The Universe Might Last Forever, Astronomers Say, but Life Might Not

	Barney Taxel for The New York Times Dr. Freeman Dyson, above, and Dr. Lawrence M. Krauss and Dr. Glenn D. Starkman, top, have investigated the future.

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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."