n the decades that astronomers have debated the fate of the expanding universe — whether it will all end one day in a big crunch, or whether the galaxies will sail apart forever — aficionados of eternal expansion have always been braced by its seemingly endless possibilities for development and evolution. As the Yale cosmologist Dr. Beatrice Tinsley once wrote, "I think I am tied to the idea of expanding forever."

Life and intelligence could sustain themselves indefinitely in such a universe, even as the stars winked out and the galaxies were all swallowed by black holes, Dr. Freeman Dyson, a physicist at the Institute for Advanced Study, argued in a landmark paper in 1979. "If my view of the future is correct," he wrote, "it means that the world of physics and astronomy is also inexhaustible; no matter how far we go into the future, there will always be new things happening, new information coming in, new worlds to explore, a constantly expanding domain of life, consciousness, and memory."

Now, however, even Dr. Dyson admits that all bets are off. If recent astronomical observations are correct, the future of life and the universe will be far bleaker.

In the last four years astronomers have reported evidence that the expansion of the universe is not just continuing but is speeding up, under the influence of a mysterious "dark energy," an antigravity that seems to be embedded in space itself. If that is true and the universe goes on accelerating, astronomers say, rather than coasting gently into the night, distant galaxies will eventually be moving apart so quickly that they cannot communicate with one another. In effect, it would be like living in the middle of a black hole that kept getting emptier and colder.

In such a universe, some physicists say, the usual methods of formulating physics may not all apply. Instead of new worlds coming into view, old ones would constantly be disappearing over the horizon, lost from view forever.

Cosmological knowledge would be fragmented, with different observers doomed to seeing different pieces of the puzzle and no single observer able to know the fate of the whole universe or arrive at a theory of physics that was more than approximate.

"There would be a lot of things about the universe that we simply couldn't predict," said Dr. Thomas Banks, a physicist at the University of California at Santa Cruz.

And perhaps most important, starved finally of the energy even to complete a thought or a computation, the domain of life and intelligence would not expand, but constrict and eventually vanish like a dwindling echo into the silence of eternity. "I find the fate of a universe that is accelerating forever not very appealing," said Dr. Edward Witten, a theorist at the Institute for Advanced Study.

That is an understatement, in the view of Dr. Lawrence M. Krauss, an astrophysicist at Case Western Reserve University in Cleveland, who along with his colleague Dr. Glenn D. Starkman has recently tried to limn the possibilities of the far future. An accelerating universe "would be the worst possible universe, both for the quality and quantity of life," Dr. Krauss said, adding: "All our knowledge, civilization and culture are destined to be forgotten. There's no long-term future."

Einstein's Last Laugh

Until about four years ago, an overwhelming preponderance of astronomers subscribed to the view that the cosmic expansion was probably slowing down because of the collective gravity of the galaxies and everything else in the universe, the way a handful of stones tossed in the air gradually slow their ascent. The only question was whether the universe had enough gravitational oomph to stop expanding and bring itself back together in a "big crunch," or whether the galaxies would sail ever more slowly outward forever.

It was to measure that rate of slowing of this outward flight, and thus find the long- sought and elusive answer to the cosmic question, that two teams of astronomers started competing projects in the 1990's using distant exploding stars, supernovas, as cosmic beacons.

In 1998 the two teams announced that instead of the expected slowing, the galaxies actually seem to have speeded up over the last five or six billion years, as if some "dark energy" was pushing them outward.

"It's definitely the strangest experimental finding since I've been in physics," Dr. Witten said. "People find it difficult to accept. I've stopped expecting that the finding will be proved wrong, but it's an extremely uncomfortable result."

To astronomers this dark energy bears a haunting resemblance to an idea that Albert Einstein had back in 1917 and then abandoned, later calling it his biggest blunder. In that year he inserted a mathematical fudge factor that came to be known as the cosmological constant into his equations of general relativity in order to stabilize the universe against collapse; Einstein's constant acted as a kind of cosmic repulsion to balance the gravitational pull of the galaxies on one another.

Einstein gave up the cosmological constant after the American astronomer Edwin Hubble discovered that the universe was expanding and thus did not need stabilizing. But his fudge factor refused to die. It gained a new identity with the advent of quantum mechanics, the bizarre-sounding rules that govern the subatomic realm. According to those rules, empty space is not empty, but rather foaming with energy. Inserted into Einstein's equations, this energy would act like a cosmological constant, and try to blow the universe apart.

