It has always been easy to make fun of cosmologists, confined to a dust mote lost in space, pronouncing judgment on the fate of the universe or the behavior of galaxies billions of light-years away, with only a few scraps of light as evidence.
''Cosmologists are often wrong,'' the Russian physicist Lev Landau put it, ''but never in doubt.''
For most of the 20th century, cosmology seemed less a science than a religious war over, say, whether the universe had a beginning, in a fiery Big Bang billions of years ago, or whether it exists eternally in the so-called Steady State.
In the last few years, however, a funny thing has happened. Cosmologists are beginning to agree with one another. Blessed with new instruments like the Hubble Space Telescope and other space-based observatories, a new generation of their giant cousins on the ground and ever-faster computer networks, cosmology is entering ''a golden age'' in which data are finally outrunning speculation.
''The rate at which we are learning and discovering new things is just extraordinary,'' said Dr. Charles Bennett, an astronomer at the Goddard Space Flight Center in Greenbelt, Md.Continue reading the main story
As a result, cosmologists are beginning to converge on what they call a ''standard model'' of the universe that is towering in its ambition. It purports to trace, at least in broad strokes, cosmic history from the millisecond after time began, when the universe was a boiling stew of energy and subatomic particles, through the formation of atoms, stars, galaxies and planets to the vast, dilute, dark future in which all of these will have died.
The universe, the cosmologists say, was born 14 billion years ago in the Big Bang. Most of its material remains resides in huge clouds of invisible so-called dark matter, perhaps elementary particles left over from the primordial explosion and not yet identified.
Within these invisible clouds, the glittery lights in the sky that have defined creation for generations of humans are swamped, like flecks of foam on a rolling sea. A good case can be made, scientists now agree, that the universe will go on expanding forever.
In fact, recent observations have suggested that the expansion of the universe is speeding up over cosmic time, under the influence of a ''dark energy'' even more mysterious than dark matter.
Recently, a group of astronomers led by Dr. William Percival at the University of Edinburgh combined data from a variety of observations to compile, based on the simplest theoretical model, what they say is the most precise enumeration yet of the parameters that cosmologists have been fighting about for all these decades.
The universe, they calculated, is 13.89 billion years old, plus or minus half a billion years. Only 4.8 percent of it is made of ordinary matter. Matter of all types, known and unknown, luminous and dark, accounts for just 27.5 percent. The rest of creation, 72.5 percent, is the mysterious dark energy, they reported in a paper submitted last month to The Monthly Notices of the Royal Astronomical Society.
It is a picture that in some ways is surprisingly simple, satisfying long-held theoretical prejudices about how the universe should be designed. Continued agreement with coming experiments may mean that science is approaching the end of a ''great program'' of cosmological tests that began in the 1930's, Dr. P. J. E. Peebles of Princeton and Dr. Bharat Ratra of Kansas State University said in the draft of a coming article for The Reviews of Modern Physics.
In other ways this new dark universe is utterly baffling, a road map to new mysteries. Dr. Marc Davis, a cosmologist at the University of California at Berkeley, called it ''a universe chock full of exotics that don't make sense to anybody.''
Moreover there are some questions that scientists still do not know how to ask, let alone answer, scientifically. Was there anything before the Big Bang? Is there a role for life in the cosmos? Why is there something rather than nothing at all? Will we ever know?
''We know much, but we still understand very little,'' said Dr. Michael Turner, a cosmologist at the University of Chicago.
The Big Question
Expanding Forever, Or Big Crunch?
The dim caves of Lascaux, the plains of Stonehenge and the dreamtime tales of Australian aborigines all testify to the need to explain the world and existence. This quest took its present form in 1917. That was when Albert Einstein took his new general theory of relativity, which explained how matter and energy warp space-time to produce gravity, and applied it to the universe.
Einstein discovered that the cosmos as his theory described it would be unstable, prone to collapse under its own gravity. Astronomers, however, were sure that the universe was stable. So Einstein added a fudge factor that he called the cosmological constant to his equations. It acted as a long-range repulsive force to counterbalance gravity.
