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…

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.

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…

The universe, cosmologists calculate, 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…

It is a picture that in some ways is surprisingly simple… 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…

But 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 [or] "missing matter."

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… 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…

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… 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…

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…

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…

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, 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.

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…

In April 1992 NASA's Cosmic Background Explorer, or COBE, satellite 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…

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 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…

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…

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, arrived at the same answer at the same time: the cosmos was not slowing at all; it was speeding up…The results have sent Einstein's old cosmological constant to the forefront of cosmology.

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…

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…

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."