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Special section | cosmic questions, answers pending Cosmic questions, answers pending Throughout human history, great missions of mission reveal the exploration have been inspired by curiosity, : the desire to find out about unknown realms. secrets of the Such missions have taken explorers across wide oceans and far below their surfaces, THE OBJECTIVE deep into jungles, high onto mountain peaks For millennia, people have turned to the heavens in search of clues to nature’s mysteries. Truth seekers from ages past to the present day and over vast stretches of ice to the Earth’s have found that the Earth is not the center of the universe, that polar extremities. countless galaxies dot the abyss of space, that an unknown form of Today’s greatest exploratory mission is no matter and dark forces are at work in shaping the cosmos. Yet despite longer Earthbound. It’s the scientific quest to these heroic efforts, big cosmological questions remain unresolved: explain the cosmos, to answer the grandest What happened before the ?...... Page 22 questions about the universe as a whole. What is the universe made of?...... Page 24 What is the identity, for example, of the Is there a theory of everything?...... Page 26 Are space and time fundamental?...... Page 28 “dark” ingredients in the cosmic recipe, com- What is the fate of the universe?...... Page 30 posing 95 percent of the universe’s content? And just what, if anything, occurred more than Find tools for the mission on Page 32. For pdfs of this section, and more resources, visit www.sciencenews.org/cosmicquestions 13.7 billion years ago, when the universe acces- sible to astronomical observation was born? THE WHEREABOUTS Will physicists ever succeed in devising a theory to encompass all the forces and particles of nature in one neat mathematical package Manhattan (and in so doing, perhaps, help answer some of these other questions)? Will that package include the supposedly basic notions of space and time, or will such presumed preexisting ele- 4 km across 1 x 104 km ments of reality turn out to be mere illusions emerging from ur-material of impenetrable obscurity? And finally (fittingly), what about cos- mic finality?W ill the universe end in a bang, a whimper or the cosmic equivalent of a Bruce Willis movie (everything getting blown apart)? 6 x 1020 km In the pages that follow, Science News writ- Understanding the universe requires recognizing ers assess the state of the evidence on these its immense scale. Zooming out from Manhattan momentous issues. In none of these arenas reveals the Earth, solar system, galaxies and then are the results yet firm. But as string theorist walls of galaxies separated by voids. At the most distant scales, the universe looks uniform. Brian Greene wrote in his book The Elegant Universe, “sometimes attaining the deepest familiarity with a question is our best substi- tute for actually having the answer.” — Tom Siegfried, Editor in Chief

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THE VITAL STATISTICS PAST MISSION FINDINGS 13.75 billion years (uncertainty +/–0.11): Time since the Big Bang, 1543 publishes a mathemati- the creation of the universe. cal description of planetary motion, assuming that the sun is the center of the solar system. Later 377,730 years (+3,205/–3,200): Time after the Big Bang when work by Johannes Kepler, and Isaac photons stopped interacting with charged matter and produced the Newton provides further evidence. relic radiation known as the cosmic microwave background. 1666 formulates the law of 70.4 kilometers/second/megaparsec (+1.3/–1.4): Expansion rate and laws of motion, published in 1687. of the universe assuming its spacetime geometry is flat. Also known urt/ssc, as the Hubble constant. 1900 Max formulates the first description h ; r.

of quantum theory, which will eventually explain the É b 90 billion light-years: Rough diameter of the known universe. nature of matter and energy on the subatomic scale. .com t. du k by –0.980 (+/–0.053): Equation of state, a measure of the (negative) 1917 applies to pted pressure exerted by divided by its density. An unvarying value the universe. Later work by and a d a utterstoc of –1 suggests that dark energy is Einstein’s . independently by Aleksandr Friedmann implies the h /s a e/ssc, possibility that the universe is expanding. ri 1.0023 (+0.0056/–0.0054): Value of omega, the total mass-energy y m Pyl G . T

density relative to the critical mass-. Omega equal to 1 ,

1924 announces that the “spiral a tor s k signifies a universe with flat spatial geometry. nebulae” sit beyond the Milky Way and later that the a /n h Milky Way is just one of many galaxies. et: Vi tec abl Cal L- er; t Hubble finds that the universe is expanding, P

1929 ll u after analyzing the of distant galaxies. f ger a

1933 Fritz Zwicky examines galaxies in the Coma e r ll A, usgs, dmsp; J

cluster and determines that there is unseen mass, S A N /

what scientists call “.” C er; nico F ll u GS f 1960s Steven Weinberg, Abdus Salam and ger 1 x 109 km 1 x 1018 km Sheldon Glashow independently propose a theory to a e r

unify electromagnetism and the weak nuclear force. ll

1964 Arno Penzias and Robert Wilson discover the ert simmon, modis/ ers; nico b h

cosmic microwave background radiation; in 1990 rc a nd ro a

NASA’s COBE mission confirms that the radiation’s i kl

properties verify the universe’s birth in a Big Bang. c Ö oto rese Ph

1986 Astronomers Margaret Geller, John Huchra er/ k ; reto st r y and Valérie de Lapparent map a section of the a tor id p va