In 1929, the astronomer Edwin Hubble discovered that the universe was expanding. The sky was full of distant galaxies all rushing away from us and one another, as if propelled by what the British astronomer Dr. Fred Hoyle later called derisively a ''big bang.'' The universe was not stable and, thus, did not require counterbalancing. Einstein abandoned his constant, referring to it as his biggest blunder. But it would return to haunt cosmologists, and the universe.
Hoyle's term stuck, and the notion of an explosive genesis became orthodoxy in 1965, when Dr. Arno Penzias and Dr. Robert Wilson, radio astronomers at Bell Laboratories, discovered a faint uniform radio glow that pervaded the sky. It was, cosmologists concluded, the fading remnant of the primordial fireball itself.
Relieved of their fudge factor, the equations describing Einstein's universe were simple. Dr. Allan Sandage, the Carnegie Observatories astronomer, once called cosmology ''the search for two numbers'' -- one, the Hubble constant, telling how fast the universe is expanding, and the other telling how fast the expansion is slowing, and thus whether the universe will expand forever or not.
The second number, known as the deceleration parameter, indicated how much the cosmos had been warped by the density of its contents. In a high-density universe, space would be curved around on itself like a ball. Such a universe would eventually stop expanding and fall back together in a big crunch that would extinguish space and time, as well as the galaxies and stars that inhabit them. A low-density universe, on the other hand, would have an opposite or ''open'' curvature like a saddle, harder to envision, and would expand forever.
In between with no overall warpage at all was a ''Goldilocks'' universe with just the right density to expand forever but more and more slowly, so that after an infinite time it would coast to a stop. This was a ''flat'' universe in the cosmological parlance, and to many theorists the simplest and most mathematically beautiful solution of all.
But the sky did not yield those cosmic numbers easily, even with the help of the 200-inch Hale telescope on Palomar Mountain in Southern California, dedicated in 1948, which had been built largely for that task. Dr. Hubble wrote of measuring shadows and searching ''among ghostly errors of measurement for landmarks that are scarcely more substantial.''
The Dark Side
Invisible Matter Molds Galaxies
It was not till the mid-70's, a quarter-century after the Palomar giant began operating, that groups of astronomers reached the tentative conclusion that the universe they could see -- stars, gas, planets and galaxies -- did not have nearly enough gravitational oomph to stop the cosmic expansion.
''So the universe will continue to expand forever and galaxies will get farther and farther apart, and things will just die,'' Dr. Sandage said at the time.
But the Great Argument was just beginning. Apparently there was a lot of the universe that astronomers could not see. The stars and galaxies, were moving as if immersed in the gravity of giant invisible clouds of so-called dark matter -- ''missing matter'' the Swiss astronomer Dr. Fritz Zwicky labeled it in the 1930's.
Many galaxies, for example, are rotating so fast that they would fly apart unless they were being reined in by the gravity of halos of dark matter, according to pioneering observations by Dr. Vera Rubin of the Carnegie Institution of Washington and her colleagues. Her measurements indicated that these dark halos outweighed the visible galaxies themselves from 5 to 10 times. But there could be even more dark matter farther out in space, perhaps enough to stop the expansion of the universe, eventually, some theorists suggested. Luminous matter, the Darth Vaders of the sky said, is like the snow on mountaintops.
But what is the dark matter? While some of it is gas or dark dim objects like stars and planets, cosmologists speculate that most of it is subatomic particles left over from the Big Bang.
Many varieties of these particles are predicted by theories of high-energy physics. But their existence has not been confirmed or detected in particle accelerators.
''We theorists can invent all sorts of garbage to fill the universe,'' Dr. Sheldon Glashow, a Harvard physicist and Nobel laureate, told a gathering on dark matter in 1981.
Collectively known as WIMP's, for weakly interacting massive particles, such particles would not respond to electromagnetism, the force responsible for light, and thus would be unable to radiate or reflect light. They would also be relatively slow-moving, or ''cold'' in physics jargon, and thus also go by the name of cold dark matter.
As Earth in its travels passed through the dark-matter cloud that presumably envelops the Milky Way, the particles would shoot through our bodies, rarely leaving a trace, like moonlight through a window.
But the collective gravity of such particles, cosmologists say, would shape the cosmos and its contents.
