free, and for the most part have never been bothered by all we would have to do is look at the angular of any given gal- outer ring (it’s really a sphere, but we show it in two dimensions) repre- such an oscillation has remained visible as the larger white rings. E.M. HUFF/ THE SDSS-III TEAM/THE TEAM/GRAPHIC BY ZOSIA ROSTOMIAN sents the last point of contact between light and baryons. The red cir- matter since. In fact, this radiation — stretched to microwaves by axy, and we could figure out its distance. cle is a reference to show a standard distance from the center. 2) After the expansion of the universe — now fills the entire sky as the cos- BAOs are not that well behaved, though. When astronomers some time (millions of years), the outer shell, where matter () mic microwave background (CMB). Without the snowplows, how- look at the distribution of galaxies in the universe, it’s not clear has accumulated, has expanded much more than the inner portion. ever, a pileup of snow was left at a specific distance away from that which galaxies belong on which ripple. Furthermore, in the 13.8 generation. This reliable ruler helps astronomers precisely measure Different shells have different shapes. Add to that the fact that shells initial central dark matter concentration — the sound horizon. billion years of cosmic evolution, galaxies and dark matter have the expansion rate of the universe — the Hubble constant. interact with other shells over time, and you get an idea how difficult Since the late 1960s, cosmologists have known there should be a been jostling each other around gravitationally. The average sepa- Since the discovery in 1998 of the accelerating expansion of the they are for astronomers to detect. 3) This illustration, the same as #2, greater concentration of matter at the last point of contact between ration is still 150 Mpc, but individual distances might be anywhere universe, the Hubble constant has emerged as the key to under- adds the gravitational influence of material in the space around the sphere and why it’s no longer uniform. Longer arrows represent photons and baryons, a spherical shell at the sound horizon from a from 140 to 160 Mpc, smearing out the signature. standing the nature of dark energy, a component that scientists regions of greater gravity. 4) Several billion years have passed. The central condensation where dark matter tried to gather everything One of the great strengths of BAOs, however, is that the cosmic know little about despite it making up nearly three-quarters of the outer shell now has an even different shape because it has continued to itself. In some respects, it would look much like a snapshot of a sound waves propagated in every direction. Astronomers are universe. But the value of the Hubble constant has to be pre- to interact with other matter in the universe. N. PADMANABHAN, ET AL., MNRAS 2012 pond just after a pebble hit — a central uplift and a shell (instead of able to measure distances using the “side-to-side” cise. Knowing it to within a few percent of the right a ring in the pond) whose radius astronomers could determine by ruler (across our line of sight) to precisely link answer allows for too many theoretical possibili- knowing the time the wave had traveled and its speed, which cosmic distances to galactic speeds, and they ties. According to Wayne Hu of the University matter made up of protons and neutrons, but not electrons; those depended on the conditions of the fluid. can use the “front-to-back” ruler (toward of Chicago, “The Hubble constant is in the are a different class of particle called leptons. Because protons and In an ideal cosmos, there would be only a single pebble dropped and away from us) to glean information category of things we want to know to neutrons outweigh electrons by a factor of about 1,800, virtually all into the pond, creating a single expanding sphere. In the real uni- about subtle changes in the oscillations. When you hear within 1 percent.” the stuff you encounter is baryons. When you hear a bell ring, verse, though, things are much messier. The many “pebbles” cre- Making BAOs an even more attrac- you’re really detecting baryon acoustic oscillations — periodic ated a cacophony of interfering waves whose patterns imprinted on tive tool is that, even if the absolute a bell ring, you’re really A survey of surveys compressions in the air molecules. both the material in the universe and the CMB. scale proves to be in error, the geo- detecting baryon acoustic One of the first attempts at spotting Everyday sound waves are simultaneously vastly different from Finding the cosmic ruler — the scale of those oscillations — metric information alone tells a pre- the BAO signature was the Australian and incredibly similar to those that rippled through the hot, dense required investigators to be able to detect and map minuscule tem- cise history of cosmic expansion. oscillations — periodic 2-degree Field Galaxy Sur- medium of the early universe. Tamara Davis at the University of perature variations (ten-thousandths