, revealing a structure that av ser t: d

encompasses large walls and giant voids. f b e o l h rt

1998 Researchers discover that the universe is rom f Ea A

expanding at an accelerating rate, suggesting a mys- S 22 23 A 1 x 10 km 4 x 10 km N , terious force dubbed “dark energy” might be at work. ottom, b ; a USGS s a t: f /n e l h tec rom al f -c l op, p j T

www.sciencenews.org April 23, 2011 | science news | 21 special section | cosmic questions, answers pending

If the universe occupies a sheetlike membrane, the Big Bang may have been just one in a series of collisions, each “” refreshing the cosmos.

The modern-day notion of the cos- mos’s tumultuous beginning — known as the Big Bang — has its roots in Edwin Hubble’s 1929 discovery that the uni- verse is expanding. At the time, scientists envisioned the universe explosively fly- ing outward from a single point in space and time. Though this simple version of the Big Bang idea can’t fully explain what people see in the cosmos today, of MIT added a new ingredient in 1981. Early in its history, the universe underwent a brief period of faster-than-light expansion, known as , he proposed. In the years since Guth’s suggestion, inflation has been wildly successful in explain- ing the structure of the universe and its arrangement of galaxies.

Bubbling over Some scientists think that if inflation happened once, it could happen many more times — hinting at a cosmos alive and well eons before the Big Bang. Rapid expansion, in these interpretations, isn’t confined to just one neck of the cosmic woods, like a single expanding balloon. Instead, distant patches of space keep inflating, like a child continually blow- what happened before the big bang? ing soap bubbles, says Alex Vilenkin of Tufts University in Medford, Mass. Every inflated patch becomes a sepa- rate universe, with its own Big Bang beginning (SN: 6/7/08, p. 22). In this Pre-Bang branes “” scenario, the fireball that begot the universe seen with today’s telescopes was preceded by a multitude of and bubbles others just as surely as it will be followed by many more, each popping off at differ- By Ron Cowen s Illustration by Nicolle Rager Fuller ent times in different parts of the cosmos, Vilenkin says. osmologists Paul Steinhardt and 13.7 billion years too late to know what Just as the sun is merely one of billions Neil Turok liken the early his- happened. of stars in the Milky Way galaxy, the vis- Ctory of the universe to a play in But that hasn’t stopped Steinhardt, ible universe may be one of countless in which the protagonists — matter and Turok and other researchers from pon- the cosmic firmament. Cosmologists call radiation — move across the stage accord- dering whether the universe was born in this ensemble of the . ing to the laws of physics. Astronomers a giant fireball around that time or might Not only might there have been a are actors who arrived on the scene have existed before that. plethora of universes that came before

22 | science news | April 23, 2011 www.sciencenews.org

E. feliciano www.sciencenews.org small distance away. Another branesitsa surface, a “brane.” occupies asheetlike the known universe In thecyclicmodel, series ofcontractionsandbounces. cyclesthrougha that theuniverse Another model(below)suggests (right). out ofalargermultiverse nario, bubbled the known universe inflationsce - Intheeternal theorizing. Bang doesn’t stopphysicists from Not knowingwhatcamebeforetheBig In itself theory String Bounce been tooweaktoformgalaxies. have could universe earlier another, in block with galaxies, for example, gravity Although the known universe is chocka properties. physical of set own its have would universe inflated each 26), (see Page nature in forces unify known four help the could it because popular become has that idea an theory, string with inflation eternal combining In ers. oth the from different been have also may one each know,but people one the sae ih ay oldu dimen sions rolled-up many with space a of a sheet, called a brane, which can can which brane, a called sheet, a of version three-dimensional a on resides time since the last “Big Bounce.” and themerely bounces;is13.7billion years contractions of cycle endless an with replaced is Bang Big the Canada, Waterloo, in Institute Perimeter the of director now Turok, and University, Princeton at now Steinhardt, by oped develmodel a picture.Inpre–BigBang source: p. steinhardt p. source: In this picture, the known universe universe known the picture, this In the

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Big Bang. Big a like looks what in energy releasing and then rebound, T in the previous cycle. Though the details tence to the composition of the universe exis their owe present now structures stars,large-scale galaxies,the and of All retains the memory of what came before. it Bang, Big the before existed only not more. or years lion tril a last may cycle Each bounces. and for together another round of collisions back pulled are branes two the tually, even But today. is it as just expanding is cosmos the out; smooth and stretch Bang ( this Big Bounce would look just like a Big tion. To an observer on one of the branes, radia andmatter convertedto is which enormousan amountenergy, of someof unleash they apart, bounce and collide seeds of galaxies. the When are the branes wrinkles finally These wrinkles. tiny As the two branes pull closer, they develop senting a universe nearly devoid of matter. branesareperfectly wrinkle-free, repre brane residesatiny distance away. travel along an extra dimension. Another he branes collide collide branes he In the Big Bounce model, the universe they separated, are branes the While two the separated, are they When SN: 9/22/01, p. 184

