exploring our universe and others

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Exploring OurUniverse

and OthersIn the 21st century cosmologists will unravel the mystery

of our universe’s birth—and perhaps prove the existence

of other universes as well

Cosmic exploration is preeminently a 20th-century achievement. Only in the 1920s did we real-

ize that our Milky Way, with its 100 billion stars, is just one galaxy among millions. Our em-

pirical knowledge of the universe has been accumulating ever since. We can now set our entire

solar system in a grand evolutionary context, tracing its constituent atoms back to the initial

instants of the big bang. If we were ever to discover alien intelligences, one thing we might share with them—

perhaps the only thing—would be a common interest in the cosmos from which we have all emerged.

Using the current generation of ground-based and orbital observatories, astronomers can look back into

the past and see plain evidence of the evolution of the universe. Marvelous images from the Hubble Space

Telescope reveal galaxies as they were in remote times: balls of glowing, diffuse gas dotted with massive, fast-

burning blue stars. These stars transmuted the pristine hydrogen from the big bang into heavier atoms, and

when the stars died they seeded their galaxies with the basic building blocks of planets and life—carbon,

oxygen, iron and so on. A Creator didn’t have to turn 92 different knobs to make all the naturally occurring

elements in the periodic table. Instead the galaxies act as immense ecosystems, forging elements and recy-

cling gas through successive generations of stars. The human race itself is composed of stardust—or, less ro-

mantically, the nuclear waste from the fuel that makes stars shine.

Astronomers have also learned much about the earlier, pregalactic era by

studying the microwave background radiation that makes even intergalac-

tic space slightly warm. This afterglow of creation tells us that the entire

Large-scale structure of the uni-verse can be simulated by runningcosmological models on a super-computer. In this simulation, pro-duced by the Virgo Consortium,each particle represents a galaxy.

by Martin Rees

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universe was once hotter than the centers ofstars. Scientists can use laboratory data to calcu-late how much nuclear fusion would have hap-pened during the first few minutes after the bigbang. The predicted proportions of hydrogen,deuterium and helium accord well with what as-tronomers have observed, thereby corroboratingthe big bang theory.

At first sight, attempts to fathom the cosmosmight seem presumptuous and premature, evenin the closing days of the 20th century. Cosmolo-gists have, nonetheless, made real progress in re-cent years. This is because what makes thingsbaffling is their degree of complexity, not theirsheer size—and a star is simpler than an insect.The fierce heat within stars, and in the early uni-verse, guarantees that everything breaks downinto its simplest constituents. It is the biologists,

whose role it is to study the intri-cate multilayered structure oftrees, butterflies and brains, whoface the tougher challenge.

The progress in cosmology hasbrought new mysteries intosharper focus and raised ques-tions that will challenge as-tronomers well into the nextcentury. For example, why doesour universe contain its observedmix of ingredients? And how,from its dense beginnings, did it

heave itself up to such a vast size? The answers willtake us beyond the physics with which we are fa-miliar and will require new insights into the natureof space and time. To truly understand the historyof the universe, scientists must discover the pro-found links between the cosmic realm of the verylarge and the quantum world of the very small.

It is embarrassing to admit, but astronomersstill don’t know what our universe is made of.The objects that emit radiation that we can ob-serve—such as stars, quasars and galaxies—consti-tute only a small fraction of the universe’s matter.The vast bulk of matter is dark and unaccountedfor. Most cosmologists believe dark matter iscomposed of weakly interacting particles left overfrom the big bang, but it could be something evenmore exotic. Whatever the case, it is clear thatgalaxies, stars and planets are a mere afterthoughtin a cosmos dominated by quite different stuff.Searches for dark matter, mainly via sensitive un-derground experiments designed to detect elu-sive subatomic particles, will continue apace inthe coming decade. The stakes are high: successwould not only tell us what most of the universeis made of but would also probably reveal somefundamentally new kinds of particles.