Gathering along the fault lines laid down by random perturbations of density in the early universe, dark matter would congeal into clouds with about the mass of 100,000 Suns. The ordinary matter that was mixed in with it would cool and fall to the centers of the clouds and light up as stars.
The clouds would then attract other clouds. Through a series of mergers over billions of years, smaller clouds would assemble into galaxies, and the galaxies would then assemble themselves into clusters of thousands of galaxies, and so forth.
Using the Hubble and other telescopes as time machines -- light travels at a finite speed, so the farther out astronomers look the farther back in time they see -- cosmologists have begun to confirm that the universe did assemble itself from the ''bottom up,'' as the dark matter model predicts.
Last year, two teams of astronomers reported seeing the first stars burning their way out of the cloudy aftermath of the Big Bang, when the universe was only 900 million years old. The bulk of galaxy formation occurred when the universe was a half to a quarter its present age, cosmologists say.
''The big news in the last decade is that even half a universe ago the universe looked pretty different,'' said Dr. Alan Dressler of the Carnegie Observatories in Pasadena. Galaxies before then were small and irregular, with no sign of the majestic spiral spider webs that decorate the sky today.
We would barely recognize our own Milky Way galaxy, if we could see it five billion years ago when the Sun formed, he said.
''By eight billion years back, it would be unrecognizable,'' said Dr. Dressler.
Yet there are still many questions that the cold dark matter model does not answer. Astronomers still do not know, for example, how the first stars formed or why the models of dark matter distribution don't quite fit in the cores of some kinds of galaxies. Nor have the dark matter particles themselves been unambiguously detected or identified, despite continuing experiments.
Some astronomers suggest that the discrepancies stem from the inability of simple mathematical models to deal with messy details of the real world.
''It's a huge mystery exactly how stars form,'' Dr. Richard Bond of the Canadian Institute for Theoretical Astrophysics said. ''We can't solve it now. So it's even harder to try to solve them back then.''
But others, notably Dr. Mordehai Milgrom, a theorist at the Weizmann Institute in Israel, have suggested that modifying the gravitational laws by which dark matter was deduced in the first place would alleviate the need for dark matter altogether.
The Bang's Fuel
Inflating One Ounce To a Whole Universe
Clues to what had actually exploded in the Big Bang emerged as an unexpected gift from another great scientific quest: physicists' pursuit for a so-called theory of everything that would unite all physical phenomena in a single equation. Unable to build machines powerful enough to test their most ambitious notions on Earth, some theorists turned to the sky.
''The Big Bang is the poor man's particle accelerator,'' Dr. Jakob Zeldovich, an influential Russian cosmologist, said.
Physicists recognize four forces at work in the world today -- gravity, electromagnetism, and the strong and weak nuclear forces. But they suspect, based on data from particle accelerators and high-powered theory, that those are simply different manifestations of a single unified force that ruled the universe in its earliest, hottest moments.
As the universe cooled, according to this theory, there was a fall from grace, and the laws of physics evolved, with one force after another ''freezing out,'' or splitting away.
In 1979, Dr. Alan Guth, now at the Massachusetts Institute of Technology, realized that a hypothesized glitch in this process would have had drastic consequences for the universe. Under some circumstances, a glass of water can stay liquid as the temperature falls below 32 degrees, until it is disturbed, at which point it will rapidly freeze, releasing latent heat in the process. Similarly, the universe could ''supercool'' and stay in a unified state too long. In that case, space itself would become temporarily imbued with a mysterious kind of latent heat, or energy.
Inserted into Einstein's equations, the latent energy would act as a kind of antigravity, and the universe would blow itself apart, Dr. Guth discovered in a calculation in 1979.
In far less than the blink of an eye, 10-37 second, a speck much smaller than a proton would have swollen to the size of a grapefruit and then resumed its more stately expansion, with all of normal cosmic history before it, resulting in today's observable universe -- a patch of sky and stars 14 billion light-years across. All, by the magical-seeming logic of Einstein's equations, from about an ounce of primordial stuff.
''The universe,'' Dr. Guth liked to say, ''might be the ultimate free lunch.''
Dr. Guth called his theory inflation. Inflation, as Dr. Guth pointed out, explains why the universe is expanding. Dr. Turner of the University of Chicago referred to it as ''the dynamite behind the Big Bang.''