and the cycle repeats. matter spreads out The form. structures and othercosmic galaxies separate, Once thebranes ). - - - - - not know.’ I have answered, ‘I do not know what I do answer facetious: “More willingly would for thosewhoprytoodeep.” the man replied: “He was preparing Hell Earth, and heaven making before doing an of was God what asked When Confessions. words the unnamed man quoted in St. Augustine’s consider pur gatory, like bit a sounds genesis cosmic wait amillenniumtofindthepattern. infrequent that observers might have to verses may still exist. Bumps could be so multiverse is another story, but Planck Planck but story, another is multiverse The Big Bounce would fall flat. Steinhardt. says done,” “we’re found, is ground back ( microwave cosmic the in leave tale signs that gravitational waves would tell the for searching now is spacecraft the Big Bang. background,theradiation left over from microwave cosmic the on imprint able waves produce a much more powerful set of the be detected. Inflation, in contrast, would scenario, the waves would be too weak to ButBounce. Big the inof thiscycle each fabric of spacetime, are generated during notes. the in ripples tiny waves, Gravitational Steinhardt scenario, tionary infla any and Bounce Big the between distinguish could though, observation, reality captures verse multi the or Bounce Big the Whether Cosmic os t all at does cal lawswouldremainthesame. might be different, the underlying physi verse happened to have struck the one one the struck have to happened verse may offercluesaboutthattoo. in which people reside, Planck might be collide with each other. If another uni- be spotted, though, other bubble uni bubble other though, spotted, be St. Augustine, himself, found the the found himself, Augustine, St. about uncertainty prolonged that If hte o nt nlto ipis a implies inflation not or Whether The EuropeanAgency’sSpaceThePlanck able to detect a particular patternof particular detecta to able As bubble universes expand, they can background. hot and cold spots in the microwave April 23, 2011 Even if no sign of a collision can can collision a of sign no if Even

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special section | cosmic questions, answers pending

Without an as-yet-unidentified material called dark matter, clusters of galaxies wouldn’t hold together.

what is the universe made of?

scientists have little insight into where dark matter and dark energy come from. In the dark But figuring out dark matter would illu- minate what holds galaxies together. By Alexandra Witze s Illustration by Nicolle Rager Fuller Figuring out dark energy might help reveal the universe’s ultimate fate (see n ancient times, listing the ingre- that we see around us is just the tip Page 30). dients of the universe was simple: of the iceberg,” says Joshua Frieman, an It’s little wonder that scientists regard Iearth, air, fire and water. Today, astrophysicist at the University of Chi- the identities of dark matter and dark scientists know that naming all of that, cago and the Fermi National Accelerator energy as among today’s biggest astro- plus everything else familiar in every- Laboratory in Batavia, Ill. nomical puzzles. day life, leaves out 95 percent of the The rest is, quite literally, dark. Nearly cosmos’s contents. one-quarter of the universe’s composi- A different matter From the that make up an tion is as-yet-unidentified material called Dark matter made its debut in 1933, astronomer, to the glass and steel of a dark matter. The remaining 70 percent when Swiss astronomer Fritz Zwicky telescope, to the hot plasma of the stars or so is a mysterious entity — known as measured the velocities of galaxies in a above — all ordinary stuff accounts for dark energy — that pervades all of space, group known as the Coma cluster and less than 5 percent of the mass and pushing it apart at an ever-faster rate. found them moving at different rates energy in the universe. “All the visible “Dark” is an appropriate adjective, as than expected. Some unseen and large

24 | science news | April 23, 2011 www.sciencenews.org

amount of “dunkle Materie,” he pro- Mostly unfamiliar The stuff that makes tant galaxies were flying away from each posed in German, must exist, exerting up people, planets, stars and interstellar gas other. The universe, Hubble showed, was accounts for just under 5 percent of the uni- its gravitational effects on the galaxies verse. The rest is made of mysterious entities expanding. It had been zooming outward within the cluster. dubbed dark matter and dark energy. ever since the Big Bang gave birth to it. Astronomer confirmed Mass-energy content of the universe Einstein happily ditched his cosmo- dark matter’s existence in the 1970s, 4.6% ? logical constant, but in 1998 astronomers after she and colleagues had measured showed that it should have been recy-