According to astronomers the recently discovered dark energy now accounts for about two-thirds of the mass of the universe. But is this Einstein's old fudge factor, the cosmological constant, come home to roost — in which case the universe will accelerate eternally? Or is the presumed acceleration only temporary, driven by one of the many mysterious force fields, dubbed quintessence, allowed by various theories of high energy physics?

Or is the acceleration even real?

"It's important to find out if the cosmological constant is really constant," said Dr. Witten.

Because the repulsive force resides in space itself, as the universe grows, the push from dark energy grows as well. "If dark energy is the cosmological constant then it is a property of the vacuum that will always be with us, getting more powerful as the universe gets bigger and the universe will expand forever," explained Dr. Adam Riess of the Space Telescope Science Institute in Baltimore. But if the dark energy is some form of quintessence, "then there may be more such fields which arise in the future, possibly of the opposite sign, and then all bets are off for the future of the universe."

Dr. Krauss said, "The good news is that we can't prove that this is the worst of all possible universes."

The Long Goodbye

It might seem strange or presumptuous for astronomers to try to describe events all the way to the end of time when physicists are still groping for a "theory of everything." But to Dr. Krauss, this is testimony to the power of ordinary physics. "We can still put ultimate limits on things without even knowing the ultimate theory," he said. "We can put limits on things based on ordinary physics."

Dr. Dyson said his venture into eschatology was inspired partly by a 1977 paper on the future of an ever expanding universe by Dr. J. N. Islam, now at the University of Chittagong in Bangladesh, in The Quarterly Journal of the Royal Astronomical Society. Dr. Dyson was also motivated, he wrote in his paper, to provide a counterpoint to a famously dour statement by Dr. Steven Weinberg, who wrote in his book "The First Three Minutes," "The more the universe seems comprehensible, the more it also seems pointless."

Dr. Dyson wrote, "If Weinberg is speaking for the 20th century, I prefer the 18th."

If the present trend of acceleration continues this is the forecast:

In about two billion years Earth will become uninhabitable as a gradually warming Sun produces a runaway greenhouse effect. In five billion years the Sun will swell up and die, burning the Earth to a crisp in the process. At about the same time the Milky Way will collide with its twin the Andromeda galaxy, now about two million light- years away and closing fast, spewing stars, gas and planets across intergalactic space.

Any civilization that managed to survive these events would face a future of increasing ignorance and darkness as the accelerating cosmic expansion rushes most of the universe away from us. "Our ability to know about the universe will decrease with time," said Dr. Krauss. "The longer you wait, the less you see, the opposite of what we always thought."

As he explains it, the disappearance of the universe is a gradual process. The faster a galaxy flies away from us, the dimmer and dimmer it will appear, as its light is "redshifted" to lower frequencies and energies, the way a police siren sounds lower when it is receding. When it reaches the speed of light, the galaxy will appear to "freeze," like a dancer caught in midair in a photograph, in accordance to Einstein's theory of relativity, and we will never see it get older, said Dr. Abraham Loeb, an astronomer at Harvard. Rather it will simply seem dimmer. The farther away an object is in the sky, he said, the younger it will appear as it fades out of sight. "There is a finite amount of information we can collect from the universe," Dr. Loeb said.

About 150 billion years from now almost all of the galaxies in the universe will be receding fast enough to be invisible from the Milky Way. The exceptions will be galaxies that are gravitationally bound to the cloud of galaxies, known as the Local Group, to which the Milky Way belongs. Within this cloud, life would look much the same at first. There would be galaxies in the sky. "When you look at the night the stars will still be there," said Dr. Krauss. "To the astronomer who wants to see beyond, the sky will be sadly empty. Lovers won't be disturbed — scientists will be."

But about 100 trillion years from now, when the interstellar gas and dust from which new stars condense is finally used up, new stars will cease to be born. From that time on, the sky will grow darker and darker. The galaxies themselves, astronomers say, will collapse in black holes within about 10³⁰ years.

But even a black hole is not forever, as Dr. Stephen Hawking, the Cambridge University physicist and best-selling author, showed in path-breaking calculations back in 1973. Applying the principles of quantum mechanics to these dread-sounding objects, Dr. Hawking discovered that a black hole's surface, its so-called event horizon, would fluctuate and exude energy in the form of random bursts of particles and radiation, growing hotter and hotter until the black hole eventually exploded and vanished.

Black holes the mass of the sun would take 10⁶⁴ years to explode. For black holes the mass of a galaxy those fireworks would light up space-time 1098 years from now.

Against the Fall of Night

Will there be anything or anyone around to see these quantum fireworks?