As modified and improved by Dr. Andrei Linde, now at Stanford, and by Dr. Paul Steinhardt, now at Princeton and Dr. Andreas Albrecht now at the University of California at Davis, inflation has been the workhorse of cosmology ever since. One of its great virtues, cosmologists say, is that inflation explains the origin of galaxies, the main citizens of the cosmos. The answer comes from the paradoxical-sounding quantum rules that govern subatomic affairs. On the smallest scales, according to quantum theory, nature is lumpy, emitting even energy in little bits and subject to an irreducible randomness. As a result, so-called quantum fluctuations would leave faint lumps in the early universe. These would serve as the gravitational seeds for future galaxies and other cosmic structures.
As a result of such successes, cosmologists have stuck with the idea of inflation, even though, lacking the ability to test their theories at the high energies of the Big Bang, they have no precise theory about what might have actually caused it. ''Inflation is actually a class of theories,'' said Dr. Guth.
In the latest version, called ''chaotic inflation,'' Dr. Linde has argued that quantum fluctuations in a myriad of theorized force fields could have done the trick.
Indeed, he and others now say they believe that inflation can occur over and over, spawning an endless chain of universes out of one another, like bubbles within bubbles.
''The universe inflates on top of itself,'' Dr. Linde told a physics conference this spring in Princeton. ''It's happening right now.''
The Golden Age
New Devices Detect Primordial Glow
If the inflationary theorists are right, the universe we see, the 14 billion light-years, is just a tiny piece of a much vaster universe, or even a whole ensemble of them, forever out of our view.
According to the theory, therefore, our own little patch of the cosmos should appear geometrically ''flat,'' the way a section of a balloon looks flat when viewed close up. This was the universe long thought to be the most beautiful and simple.
But it required, by the logic of Einstein's general relativity, that there be much more dark matter, or something, to the universe, enough to ''flatten'' space-time, than astronomers had found.
In fact, this prescription was so hard to reconcile with other observations, of galaxies and their evolutions, that by 1991 some astronomers and press reports suggested that the entire theoretical edifice of inflation to blow up the universe and cold dark matter to fill it, not to say the Big Bang itself, might have to be junked.
So it was with a sigh of relief that cosmologists greeted the announcement in April 1992 that NASA's Cosmic Background Explorer, or COBE, satellite had succeeded in discerning faint blotches in the primordial cosmic radio glow.
These were the seeds from which, inflation predicted, large cosmic structures would eventually grow.
''If you're religious, it's like seeing God,'' said Dr. George Smoot, a physicist from the Lawrence Berkeley National Laboratory who led the COBE team.
Astronomers say COBE signaled a transition in which heroic ideas about the universe began to be replaced by heroic data, as long-planned new telescopes and other instruments went into operation.
A year later, skywalking astronauts corrected the Hubble telescope's myopic vision. The cosmic background radiation has come in for particular scrutiny from new radio telescopes mounted in balloons and on mountaintops. The news has been good, though not decisive, for inflation.
For three years, a series of increasingly high-resolution observations has confirmed that the pattern of blotches stippling the remnant of the primordial fireball is consistent with the predictions from inflation and cold dark matter. The instruments have now mapped details small enough to have been the seeds of modern clusters of galaxies.
''I'm completely snowed by the cosmic background radiation,'' Dr. Guth said. ''The signal was so weak it wasn't even detected until 1965, and now they're measuring fluctuations of one part in 100,000.''
Perhaps most important, the analysis of the fluctuations indicates that the universe has a ''flat'' geometry, as predicted by inflation. That was a triumph. Although observations could not prove that inflation was right, a nonflat universe would have been a blow to the theory, and to cosmological orthodoxy.
''Inflation, our boldest and most promising theory of the earliest moments of creation, passed its first very important test,'' Dr. Turner said at the time.
The most precise measurements of the cosmic background, at least in the near future, are generally expected to come late this year from NASA's Microwave Anisotropy Project, or MAP, satellite, which was launched into space last year on June 30. MAP will be followed by the European Space Agency's Planck satellite, in 2006.