the speeds of stars rotating around the Dark energy cled rather than trashed. That year, two 23% centers of dozens of galaxies. She found Dark matter research teams reported their studies that, counterintuitively, stars on the gal- (non-atomic) of distant . These exploded Atoms axies’ outer fringes moved just as rapidly 72% stars can be calibrated to serve like stan- Neutrinos* as those closer in — as if Pluto orbited the *neutrino mass dard light bulbs, shining with a particular not precisely sun as quickly as Mercury. Rubin’s work known brightness. The scientists reported that demonstrated that each galaxy must be source: WMAP many distant super­novas were dimmer embedded in some much larger gravita- than expected, even accounting for an tional scaffold. experiments is now improving to the expanding universe. It was as if someone Ever since, other lines of evidence point that WIMPs and other candidate had quickly moved the light bulbs into a have strengthened the case for dark particles should be either spotted or more distant room. The universe was not matter. It resembles ordinary matter ruled out in the near future. only getting bigger — it was doing so at an in that it interacts via the well-under- accelerating rate. stood gravitational force; that’s why it Mysterious forces Something funny was going on, giving affected Zwicky’s and Rubin’s galaxies. Spotting dark matter may prove to be the cosmos a repulsive push. So Michael But scientists know that dark matter is easier than understanding dark energy, Turner, a cosmologist at the University of not ordinary; if it were, it would have whose mysteries make scientists feel like Chicago, dubbed the thing “funny energy” affected ratios of chemical elements mental wimps. at first, before settling on “dark energy.” born in the early universe and thus Albert Einstein unknowingly ush- More than a decade later, scientists thrown off the abundances of such ele- ered dark energy onto the stage in 1917, still don’t have a concrete clue to what ments observed today. while modifying his new equations of dark energy is (SN: 2/2/08, p. 74). The- The leading candidate for a dark general relativity. Einstein wondered orists have done their best to explain matter particle is the vaguely named why gravity didn’t make the universe it, putting forward ideas including a “weakly interacting massive particle,” or contract in on itself, like a balloon with seething “vacuum energy” created as rizona A

f WIMP. Such particles would be “weakly the air sucked out of it. He thus made particles pop in and out of existence, and et al, interacting” because they rarely affect up a “cosmological constant,” a fixed “quintessence” — named after Aristotle’s niv. o lowe U C

. ordinary matter, and “massive” because amount of energy in the vacuum of space postulated fifth element — that changes D , they must exceed the mass of most that would provide an outward push to its strength depending on its place or agellan/ NASA / M

I known particles, possibly weighing in at counter gravity’s inward pull. time in the universe. , c as much as 1,000 times the mass of the In 1929, though, Edwin Hubble solved Meanwhile, observers have spent STS et al proton. But nobody has yet definitively Einstein’s problem by reporting that dis- the last decade dreaming up ways to lowe ptical: C detected a WIMP, despite decades of probe dark energy from O . , D A , I F experiments designed to spot one. the ground and in space F C , W Results from dark matter experiments (see Page 32). In particular, ESO NASA , / are mixed: One group in Italy claims to precision measurements C X C NASA , see a WIMP signal seasonally, with more of many distant galaxies / I c

et al WIMPs hitting detectors as the Earth could help pin down the STS moves into a stream of galactic dark mat- nature and distribution of ap: M ter debris, and fewer when Earth moves dark energy. A new camera, arkevitch

M away. But other researchers haven’t been optimistically called the . ensing L M able to confirm those results. Recent Dark Energy Survey, will reports from other experiments, includ- see first light this autumn rizona, A

f ing one buried in Minnesota’s Soudan at the Cerro Tololo Inter- mine, hint that WIMPs might be lighter American Observatory in niv. o ; bottom, X-ray: U É than theorists had expected, on the order In this false-color image of galaxies colliding, the Chile. Real light — insight of 10 proton masses (SN: 8/28/10, p. 22). majority of the mass (blue) is separate from most into the dark — may take agellan/ M top: t. dub The sensitivity of many long-running normal matter (pink), direct evidence of dark matter. some time. s

www.sciencenews.org April 23, 2011 | science news | 25 special section | cosmic questions, answers pending

is there a theory of everything?

for short — ties all of physics into one neat package by reducing the bewilder- Strung together ing taxonomy of particles in the current bestiary of physics, the Standard Model, By Matt Crenson s Illustration by Nicolle Rager Fuller to identical snippets of string, each less than a billionth of a billionth of a bil- hysics is really two sciences. The clues are expected to come from lionth of a centimeter long. According There’s quantum mechanics, the the Large Hadron Collider, a ring of to string theory, the particles that carry Pweird tumultuous world where superconducting magnets in the Alps the three forces included in the Standard particles pop into and out of nothingness designed to smash protons together at Model — the photon (electromagnetism), and cats can be simultaneously living energies never before seen on Earth. the gluon (strong force) and the W and and dead. And there’s general relativ- The collider began operating in March Z bosons (weak force) — are all just the ity, Einstein’s majestic vision of massive 2010 and is expected to reach full power same tiny dancers each following their objects bending space and time. in 2014, when it will attempt to smash own distinct rhythms. Ever since these two very different its protons together with double the vio- And unlike the Standard Model, string views of the universe emerged early in lence it does today. theory has room for gravity. the 20th century, generations of physi- Even then, the LHC will be far from Though proposals besides string cists have tried to unite them in a single powerful enough to re-create the sin- theory attempt to explain how all the theory that would ideally describe all gle, unified force that physicists believe forces of nature might fit together, most four of nature’s basic forces to boot. Even existed for a fraction of a second after the of those other theories come with major Einstein tried, and failed. Now, after Big Bang — you’d need a collider as big as flaws. Some predict the existence of par- an especially frustrating few decades the universe itself for that. But the LHC ticles that can’t exist, for example. with little new evidence to guide them, might be able to test some of the predic- String theory’s primary drawback is today’s physicists may be about to get tions made by the leading theory that that it requires there be much more to some tantalizing hints about how the joins gravity and the other forces. the universe than physicists can probe, forces fit together. — string theory making the theory very difficult to test.