Astronomers are also unsure how much darkmatter there is. The ultimate fate of our universe—

whether it continues expanding indefinitely oreventually changes course and collapses to the so-called big crunch—depends on the total amountof dark matter and the gravity it exerts. Currentdata indicate that the universe contains onlyabout 30 percent of the matter that would beneeded to halt the expansion. (In cosmologists’jargon, omega—the ratio of observed density tocritical density—is 0.3.) The odds favoring perpet-ual growth have recently strengthened further:tantalizing observations of distant supernovaesuggest that the expansion of the universe maybe speeding up rather than slowing down. Someastronomers say the observations are evidence ofan extra repulsive force that overwhelms gravityon cosmic scales—what Albert Einstein called thecosmological constant. The jury is still out on this

80 SCIENTIFIC AMERICAN December 1999 Exploring Our Universe and Others

Cosmic timeline showsthe evolution of our universe from the bigbang to the present day.In the first instant ofcreation—the epoch ofinflation—the universeexpanded at a staggering rate. Afterabout three minutes,the plasma of particlesand radiation cooledenough to allow the for-mation of simple atomicnuclei; after another300,000 years, atoms ofhydrogen and heliumbegan to form. The firststars and galaxiesappeared about a billionyears later. The ultimatefate of the universe—

whether it will expandforever or recollapse—isstill unknown, althoughcurrent evidence favorsperpetual expansion.

The Big Bang

QuantumGravity Era

10–43 second10–36 second

10–5 second3 minutes

300,000 years

Probable Eraof Inflation Formation of

protons andneutrons fromquarks

Synthesis ofatomic nuclei First atoms form

The great mystery forcosmologists is the series of events thatoccurred less than one millisecond after the big bang.


issue, but if the existence of the repulsive force isconfirmed, physicists will learn something radi-cally new about the energy latent in empty space.

Research is also likely to focus on the evolutionof the universe’s large-scale structure. If one hadto answer the question “What’s been happeningsince the big bang?” in just one sentence, the bestresponse might be to take a deep breath and say,“Ever since the beginning, gravity has been am-plifying inhomogeneities, building up structuresand enhancing temperature contrasts—a prereq-uisite for the emergence of the complexity thatlies around us now and of which we’re a part.” As-tronomers are now learning more about this 10-billion-year process by creating “virtual universes”on their computers. In the coming years, they willbe able to simulate the history of the universewith ever improving realism and then comparethe results with what telescopes reveal.

Questions of structure have preoccupied as-tronomers since the time of Isaac Newton, whowondered why all the planets circled the sun inthe same direction and in almost the same plane.In his 1704 work Opticks he wrote: “Blind fatecould never make all the planets move one andthe same way in orbits concentrick.” Such a won-derful uniformity in the planetary system, Newtonbelieved, must be the effect of divine providence.

Now astronomers know that the coplanarity ofthe planets is a natural outcome of the solar sys-tem’s origin as a spinning disk of gas and dust.Indeed, we have extended the frontiers of ourknowledge to far earlier times; cosmologists canroughly outline the history of the universe backto the very first second after the big bang. Con-ceptually, however, we’re in little better shape

than Newton was. Our understanding of thecausal chain of events now stretches further backin time, but we still run into a barrier, just as sure-ly as Newton did. The great mystery for cosmolo-gists is the series of events that occurred less thanone millisecond after the big bang, when the uni-verse was extraordinarily small, hot and dense.The laws of physics with which we are familiaroffer little firm guidance for explaining whathappened during this critical period.

To unravel this mystery, cosmologists mustfirst pin down—by improving and refining cur-rent observations—some of the characteristics ofthe universe when it was only one second old: itsexpansion rate, the size of its density fluctuations,and its proportions of ordinary atoms, dark mat-ter and radiation. But to comprehend why ouruniverse was set up thisway, we must probe fur-ther back, to the veryfirst tiny fraction of a mi-crosecond. Such an ef-fort will require theoreti-cal advances. Physicistsmust discover a way torelate Einstein’s theory ofgeneral relativity, whichgoverns large-scale inter-actions in the cosmos,with the quantum prin-ciples that apply at veryshort distances [see “AUnified Physics by2050?,” by Steven Wein-berg, on page 68]. A uni-fied theory would be

Exploring Our Universe and Others

Multiple universes arecontinually being born,according to some cos-

mologists. Each universeis shown here as an ex-

panding bubble branch-ing off from its “parent”

universe. The changes incolor represent shifts inthe laws of physics fromone universe to another. IL

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1 billion years

10 to 15 billion yearsFirst stars, galaxiesand quasars appear

Modern galaxies appear

PERPETUAL EXPANSION

RECOLLAPSE


needed to explain what happened in the first cru-cial moments after the big bang, when the entireuniverse was squeezed into a space smaller than asingle atom.