Dr. Dyson argued in his 1979 paper that life and intelligence could survive the desert of darkness and cold in a universe that was expanding infinitely but ever more slowly by adopting ever slower and cooler forms of existence. Intelligence, could reside, for example, in the pattern of electrically charged dust grains in an interstellar cloud, a situation described in the 1957 science fiction novel "The Black Cloud," by the British astronomer Sir Fred Hoyle, who died in August.

As an organism like the black cloud cooled, he argued, it would think more slowly, but it would always metabolize energy even more slowly, so its appetite would always be less than its output. In fact, Dr. Dyson concluded, by making the amount of energy expended per thought smaller and smaller the cloud could have an infinite number of thoughts while consuming only a finite amount of energy.

But there was a hitch. Even just thinking requires energy and generates heat, which is why computers have fans. Dr. Dyson suggested that creatures would have to stop thinking and hibernate periodically to radiate away their heat.

In an accelerating universe, however, there is an additional source of heat that cannot be gotten rid of. The same calculations that predict black holes should explode also predict that in an accelerating universe space should be filled with so-called Hawking radiation. In effect, the horizon — the farthest distance we can see — looks mathematically like the surface of a black hole. The amount of this radiation is expected to be incredibly small — corresponding to a fraction of a billionth of a billionth of a billionth of a degree above absolute zero, but that is enough to doom sentient life.

"The Hawking radiation kills us because it gives a minimum temperature below which you cannot cool anything," said Dr. Krauss. Once an organism cools to that temperature, he explained, it would dissipate energy at some fixed rate. "Since there is a finite total energy, this means a finite lifetime."

Infinity on Trial

Although Dr. Dyson agrees with this gloomy view of life in an accelerating universe, he and Dr. Krauss and Dr. Starkman are still arguing about whether life is also doomed in a universe that is not accelerating, but just expanding and getting slower and colder.

Quantum theory, the Case Western authors point out, limits how finely the energy for new thoughts can be shaved. The theory decrees that energy is emitted and absorbed in tiny indivisible lumps called "quanta." Any computation must spend at least this much energy out of a limited supply. Each new thought is a step down an energy ladder with a finite number of steps. "So you can only have a finite number of thoughts," said Dr. Krauss.

"If you want to stare at your navel and not think any new thoughts, you won't dissipate energy, " he explained. But that would be a boring way to spend eternity. If life is to involve more than the eternal reshuffling of the same data, he and Dr. Starkman say, it cannot be eternal.

Dr. Dyson, however, says this argument applies only to so-called digital life, in which there is a fixed number of quantum states. Creatures like the black cloud, which could grow along with the universe, he said, would have an increasing number of quantum states, and so there would always be more rungs of the ladder to step down. So the bottom need never be reached and life and thought could go on indefinitely.

But nobody knows whether such a life form can exist, said Dr. Krauss.

Compared with the sight of the World Trade Center towers collapsing or the plight of a sick child, this future extinction may seem a remote concern. Dr. Allan Sandage, an astronomer at Carnegie Observatories in Pasadena, Calif., who has spent his life investigating the expansion and fate of the universe, said: "Life on this earth is going to vanish in 4.5 billion years. I wouldn't get hung up on the fact that the lights are all going out in 30 billion years."

Dr. Dyson said he was still an optimist. It is too soon to start panicking, he counseled in an e-mail message. The observations could be wrong.

"At present all possibilities are open," he wrote. "The recent observations are important, not because they answer the big questions about the history of the universe, but because they give us new tools with which to explore the history."

Even in an accelerating universe, Dr. Dyson said, humans or their descendants might one day be able to rearrange the galaxies and save more of them from disappearing. Another glimmer of hope comes from the deadly and chilling Hawking radiation itself, said Dr. Raphael Bousso, from the Institute of Theoretical Physics at the University of California at Santa Barbara. Since that radiation is produced by unpredictable quantum fluctuations, he pointed out, if you wait long enough anything can appear in it, even a new universe. "Sooner or later one of those quantum fluctuations will look like a Big Bang," he said.

In that case there is the possibility of a future, if not for us, at least for something or somebody. In the fullness of time, after all, physics teaches that the improbable and even the seemingly impossible can become the inevitable. Nature is not done with us yet, nor, as Dr. Dyson indicates, are we necessarily done with nature.

We all die, and it is up to us to decide who and what to love, but, as Dr. Weinberg pointed out in a recent article in The New York Review of Books, there is a certain nobility in that prospect.

"Though aware that there is nothing in the universe that suggests any purpose for humanity," he wrote, "one way that we can find a purpose is to study the universe by the methods of science, without consoling ourselves with fairy tales about its future, or about our own."