26 | science news | April 23, 2011 www.sciencenews.org

Superstring theory attempts to unify ing,” Randall says. “But this is one of the gravity with quantum mechanics by things we could find, and this is one of describing particles and forces as tiny the things they should be looking for.” vibrating strands and loops. Most physicists think it’s more likely that the LHC will find evidence for super- For string theory to say anything symmetric partners of the particles in about how the forces arise, physicists the Standard Model. Which partners have to figure out how all those extra appear, and their properties, would put dimensions roll up, or “compactify,” into some helpful constraints on how the the four familiar ones. universe compactifies the 11 dimensions String theory also conjures up a predicted by string theory. shadow population of partner particles For example, if the lightest super- for all of the ones currently known to particle turned out to be the wino, the exist — a notion called supersymmetry. superpartner of the weak force–carrying In fact, supersymmetry may be neces- W boson, that would be consistent with sary to join the strong, weak and electro­ a version of string theory known by the magnetic forces, so it is important even pithy moniker “M-theory compactified

if string theory isn’t correct. on 7-D manifold of G2 holonomy.” Such supersymmetric particles may When forces collide already have been observed, in fact — not Many physicists have high hopes that the on Earth, but in space. Some of the dark LHC will find evidence for both super- matter that is thought to make up more symmetric particles and extra spatial than 80 percent of the matter in the dimensions. universe could be composed of super- “Even if we don’t go out to the other symmetric particles left over from the dimensions, in some sense the other universe’s earliest moments (see Page dimensions can come to us,” says Harvard 24). In the last few years two space-based physicist Lisa Randall. instruments, the Fermi Gamma-ray For example, most versions of string Working in the 1990s with colleague Telescope and the Italian-led PAMELA theory require that the universe have Raman Sundrum, now at the University mission, have seen evidence of dark 10 or 11 dimensions — nine or 10 of space of Maryland in College Park, Randall matter in the Milky Way in the form of and one of time, rather than the four showed that it might be possible to detect gamma rays and antimatter that could that people experience: up-down, front- the decay of a gravity-carrying particle have been produced by supersymmetric­ back, left-right and past-future. coming from an extra dimension. Find- particles colliding. “The forces are unified in 11 dimen- ing such a particle at the LHC would both Because the LHC and any future col- sions, but they split apart when you go verify the existence of extra dimensions liders can carry physicists only so far to four dimensions,” says Gordon Kane, and suggest why gravity is much weaker back toward the moment just after the a physicist at the University of Michigan than the other three forces. Big Bang, science’s understanding of a in Ann Arbor. “I think it would be somewhat surpris- unified theory is ultimately going to have to come from exploring the vastness of Back to one Strength of the four forces back in time the universe. Some physicists wonder One of the enduring if such a strategy, which relies on find- puzzles in physics is Strong nuclear force why gravity — which ing and interpreting clues left behind by guides matter on the Electromagnetism Unied nature, can produce results comparable force scale of planets and orce to the high-precision experimental data galaxies — is so much that led to the Standard Model during intrinsically weaker Weak nuclear force than the other three the 20th century. forces. In the moments But string theory is not 20th century just after the Big Bang, science — in fact, string theorist Edward some researchers Witten has described it as “21st century think, the forces may Relative strength of f Gravity have been united as Temp. (kelvins) physics that fell accidentally into the one, separating into 1015 1029 1032 20th century.” Now that the 21st century forces with differing (sec) has arrived, it’s string theory’s time to be strengths as tempera- 10 –12 10 –35 10 –43 tures decreased. Recent era Distant past put to the test. s www.sciencenews.org April 23, 2011 | science news | 27 special section | cosmic questions, answers pending

A 2-D projection contains all the details needed to map a 3-D . Some physicists think space and time may emerge via a similar correspondence.