Astronomy is a subject in which observation isking. Now the same is true for cosmology—incontrast with the pre-1965 era, when speculationwas largely unconstrained. The answers to manyof cosmology’s long-standing questions are mostlikely to come from the new telescopes now go-ing into use. The two Keck Telescopes on MaunaKea in Hawaii are far more sensitive than earlierobservatories and thus can glimpse fainter ob-jects. Still more impressive is the Very Large Tele-scope being built in northern Chile, which willbe the world’s premier optical facility when it iscompleted. Astronomers can take advantage ofthe Chandra X-ray Observatory, launched intoorbit this past summer, and several new radio ar-rays on the ground. And a decade from now next-generation space telescopes will carry the enter-prise far beyond what the Hubble can achieve.

Well before 2050 we are likely to see the con-struction of giant observatories in space or per-haps on the far side of the moon. The sensitivityand imaging power of these arrays will vastly sur-pass that of any instruments now in use. The newtelescopes will target black holes and planets inother solar systems. They will also provide snap-shots of every cosmological era going back to thevery first light, when the earliest stars (or maybequasars) condensed out of the expanding debrisfrom the big bang. Some of these observatoriesmay even be able to measure gravitational waves,allowing scientists to probe vibrations in the fab-ric of space-time itself.

The amount of data provided by all these in-struments will be so colossal that the entire pro-

cess of analysis and discovery will most likely beautomated. Astronomers will focus their atten-tion on heavily processed statistics for each pop-ulation of objects they are studying and in thisway find the best examples—for instance, theplanets in other solar systems that are most likeEarth. Researchers will also concentrate on ex-treme objects that may hold clues to physicalprocesses that are not yet fully understood. Onesuch object is the gamma-ray burster, whichemits, for a few seconds, as much power as a bil-lion galaxies. Increasingly, astronomers will usethe heavens as a cosmic laboratory to probe phe-nomena that cannot be simulated on Earth.

Another benefit of automation will be open ac-cess to astronomical data that in the past wereavailable only to a privileged few. Detailed maps ofthe sky will be available to anyone who can accessor download them. Enthusiasts anywhere in theworld will be able to check their own hunches, seeknew patterns and discover unusual objects.

Intimations of a Multiverse?

Cosmologists view the universe as an intricatetapestry that has evolved from initial condi-

tions that were imprinted in the first microsec-ond after the big bang. Complex structures andphenomena have unfolded from simple physicallaws—we wouldn’t be here if they hadn’t. Simplelaws, however, do not necessarily lead to complexconsequences. Consider an analogue from thefield of fractal mathematics: the Mandelbrot set, apattern with an infinite depth of structure, is en-coded by a short algorithm, but other simple algo-rithms that are superficially similar yield very bor-ing patterns.

Our universe could not have become structured

82 SCIENTIFIC AMERICAN December 1999 Exploring Our Universe and Others

Lunar observatories willgreatly extend the reachof 21st-century astron-omers. The far side ofthe moon is an idealplace for telescopesbecause of its absenceof atmosphere and itsutterly dark nights.(Sunlight reflected offEarth’s surface cannotreach the far side, whichnever faces our planet.)Lunar ores can be usedto build the instruments.

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if it were not expanding at a special rate. If the bigbang had produced fewer density fluctuations,the universe would have remained dark and fea-tureless, with no galaxies or stars. And there areother prerequisites for complexity. If our universehad more than three spatial dimensions, planetscould not stay in orbits around stars. If gravitywere much stronger, it would crush living organ-isms of human size, and stars would be small andshort-lived. If nuclear forces were a few percentweaker, only hydrogen would be stable: therewould be no periodic table, no chemistry and nolife. On the other hand, if nuclear forces wereslightly stronger, hydrogen itself could not exist.

Some would argue that this fine-tuning of theuniverse, which seems so providential, is nothingto be surprised about, because we could not existotherwise. There is, however, another interpreta-tion: many universes may exist, but only somewould allow creatures like us to emerge, and weobviously find ourselves in one of that subset. Theseemingly designed features of our universe needthen occasion no surprise.

Perhaps, then, our big bang wasn’t the only one.This speculation dramatically enlarges our conceptof reality. The entire history of our universe be-comes just an episode, a single facet, of the infinitemultiverse. Some universes might resemble ours,but most would be “stillborn.” They would recol-lapse after a brief existence, or the laws governingthem would not permit complex consequences.

