STOCKHOLM » Three U.S.-born scientists won the Nobel Prize in physics Tuesday for discovering that the universe is expanding at an accelerating pace, a stunning revelation that suggests the cosmos will eventually freeze to ice.

The Royal Swedish Academy of Sciences said American Saul Perlmutter would share the 10 million kronor ($1.5 million) award with U.S.-Australian Brian Schmidt and U.S. scientist Adam Riess. Working in two separate research teams during the 1990s — Perlmutter in one and Schmidt and Riess in the other — the scientists raced to map the universe’s expansion by analyzing a particular type of supernovas, or exploding stars.

They found that the light emitted by more than 50 distant supernovas was weaker than expected, a sign that the universe was expanding at an accelerating rate, the academy said.

"For almost a century the universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago," the citation said. "However the discovery that this expansion is accelerating is astounding. If the expansion will continue to speed up the universe will end in ice."

Perlmutter, 52, heads the Supernova Cosmology Project at the Lawrence Berkeley National Laboratory and University of California, Berkeley.

Schmidt, 44, is the head of the High-z Supernova Search Team at the Australian National University in Weston Creek, Australia.

Riess, 41, is an astronomy professor at Johns Hopkins University and Space Telescope Science Institute in Baltimore, Maryland.

Schmidt said he was just sitting down to have dinner with his family in Canberra, Australia, when the phone call came.

"I was somewhat suspicious when the Swedish voice came on," Schmidt told The Associated Press. "My knees sort of went weak and I had to walk around and sort my senses out."

The academy said the three researchers were stunned by their own discoveries — they had expected to find that the expansion of the universe was slowing down. But both teams reached the opposite conclusion: faraway galaxies were racing away from each other at an ever-increasing speed.

The discovery was "the biggest shakeup in physics, in my opinion, in the last 30 years," said Phillip Schewe, a physicist and spokesman at the Joint Quantum Institute, which is operated by the University of Maryland and the federal government.

"I remember everyone thinking at the time (that) there was some mistake," Schewe said. But there was no mistake, and in fact the basic finding was confirmed later by other measurements. Other scientists found evidence for it when they analyzed the microwave radiation left over from the big bang that still bathes the universe, he said.

Perlmutter told AP his team made the discovery in steps, analyzing the data and assuming it was wrong.

"And after months, you finally believe it," he said. "It’s not quite a surprise anymore. I tell people it’s the longest "ah-ha" experience that you’ve ever had."

An accelerating universe means it will get increasingly colder as matter is spread out across ever-vaster distances in space, said Lars Bergstrom, secretary of the Nobel physics committee. The acceleration is believed to be driven by an unknown cosmic power, called dark energy, one of the great mysteries of the universe.

Fred Dylla, executive director of the American Institute of Physics, said the prize confirmed an idea from Albert Einstein, called the cosmological constant, that Einstein inserted in his general theory of relativity, a cornerstone of modern physics.

Einstein later repudiated that idea as his "biggest blunder," but it did lead to a lot of theoretical and experimental studies, Dylla said.

The research implies that billions of years from now, the universe will become "a very, very large, but very cold and lonely place," said Charles Blue, spokesman for the American Institute of Physics.

In contrast to the big bang, that fate has been called the "big rip" to indicate how galaxies would be torn apart, he said. Galaxies will be flying away so quickly that their light could not travel across the universe to distant observers as it does today, making the sky appear black, he said.

The Nobel committee’s comment that the universe would "end in ice" is "an eloquent way of putting it," Blue said.

The physics prize was the second Nobel to be announced this year. On Monday the medicine prize went to American Bruce Beutler and French scientist Jules Hoffmann who shared it with Canadian-born Ralph Steinman for their discoveries about the immune system. Steinman died three days before the announcement, but since his death was not known to the committee, they decided he should keep the Nobel. Since 1974, Nobels have been awarded only to living scientists.

The prestigious Nobel Prizes were established in the will of Swedish industrialist Alfred Nobel, and have been handed out since 1901.

Last year’s physics award went to Russian-born scientists Andre Geim and Konstantin Novoselov for groundbreaking experiments with graphene, the strongest and thinnest material known to mankind.

The prizes are handed out every year on Dec. 10, on the anniversary of Nobel’s death in 1896.