From left, Adam Riess, Saul Perlmutter and Brian Schmidt shared the Nobel Prize in physics awarded Tuesday. Credit Johns Hopkins University; University Of California At Berkeley; Australian National University

Three astronomers won the Nobel Prize in Physics on Tuesday for discovering that the universe is apparently being blown apart by a mysterious force that cosmologists now call dark energy, a finding that has thrown the fate of the universe and indeed the nature of physics into doubt.

The astronomers are Saul Perlmutter, 52, of the Lawrence Berkeley National Laboratory and the University of California, Berkeley; Brian P. Schmidt, 44, of the Australian National University in Canberra; and Adam G. Riess, 41, of the Space Telescope Science Institute and Johns Hopkins University in Baltimore.

“I’m stunned,” Dr. Riess said by e-mail, after learning of his prize by reading about it on The New York Times’s Web site.

The three men led two competing teams of astronomers who were trying to use the exploding stars known as Type 1a supernovae as cosmic lighthouses to limn the expansion of the universe. The goal of both groups was to measure how fast the cosmos, which has been expanding since its fiery birth in the Big Bang 13.7 billion years ago, was slowing down, and thus to find out if its ultimate fate was to fall back together in what is called a Big Crunch or to drift apart into the darkness.

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Instead, the two groups found in 1998 that the expansion of the universe was actually speeding up, a conclusion that nobody would have believed if not for the fact that both sets of scientists wound up with the same answer. It was as if, when you tossed your car keys in the air, instead of coming down, they flew faster and faster to the ceiling.

An exploding star known as Type 1a supernova. The Nobel prize winners used them to measure the expansion of the universe. Credit NASA, via Agence France-Presse — Getty Images

Subsequent cosmological measurements have confirmed that roughly 70 percent of the universe by mass or energy consists of this antigravitational dark energy that is pushing the galaxies apart, though astronomers and physicists have no conclusive evidence of what it is.

The most likely explanation for this bizarre behavior is a fudge factor that Albert Einstein introduced into his equations in 1917 to stabilize the universe against collapse and then abandoned as his greatest blunder.

Quantum theory predicts that empty space should exert a repulsive force, like dark energy, but one that is 10 to the 120th power times stronger than what the astronomers have measured, leaving some physicists mumbling about multiple universes. Abandoning the Einsteinian dream of a single final theory of nature, they speculate that there are a multitude of universes with different properties. We live in one, the argument goes, that is suitable for life.

“Every test we have made has come out perfectly in line with Einstein’s original cosmological constant in 1917,” Dr. Schmidt said.

If the universe continues 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 and all the energy will be sucked out of the universe.

Edward Witten, a theorist at the Institute for Advanced Study, Einstein’s old stomping grounds, called dark energy “the most startling discovery in physics since I have been in the field.” Dr. Witten continued, “It was so startling, in fact, that I personally took quite a while to become convinced that it was right.”


TimesCast | Nobel Nod to Dark Energy


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He went on, “This discovery definitely changed the way physicists look at the universe, and we probably still haven’t fully come to grips with the implications.”

Dr. Perlmutter, who led the Supernova Cosmology Project out of Berkeley, will get half of the prize of 10 million Swedish kronor ($1.4 million). The other half will go to Dr. Schmidt, leader of the rival High-Z Supernova Search Team, and Dr. Riess, who was the lead author of the 1998 paper in The Astronomical Journal, in which the dark energy result was first published.

All three astronomers were born and raised in the United States; Dr. Schmidt is also a citizen of Australia. They will get their prizes in Stockholm on Dec. 10.

Since the fate of the universe is in question, astronomers would love to do more detailed tests using supernovas and other observations. So they were dispirited last year when NASA announced that cost overruns and delays on the James Webb Space Telescope had left no room in the budget until the next decade for an American satellite mission to investigate dark energy that Dr. Perlmutter and others had been promoting for almost a decade. Indeed on Tuesday the European Space Agency announced that it would launch a mission called Euclid to study dark energy in 2019.

Cosmic expansion was discovered by Edwin Hubble, an astronomer at the Mount Wilson Observatory in Pasadena, Calif., in 1929, but the quest for precision measurements of the universe has been hindered by a lack of reliable standard candles, objects whose distance can be inferred by their brightness or some other observable characteristic. Type 1a supernovae, which are thought to result from explosions of small stars known as white dwarfs, have long been considered uniform enough to fill the bill, as well as bright enough to be seen across the universe.

In the late 1980s Dr. Perlmutter, who had just gotten a Ph.D. in physics, devised an elaborate plan involving networks of telescopes tied together by the Internet to detect and study such supernovae and use them to measure the presumed deceleration of the universe. The Supernova Cosmology Project endured criticism from other astronomers, particularly supernova experts, who doubted that particle physicists could do it right.


Saul Perlmutter on Dark Energy


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Indeed, it took seven years before Dr. Perlmutter’s team began harvesting supernovae in the numbers it needed. Meanwhile, the other astronomers had formed their own team, the High-Z team, to do the same work.

“Hey, what’s the strongest force in the universe?” Robert P. Kirshner of the Harvard-Smithsonian Center for Astrophysics, and a mentor to many of the astronomers on the new team, asked a reporter from this newspaper once. “It’s not gravity, it’s jealousy,” Dr. Kirshner said.

In an interview with The Associated Press, Dr. Perlmutter described the subsequent work of the teams as “a long aha.” The presence of dark energy showed up in an expected faintness on the part of some distant supernovae: the universe had sped up and carried them farther away from us than conventional cosmology suggested.

As recounted by the science writer Richard Panek in his recent book, “The 4% Universe, Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality,” neither team was eager to report such a strange result.

In January 1998, Dr. Riess interrupted preparations for his honeymoon to buck up his comrades. “Approach these results not with your heart or head but with your eyes,” he wrote in an e-mail. “We are observers after all!”

In the years since, the three astronomers have shared a number of awards, sometimes giving lectures in which they completed one another’s sentences. A Nobel was expected eventually.

“No more waiting!” Dr. Kirshner said Tuesday.

Correction: October 4, 2011

An earlier version of this article incorrectly stated the publication in which Adam G. Riess's 1998 paper on dark energy appeared. It was The Astronomical Journal, not Science. The article also stated incorrectly the amount of the prize. It is 10 million Swedish kronor ($1.4 million).

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