Some cosmologists, especially Andrei Linde ofStanford University and Alex Vilenkin of TuftsUniversity, have already shown how certainmathematical assumptions lead, at least in theory,to the creation of a multiverse. But such ideas willremain on the speculative fringe of cosmologyuntil we really understand—rather than just guessat—the extreme physics that prevailed immedi-ately after the big bang. Will the long-awaitedunified theory uniquely determine the masses ofparticles and the strengths of the basic forces? Orare these properties in some sense accidental out-comes of how our universe cooled—secondarymanifestations of still deeper laws governing anentire ensemble of universes?

This topic might seem arcane, but the status ofmultiverse ideas affects how we should place ourbets in some ongoing cosmological controversies.Some theorists have a strong preference for thesimplest picture of the cosmos, which would re-quire an omega of 1—the universe would be justdense enough to halt its own expansion. They areunhappy with observations suggesting that theuniverse is not nearly so dense and with extracomplications such as the cosmological constant.Perhaps we should draw a lesson from 17th-centu-ry astronomers Johannes Kepler and GalileoGalilei, who were upset to find that planetary or-bits were elliptical. Circles, they thought, weresimpler and more beautiful. But Newton later ex-plained all orbits in terms of a simple, universal lawof gravity. Had Galileo still been alive, he would

have surely been joyfully reconciled to ellipses.The parallel is obvious. If a low-density universe

with a cosmological constant seems ugly, maybethis shows our limited vision. Just as Earth followsone of the few Keplerian orbits around the sun thatallow it to be habitable, our universe may be one ofthe few habitable members of a grander ensemble.

A Challenge for the New Millennium

As the 21st century dawns, scientists are ex-panding humanity’s store of knowledge on

three great frontiers: the very big, the very smalland the very complex. Cosmology involves themall. In the coming years, researchers will focus theirefforts on pinning down the basic universal con-stants, such as omega, and on discovering whatdark matter is. I think there is a good chance ofachieving both goals within 10 years. Maybe ev-erything will fit the standard theoretical frame-work, and we will successfully determine not onlythe relative abundance of ordinary atoms and darkmatter in the universe but also the cosmologicalconstant and the primordial density fluctuations.If that happens, we will have taken the measure ofour universe just as, over the past few centuries, wehave learned the size and shape of Earth and oursun. On the other hand, our universe may turnout to be too complicated to fit into the standardframework. Some may describe the first outcomeas optimistic; others may prefer to inhabit a morecomplicated and challenging universe!

In addition, theorists must elucidate the exoticphysics of the very earliest moments of the uni-verse. If they succeed, we will learn whether thereare many universes and which features of ouruniverse are mere contingencies rather than thenecessary outcomes of the deepest laws. Our un-derstanding will still have limits, however. Physi-cists may someday discover a unified theory thatgoverns all of physical reality, but they will neverbe able to tell us what breathes fire into their equa-tions and what actualizes them in a real cosmos.

Cosmology is not only a fundamental science;it is also the grandest of the environmental sci-ences. How did a hot amorphous fireball evolve,over 10 to 15 billion years, into our complex cos-mos of galaxies, stars and planets? How did atomsassemble—here on Earth and perhaps on otherworlds—into living beings intricate enough toponder their own origins? These questions are achallenge for the new millennium. Answeringthem may well be an unending quest.

Exploring Our Universe and Others SCIENTIFIC AMERICAN December 1999 83

MARTIN REES is RoyalSociety Research Profes-sor at the University ofCambridge and holds thehonorary title of Astron-omer Royal. He was previ-ously director of the Cam-bridge Institute of Astron-omy and before that aprofessor at the Universityof Sussex. His research interests include blackholes, galaxy formationand high-energy astro-physics. He also lecturesand writes extensively forgeneral audiences. Hisrecent books are Gravity’sFatal Attraction (ScientificAmerican Library, 1996,with Mitchell C. Begelman),Before the Beginning (Perseus Books, 1998)and Just Six Numbers(Basic Books, in press).

Further Information

The Author

THE ASTRONOMY AND ASTROPHYSICS ENCYCLOPEDIA. Edited by Stephen P. Maran.Van Nostrand Reinhold, 1992.

PLANET QUEST: THE EPIC DISCOVERY OF ALIEN SOLAR SYSTEMS. Ken Croswell. FreePress, 1997.

THE LITTLE BOOK OF THE BIG BANG: A COSMIC PRIMER. Craig J. Hogan. Coperni-cus, 1998.

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