built from more primordial ingredients, so far unperceived. Newton simply declared space and time as absolute and constant, providing a convenient arena for the operation of his laws of motion and gravity. Einstein saw, and showed, that space and time actually shift shape or speed as events unfold; mass and motion warp space and alter the flow of time. Coping with these inconveniences required a merger, space and time becoming space­time. From that merger emerged a bonus: a model for the evolu­ tion of the cosmos, from an initial speck of matter and energy to a gigantic bal­ looning multi­galactic network. Nowadays, though, spacetime’s abil­ ity to accommodate nature’s phenomena has begun to fade as physicists push their probes to the limits of distance and dura­ tion. Below a certain very tiny distance, the dimension of length can no longer be explored, or even defined. Time faces a similar limit when durations approach the very brief. Today’s leading theories for answer­ ing the greatest cosmic questions suggest that neither time nor space appear in reality’s ultimate recipe. Somewhere between the stove and the table, space and time emerge, cooked up out of equations underlying an exis­ are space and time fundamental? tence without rulers and clocks. At least that is “the widespread current belief,” says physicist Joe Polchinski of the Kavli Institute for Theoretical Physics at the Out of the fabric University of California, Santa Barbara. By Tom Siegfried s Illustration by Nicolle Rager Fuller Space as society To illustrate this, Fotini Markopoulou of f all the mysteries of life and the achieved them: Euclid (who cataloged the Perimeter Institute for Theoretical universe, none resist the sleuth­ the insights preceding him), Galileo, Physics in Waterloo, Canada, compares Oing of science’s best private eyes Newton, Einstein. Yet each advance space to society. Space, like society, has more obstinately than the ultimate left deeper questions unanswered. And features that can be described — geom­ nature of space and time. now the 21st century’s best brains still etry textbooks catalog space’s proper­ Every several centuries or so, pro­ cannot say for sure whether space and ties and their implications. But space found insights do occur, immortalizing time are fundamental building blocks as reflected in geometry need not have the names of the investigators who of natural existence, or are themselves been present at the beginning. It could

28 | science news | April 23, 2011 www.sciencenews.org

have emerged from the interactions of becomes impossible to probe space to beginning, but that beginning would matter and forces, just as society mate­ any arbitrarily short distance, Seiberg be stillborn without time to get reality rializes from interactions among people. observes. That’s another way of saying going. “We have capitalist societies, agricul­ that at distances less than some (very Seiberg, though, believes time and tural societies, totalitarian societies,” short) length, the idea of space becomes space will both go down the cosmic drain Markopoulou wrote in a 2008 paper meaningless. together. (arXiv.org/abs/0909.1861). Nobody is Further study of spaceless theories “My personal prejudice is that these confused by phrases such as “our society may help solve serious problems con­ objections and questions are not obsta­ is addicted to credit.” But that doesn’t fronting physicists today, Seiberg cles to emergent time,” Seiberg writes. mean society is a fundamental feature believes. String theory implies count­ “Instead, they should be viewed as of existence. less possible vacuum states — that is, challenges and perhaps even clues to “A society does not exist independent spaces of differing physical proper­ the answers.” of its members,” Markopoulou pointed ties — with no obvious method for More intriguingly, he observes, space out. “We can see space­time geometry determining which one the visible uni­ and time’s ultimate status in nature may as the analog to society, with the role verse should have chosen. Knowing have something to say about the prac­ of individuals played by matter and its how space emerges from spacelessness tice of science. Much of modern science dynamics.” might help explain why humans exist is based on the concept of reduction­ As Polchinski notes, specifying space­ in one particular space from among the ism — explaining large-scale phenomena time’s status in relation to matter is part countless possibilities. from laws operating at smaller scales. of the quest for a theory of quantum Doing away with time poses more dif­ That notion will eventually break down gravity — the math that would unify ficult problems, Seiberg acknowledges. if there’s a smallest scale below which Einstein’s relativity theory, which Basic notions in physics, such as that of space no longer exists. describes space­time in bulk, with the causes preceding effects, or predicting “Therefore, once we understand how quantum physics that governs the micro­ the outcome of experiments before the spacetime emerges, we could still look world (see Page 26). A key clue in that experiment is done, seem to lose their for more basic fundamental laws, but quest is a correspondence between the meaning if there is no time to define these laws will not operate at shorter surface of a black hole, a gravitational before and after. So some physicists, distances,” he writes. “This follows bottomless pit from which can Markopoulou for one, have suggested from the simple fact that the notion of escape, and the space within it. It turns that even if space is emergent, time may ‘shorter distances’ will no longer make out that a mathematical description of remain fundamental. In fact, she conjec­ sense. This might mean the end of stan­ the black hole’s outer boundary (the tures, time is needed to allow quantum dard reductionism.” And the beginning point of no return for objects falling in) processes to create the illusion of space. of a new view of not only space and time, contains all the information needed to Space may not have been around at the but of science itself. s specify the three-dimensional interior. In essence, that means the 3-D space inside somehow emerges from the phys­ As small as it gets ics of the 2-D surface. Current theories are unable to describe space and time below certain limits defined by what are called “natural units.”T hese units, proposed by the Ger- Time materialized man physicist Max Planck, are derived from fundamental quantities such as the Generalizing the peculiarities of black speed of light. A theory uniting quantum mechanics with gravity will be needed holes to ordinary space and time remains to reveal whether space and time are meaningful concepts at smaller scales. a research challenge for quantum grav­ Planck length: 1.616 x 10–35 meters ity physicists. But most agree that sooner or later space and time will have The Planck length is derived from Newton’s gravitational constant, the speed of light and Planck’s own constant from quantum theory. It is unfathomably small: Comparing to go. String theory — the most-studied its size to a bacterium is like comparing the size of a bacterium to the visible universe. approach to quantum gravity — offers Many physicists believe that at shorter lengths space cannot be probed and the con- several examples of how space, rather cept of distance becomes no longer meaningful. than being fundamental, emerges into –44 existence, as physicist Nathan Seiberg Planck time: 5.391 x 10 seconds of the Institute for Advanced Study in The Planck time is also calculated from the gravitational constant, the speed of light Princeton, N.J., outlined in a 2006 paper and Planck’s constant in such a way that moving at one Planck length per one Planck time would be equal to the speed of light. Current theories are unable to describe the (arXiv.org/abs/hep-th/0601234). universe at an age younger than the Planck time; physicists hope that a theory of quan- If matter at its most basic is made of tum gravity could illuminate that epoch. tiny vibrating strings, for instance, it www.sciencenews.org April 23, 2011 | science news | 29 Special section | cosmic questions, answers pending