of a degree) and disentangle Sure, everything might have to be vey, which began in the late 1990s. It Queensland in Australia, who studies large-scale structures driven all the ripples. It wasn’t until 1992’s Cosmic Background Explorer scaled up or down, but the relative compressions in the air made an ambiguous, possible detec- by those primordial BAOs, explains, “The equations I used to results that astronomers saw a real chance of measuring the acous- picture wouldn’t change, a promise that molecules. tion of the oscillations. The Sloan Digi- describe waves on Coogee Beach [in Australia] are the same ones tic scale in the CMB. The 2001 Wilkinson Microwave Anisotropy standard candles can’t always make. tal Sky Survey (SDSS) at Apache Point that tell me how waves travel throughout the early universe; and Probe results plus observations from the 2009 mission pro- To extract a precise cosmic ruler from Observatory in New Mexico followed. the same equations I use to describe how pressure changes as I vided tighter constraints. the mess of overlapping ripples, though, SDSS kicked into action in 2000, its 2.5- pump up my bike tire also tell me how fast those early sound waves Now cosmologists can see what Daniel Eisenstein at the astronomers need to observe thousands of meter telescope carrying out a small number of should travel and what pattern they should have.” Harvard-Smithsonian Center for in Cambridge, Mas- points. Tens of thousands. Millions, even. large-scale projects that would require thousands Knowing that speed and that pattern is key. Back when the uni- sachusetts, calls “a beautiful pattern on the sky.” The CMB has hot- Or, as Eisenstein puts it, “All we need to do is go out of hours of telescope time encompassing huge swaths of verse was a dense cauldron of charged particles and light, matter ter and cooler patches with a characteristic size of about 1°, a scale and survey the universe in three dimensions.” But why? sky. Five years later, after looking for correlations in the separations interacted strongly with light. Any change in the distribution of that conveys a wealth of information about the conditions of that Once largely an observational and computational Everest chal- between 46,748 pairs of galaxies, astronomers announced that they matter would be met with a strong pushback from the photons. early photon-baryon fluid. lenge, BAOs are shaping up to be a key player in ’s next had indeed found galaxies on cosmic ripples of a preferred size.

46 ASTRONOMY • NOVEMBER 2014 WWW.ASTRONOMY.COM 47 About that time, Australian astronomers undertook the BOSS progress 6-degree Field Galaxy Survey. A similar survey called WiggleZ ran July 2012 from 2006 through 2011. Its goal was to use BAOs (hence the “Wiggle”) at vast distances, looking back at a time when the ruler was a bit smaller than it is today. But the precision required to constrain the Hubble constant was still lacking. Astronomers needed more data, especially at a variety of distances, so in 2008, SDSS progressed to the BOSS level. With upgrades to the CCD detectors and the installation of more collect- ing points, the survey obtained positions and spectra for up to 8,000 galaxies each night, year in and year out, for nearly six years. Along the way, it mapped a quarter of the sky. When all was said and done, hundreds of scientists from more than a dozen different countries were analyzing and modeling detailed information on 1.2 July 2013 million galaxies in a volume of 450 billion billion billion cubic light-years. They also obtained data on 140,000 — highly energetic and extremely distant centers of the earliest galaxies. The 1 percent Finally in January 2014, the BOSS team made the announcement

everyone had been waiting for: They had measured the distances to LABORATORY NATIONAL BERKELEY LAWRENCE ROSTOMIAN, ZOSIA BY ILLUSTRATION Astrophysicist Rich Kron of the University of Chicago and Fermi National Baryon acoustic oscillations show how galaxies and other matter tend to clus- galaxies nearly halfway across the to a preci- Accelerator Laboratory inserts optical fibers into a pre-drilled “plug-plate,” ter in spheres, which originated as waves in the early universe. Each sphere’s sion of 1 percent. Although the 1 percent precision in measuring part of the ’s spectrographic system that carried out radius (white line) is the scale of a “standard ruler,” which lets astronomers de- distances does not translate into the high precision sought in deter- the Baryon Oscillation Spectroscopic Survey. THOMAS NASH termine, with 1 percent accuracy, the structure and evolution of the cosmos. mining the Hubble constant, their observations seem to constrain July 2014 it to 67 kilometers per second per Mpc. In other words, two galax- ies separated by 1 Mpc are moving away from each other at a rate Or, perhaps, are the observations telling us something unex- requires huge, accurate samples,” says Chris Blake, the lead scien- of 67 km/s. That number is on the low side of the generally pected about our universe? tist of