It is highly unlikely that MAP or Planck will be able to detect what Dr. Turner calls ''the smoking gun signature of inflation.'' The violent stretching of the universe should roil space-time with so-called gravitational waves that would leave a faint imprint on the cosmic fireball.
Detecting those waves would not only confirm inflation, but also might help scientists establish what caused the inflation in the first place, giving science its first look at the strange physics that prevailed when creation was only about a trillionth of a trillionth of a trillionth of a second old.
The Universe's Fate
Bleak Implications Of 'Dark Energy'
In 1998, two competing teams of astronomers startled the scientific world with the news that the expansion of the universe seemed to be speeding up under the influence of a mysterious antigravity that seems embedded in space itself and that is hauntingly reminiscent of Einstein's old, presumably discredited, cosmological constant.
''Dark energy,'' the phenomenon was quickly named.
If dark energy is real and the acceleration continues, the galaxies will eventually speed away from one another so quickly that they couldn't see one another. The universe would become cold and empty as the continued acceleration sucked away the energy needed for life and thought.
It would be ''the worst possible universe,'' for the quality and quantity of life, said Dr. Lawrence Krauss, a physicist at Case Western Reserve University.
Dr. Edward Witten of the Institute for Advanced Study in Princeton, called the discovery of dark energy ''the strangest experimental finding since I've been in physics.''
The discovery was a surprise to the astronomers involved. Neither team had expected to find the universe accelerating. They had each set out to measure by how much the expansion of the universe was slowing because of the gravity of its contents and thus settle the question of its fate.
One team was led by Dr. Saul Perlmutter, a physicist at Lawrence Berkeley. The other team was a band of astronomers led by Dr. Brian Schmidt of Mount Stromlo and the Siding Spring Observatory in Australia.
Each group employed far-flung networks of telescopes, including the Hubble, and the Internet to find and monitor certain exploding stars, or supernovas, as cosmic beacons. Such explosions, the death rattles of massive stars, are powerful enough to be seen clear across the universe when the universe was younger and, presumably, expanding faster.
Leapfrogging each other across the universe, the two teams, propitiously for their credibility, arrived at the same answer at the same time: the cosmos was not slowing at all; it was speeding up.
Dr. Perlmutter, who had once resented the competition, conceded, ''With only one group, it would have been a lot harder to get the community to buy into such a surprising result.''
''This was a very strange result,'' said Dr. Adam Riess, a member of Dr. Schmidt's team. ''It was the opposite of what we thought we were doing.''
The results have sent Einstein's old cosmological constant to the forefront of cosmology. Despite his disavowal, the constant had never really gone away and had in fact been given new life by quantum physics. Einstein had famously rejected quantum's randomness, saying God didn't play dice.
But it justified, in retrospect, his fudge factor.
According to the uncertainty principle, a pillar of quantum theory, empty space was not empty, but rather foaming with the energy of so-called virtual particles as they flashed in and out of existence on borrowed energy. This so-called vacuum energy could repel, just like Einstein's old cosmological constant, or attract.
The case for dark energy got even stronger a year later, when the cosmic background observations reported evidence of a flat universe. Because astronomers had been able to find only about a third as much matter, both dark and luminous, as was needed by Einstein's laws to create a flat geometry, something else had to be adding to it.
The discovery of dark energy exemplified Dr. Zeldovich's view of the universe as the poor man's particle accelerator, and it caught the physicists flat-footed, somewhat to the pride of the astronomers.
''A coming of age of astronomy,'' Dr. Dressler called it.
What is dark energy? The question now hangs over the universe.
Is it really Einstein's old fudge factor returned to haunt his children? In that case, as the universe expands and the volume of space increases, astronomers say, the push because of dark energy will also increase, accelerating the galaxies away from one another faster and faster, leading to a dire dark future.
Other physicists, however, have pointed out that the theories of modern physics are replete with mysterious force fields, collectively called ''quintessence,'' that might or might not exist, but that could temporarily produce negative gravity and mimic the action of a cosmological constant. In that case, all bets on the future are off. The universe could accelerate and then decelerate, or vice versa as the dark energy fields rose or fell.
