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

By DENNIS OVERBYE

	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.

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."