WiggleZ. “Basically in the future everyone will use the BOSS accepted range of 65 to 75 km/s/Mpc. model because anything else is just too hard. The scale of these As if that weren’t impressive enough, BOSS scientists followed The future of understanding projects now puts them beyond the reach of smaller groups, and it’s up just three months later with distances to intergalactic gas clouds cosmic history not possible to acquire enough time on open national facilities for using quasars as the backlighting. Although the distance uncer- The precise calibration of the BAO ruler hinges on researchers individual surveys, and so dedicated telescopes are required. The tainty here was a bit larger (2.2 percent), the data provide a much understanding how far the sound wave could travel in the early move is also toward large collaborations of hundreds of people, deeper look into the universe at an age of 2.8 billion years, when universe, and that distance depends on the ratio of most of whom never go to the telescope.” the expansion rate was a zippy 222 km/s/Mpc. pressure and matter. An intriguing possibility is With the support of the astronomical commu- These figures seem to represent a huge victory to astronomers. that there is an unknown source of radiation nity, several new BAO projects are already To Do Done As BOSS team member David Schlegel proudly stated during the pressure in the baryon-photon fluid. underway. BOSS will continue with SDSS The last three public data releases from the Baryon Oscillation Spectro- January announcement, “I now know the size of the universe better Astronomers have proposed a new class As BOSS team IV, eBOSS (Extending BOSS, from 2014 scopic Survey show how much of the planned sky coverage the project than I know the size of my house.” But there’s tension. of tiny, neutral, non-interacting par- to 2020). And slated to come online in observed. Astronomers targeted the two regions shown on these all-sky While BOSS values are in line with those implied by 2013 ticles called sterile neutrinos. member David Schlegel 2018 is the Dark Energy Spectroscopic maps. SLOAN DIGITAL SKY SURVEY Planck data, they are in disagreement with those from local stan- Scientists know of three active Instrument, which will utilize the dard candles, objects that astronomers are generally more confi- families of neutrinos that result from proudly stated, “I now 4-meter Mayall Telescope at Kitt Peak dent about. Moreover, in early 2014, the long-awaited BICEP2 various nuclear events. A new class of know the size of the National Observatory in Arizona. results, which reported on the behavior of specific features of the them would open the door to the pos- Other projects in the works CMB, seemed to favor a value of 74 km/s/Mpc. This figure lies sibility of exciting new physics. universe better than include the Wide-Field Multi-Object beyond the uncertainties that BOSS scientists reported. Inserting these new neutrinos into Spectrograph, the Australian Square Myriad observational projects are currently going on with the the mix is, if nothing else, consistent I know the size Kilometre Array Pathfinder, the Trans- hopes of easing the tension, each one largely convinced that it has with the BICEP2 data. If they turn out of my house.” forming Astronomical Imaging surveys addressed the sources of uncertainty. At the conclusion of a recent to be more than a guess, the scale of the through Polychromatic Analysis of Nebu- Cosmic Distance Scale Workshop, astrophysicist Mario Livio of the BAO ruler would change, and the Hubble lae (an optical survey of the nearby uni- Space Telescope Science Institute in Baltimore posed the simple constant astronomers derived from the BAOs verse), and the European Space Agency’s question, “Are observers becoming somewhat overconfident?” would edge into agreement with values found by Euclid mission. By observing hundreds of mil- other studies. lions of objects in a huge range of , astrono- But other equally fascinating options exist, assuming the mers hope to better connect the dots, constraining models of the recent observations and computations are correct, and, as Livio history of the universe and maybe, just maybe, opening the door to The Sloan Foundation’s 2.5-meter telescope exclaimed at the Cosmic Distance Scale Workshop, “All of them new physics along the way. is the dedicated instrument of the Sloan are incredibly interesting!” Getting to the bottom of the discrepan- Looking to the future, Eisenstein muses, “We have only Digital Sky Survey (SDSS) at the Apache cies contained in the data is an enormous undertaking, though. scratched the surface of what is possible with the study of large- Point Observatory near Sunspot, New Mexico. Its unique design allows it to focus “Extracting the subtle signatures of the physics of the universe scale structure.” on a patch of sky that covers as much area SEE AN ANIMATION OF OSCILLATIONS IN THE EARLY UNIVERSE AT www.Astronomy.com/toc. as 30 Full . SLOAN DIGITAL SKY SURVEY

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