A third possibility is that dark energy does not exist at all, in which case not just the future, but the whole carefully constructed jigsaw puzzle of cosmology, might be in doubt. The effects of cosmic acceleration could be mimicked, astronomers say, by unusual dust in the far universe or by unsuspected changes in the characteristics of supernovas over cosmic time. As a result, more groups are joining the original two teams in the hunt for new supernovas and other ways to measure the effects of dark energy on the history of the universe.
Dr. Perlmutter has proposed building a special satellite telescope, the Supernova Astronomy Project, to investigate exactly when and how abruptly the cosmic acceleration kicked in.
The Nagging Questions
A Grand Synthesis, But Hardly Complete
For all the new answers being harvested, some old questions linger, and they have now been joined by new ones.
A flat universe is the most mathematically appealing solution of Einstein's equations, cosmologists agree. But they are puzzled by the specific recipe, large helpings of dark matter and dark energy, that nature has chosen. Dr. Turner called it ''a preposterous universe.''
But Dr. Martin Rees, a Cambridge University cosmologist, said that the discovery of a deeper principle governing the universe and, perhaps, life, may alter our view of what is fundamental. Some features of the universe that are now considered fundamental -- like the exact mixture of dark matter, dark energy and regular stuff in the cosmos -- may turn out to be mere accidents of evolution in one out of the many, many universes allowed by eternal inflation.
''If we had a theory, then we would know whether there were many big bangs or one,'' Dr. Rees said. The answers to these and other questions, many scientists suspect, have to await the final unification of physics, a theory that reconciles Einstein's relativity, which describes the shape of the universe, to the quantum chaos that lives inside it.
Such a theory, quantum gravity, is needed to describe the first few moments of the universe, when it was so small that even space and time should become fuzzy and discontinuous.
For two decades, many physicists have placed their bets for quantum gravity on string theory, which posits that elementary particles are tiny strings vibrating in a 10- or 11-dimensional space. Each kind of particle, in a sense, corresponds to a different note on the string.
In principle, string theory can explain all the forces of nature. But even its adherents concede that their equations are just approximations to an unknown theory that they call M-theory, with ''M'' standing for matrix, magic, mystery or even mother, as in ''mother of all theories.'' Moreover, the effects of ''stringy physics'' are only evident at energies forever beyond the limits of particle accelerators.
Some string theorists have ventured into cosmology, hoping, to discover some effect that would show up in the poor man's particle accelerator, the sky.
In addition to strings, the theory also includes membranes, or ''branes,'' of various dimensions. Our universe can be envisioned as such a brane floating in higher-dimensional space like a leaf in a fish tank, perhaps with other brane universes nearby. These branes could interact gravitationally or even collide, setting off the Big Bang.
In one version suggested last year by four cosmologists led by Dr. Steinhardt of Princeton, another brane would repeatedly collide with our own. They pass back and forth through each other, causing our universe to undergo an eternal chain of big bangs.
Such notions are probably the future for those who are paid to wonder about the universe.
And the fruits of this work could yet cause cosmologists to reconsider their new consensus, warned Dr. Peebles of Princeton, who has often acted as the conscience of the cosmological community, trying to put the brakes on faddish trends.
He wonders whether the situation today can be compared to another historical era, around 1900, when many people thought that physics was essentially finished and when the English physicist Lord Kelvin said that just a couple of ''clouds'' remained to be dealt with.
''A few annoying tidbits, which turned out to be relativity and quantum theory,'' the twin revolutions of 20th-century science, Dr. Peebles said.
Likewise, there are a few clouds today like what he called ''the dark sector,'' which could have more complicated physics than cosmologists think.
''I'm not convinced these clouds herald revolutions as deep as relativity and quantum mechanics,'' Dr. Peebles said. ''I'm not arguing that they won't.''
As for the fate of the universe, we will never have a firm answer, said Dr. Sandage, who was Hubble's protégé and has seen it all.
''It's like asking, 'Does God exist?' '' he said.
Predicting the future, he pointed out, requires faith that simple mathematical models really work to describe the universe.
''I don't think we really know how things work,'' he said.
Although Dr. Sandage does not buy into all aspects of the emerging orthodoxy, he said it was a fantastic time to be alive.
''It's all working toward a much grander synthesis than we could have imagined 100 years ago,'' he said. ''I think this is the most exciting life I could have had.''Continue reading the main story