what is the fate of the universe?

analyzing the data, both teams reported in 1998 that the universe’s expansion Hanging isn’t just cruising along — it is accelerat- ing. Some mysterious force, now known as dark energy (see Page 24), is driving in the balance space apart, faster and faster. By Elizabeth Quill s Illustration by Nicolle Rager Fuller A dark twist Before dark energy’s discovery, the he fate of the universe was sup- leftover champagne went flat. More than forecast was surprisingly simple. If the posed to be sealed by the turn of a decade later, scientists are still vigor- gravitational pull of all the matter in Tthe millennium. ously debating what their finding means the cosmos was strong enough to rein “I imagined we’d be walking around not only for the universe’s future, but in expansion — like the Earth’s pull on holding a sign saying ‘the world is com- also for all of . a rocket that can’t quite reach escape ing to an end’ or ‘the world is not coming Perlmutter, of the University of Cali- velocity — the universe would eventually to an end,’ ” recalls astrophysicist Saul fornia, Berkeley, led one of two teams come crashing in on itself. That ending, Perlmutter. that set out in the early 1990s to get a dubbed the , would mirror the But as Y2K soothsayers readied for grip on the far future by studying distant Big Bang that started the cosmic expan- impending doom, Perlmutter and his supernovas. These stellar explosions sion in the first place. If, though, the colleagues delivered a surprising dis- serve as distance markers to help astron- universe’s expansion escaped the claws covery suggesting that the world’s omers measure how fast the universe is of gravity, it would go on growing forever. fate would stay in limbo long after expanding — a key factor in determining Expansion would slow but never halt, the Times Square ball dropped and any if and when it will meet its end. But after and instead of ending, the universe would

30 | science news | April 23, 2011 www.sciencenews.org

In one end-time scenario, the entire Figuring out whether the universe of sound waves in the early universe. A universe — from galaxies down to would end with this , or a Freeze proposed mission named Euclid, from atoms — would rip apart at its seams. or Crunch, requires determining a prop- the European Space Agency, and a cam- erty of dark energy called its equation era mounted on a telescope in the Andes become a cold, dark, lonely place where of state. That quantity is the ratio of the will further the efforts. life could not survive — a Big Freeze. pressure exerted by the dark energy to its But dark energy gives the fleeing density. The most recent findings, based Beyond the end rocket some extra oomph, making end- on data that come from seven years of But others say that a theoretical break- time quite a bit fuzzier. mapping the glowing radiation left over through is necessary. Measuring the “A crucial issue is how the dark energy from the Big Bang, suggest that the equa- equation of state with enough precision, will behave in time,” says cosmologist tion of state is close to that expected for they argue, is impossible; a tiny devia- Rocky Kolb of the University of Chicago. the cosmological constant, deviating by tion could always linger. “Until we have some way to grapple with no more than 14 percent. “We don’t just want to measure a that, the fate of the universe hangs in the But over billions of years, even a much number,” Kolb says. “We want to under- balance.” tinier deviation — undetectable with stand how this crucial piece of physics If the strength of dark energy’s extra current instruments — could dramati- fits into the overall fabric of the theory push remains forever unchanging, it cally alter the universe’s fate, especially of nature. And until we do that, I am not could be the cosmological constant — a if the dark energy’s strength is not con- going to be comfortable with any expla- term Albert Einstein added to his equa- stant but can change over time. nation of dark energy.” tions for general relativity in 1917 and “The million dollar question is an Kolb thinks no current proposal later dismissed as his “biggest blunder.” experimental question,” Tegmark says. adequately explains dark energy, thus In this case, something like the Big Scientists need better measurements to no proposal decides among a Freeze, Freeze would play out. But if dark ener- determine whether dark energy’s equa- Crunch or Rip scenario. gy’s strength decays over time, then a Big tion of state is perfectly constant. Of course, the right theory might even Crunch of sorts remains an option. So some experimentalists are turn- predict that the universe meets its doom If instead dark energy grows stron- ing back to those very same stellar by some other, unknown means. One such ger, exceeding the repulsive force of explosions that revealed dark energy’s possibility presents itself if the observ- Einstein’s cosmological constant, a existence to begin with. A paper by able universe is just one of many bubble more painful scenario awaits: “In a Perlmutter and collaborators, appear- universes constantly being created and finite amount of time, dark energy gets ing in 2009 in the Astrophysical Journal, growing in some larger space. In this infinitely dense,” says cosmologist Max describes an ongoing effort to com- “multiverse” scenario, bubble universes Tegmark of MIT. “First denser than our pile the world’s datasets. can collide. If another bubble encroached galaxy, and our galaxy flies apart. Then Many scientists have their hopes set on on the bubble that people occupy, it would denser than Earth, and that flies apart. a future observatory, WFIRST, which be bad news, says Anthony Aguirre of the Then denser than atoms, and atoms would look for the signal of dark energy University of California, Santa Cruz. “We fly apart. In a finite time, everything is in the appearance of distant galaxies and would just be sitting around,” he says, ripped apart.” in the imprint of the cosmic equivalent “and this other bubble would smash into us at the speed of light with some huge Cosmic The discovery of dark energy made the fate of the universe much energy and we would die.” more difficult to forecast.S cientists typically talk about three possible endings (depicted below), Beyond predicting another possible depending on what this mysterious force actually is and how it behaves over time. end, the multiverse ushers in a new way Expansion of the universe over time of thinking about what an “end” actually Big Rip means. “We’d have to be living in a lucky Constant dark energy (for cosmologists) or simple universe for (Big Freeze) the part that we see to be telling us about se the whole thing,” Aguirre says. Imagining the death of the observable universe as the ultimate end may be just Big Crunch as naïve as imagining that the destruction Acceleration of the Earth, for that matter, means the Scale of the univer Deceleration end of all life in the galaxy. There might be much more out there. Even if the Big Bang Present Future bubble occupied by people bursts, other Time source: CXC/NASA, M. Weiss universes could live long and prosper. s www.sciencenews.org April 23, 2011 | science news | 31 special section | cosmic questions, answers pending

tools for the mission

Fermi

LHC Planck LSST

Hunting data shown), scientists hope to spot signs A number of instruments now operating or proposed can troll the skies or other- of supersymmetry and find evidence for wise help to answer some of the most puzzling questions about the universe. string theory — possibly pointing to a theory that unifies the forces of nature. Planck A European Space Agency 12/5/09, p. 9) and captured unexpected The collider, in a 27-kilometer tunnel observatory launched in 2009, Planck is changes in emissions from the Crab straddling the border of Switzerland and recording a more detailed picture of the Nebula (SN: 1/1/11, p. 11). Fermi could France, began regular operations in 2010 cosmic microwave background, the relic offer clues to the identity of dark matter but has yet to operate at full energy. radiation left over from the Big Bang, and to the birth of the universe. than its predecessors COBE and WMAP LSST Proposed to sit atop Cerro did. The mission is searching for pri- WFIRST The Wide-Field Infrared Sur- Pachón in the Chilean Andes, the Large mordial gravitational waves, which could vey Telescope, a proposed NASA obser- Synoptic Survey Telescope (mirrors and provide a test for inflation theory, and vatory, would probe a wide swath of the lenses depicted) will capture the entire looking for clues to the nature of dark sky with two main goals: to settle key visible sky twice each week, helping matter and dark energy. questions about dark energy by map- astronomers better understand the ping large-scale structures and to look large-scale structure of the universe Fermi Launched in 2008, the Fermi for signs of extrasolar planets in the throughout its history. Knowing how Gamma-ray Space Telescope has opened Milky Way’s central bulge. stars, galaxies and galaxy clusters are scientists’ eyes to astronomical objects distributed can offer insight into cosmic that emit very high-energy radiation, LHC By smashing protons together at ingredients, including dark matter’s including super­massive black holes and high speeds, the Large Hadron Collider distribution and dark energy’s strength. colliding neutron stars. Since its launch, is re-creating energies present just after Fermi has found evidence of antimatter­ the Big Bang. In the spew of particles Dark Energy Survey Atop another above thunderstorms on Earth (SN: emitted (computer-generated image mountain in the Chilean Andes — Cerro

32 | science news | April 23, 2011 www.sciencenews.org

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at the same lab, records seasonal varia- concepts shown) would measure dark how the craft move in relation to each a ncesco Arneodo/ tions in faint flashes of light from 25 matter distribution and try to under- other, scientists hope LISA will detect a sodium iodide detectors. And an experi- stand the nature of dark energy by look- gravitational waves. Background undula- ment in a mine in northern Minnesota, ing back 10 billion years, before dark tions left over from the early universe cern genev © : A; Xenon: Fr C

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www.sciencenews.org April 23, 2011 | science news | 33