The astounding discovery that the expansion of the universe is speeding up won the Nobel physics prize on Tuesday for three astronomers whose observations of exploding stars transformed our view of the world, and of how it may end.

Honoring two global teams of stargazers who shook cosmology to its foundations in 1998, the Nobel Committee said Americans Saul Perlmutter, Brian Schmidt and Adam Riess had shown how the universe that emerged from the Big Bang may fly apart so far, cooling as it goes, that it will end in ice.

Their work gave birth to the theory of dark energy, a kind of inverse gravity, that causes the expansion to accelerate. Up to three quarters of the universe seems to comprise dark energy -- but just what it is a matter of speculation, notably at facilities like the Large Hadron Collider at Geneva. Many hope an answer could reconcile apparent anomalies in physics.

The teams studied dozens of exploding stars, or supernovae, expecting to confirm theories dating back to the 1920s that the universe has been expanding for 14 billion years since Big Bang but at an ever slower rate. The discovery came as a complete surprise, even to the laureates themselves, the panel said.

We ended up telling the world we have this crazy result -- the universe is speeding up, the Montana-born Schmidt, based in Australia, said by telephone to the Royal Swedish Academy of Sciences, where the 2011 prizewinners were announced.

It seemed too crazy to be right and I think we were a little scared, added Schmidt, 44, who led the High-z Supernova Search Team that included the Baltimore-based Riess, 41. Schmidt is at the Australian National University in Canberra.

Perlmutter, of the Supernova Cosmology Project, called the findings a huge surprise, that the world works in a very different way than you thought.

Speaking from the University of California at Berkeley, he hailed the global reach of both teams: Cosmology has brought together ideas and people from all over the world, said Perlmutter, who was born in 1959.

The chain of analysis was so long that at first we were reluctant to believe our result, Perlmutter said. But the more we analyzed it, the more it wouldn't go away.

ALL BETS OFF

Riess told Reuters he was stunned and incredibly honored by the award, though cautious about the prediction that the world would eventually freeze up into ice, without energy.

It is what we see, he said. But the truth is all bets are off. The universe could still recollapse.

Describing the feeling of having expectations contradicted, he said: If you tossed a ball into the air and it kept right on going up instead of falling to the ground, you'd be pretty surprised. Well, that's about how surprised we were.

With the assumption that gravity should slow the expansion of the universe debunked, the fact the opposite was true revived an idea Albert Einstein once rejected as his biggest blunder -- that vacuum of space might create anti-gravity.

Suddenly that idea made sense, Riess said.

He and Schmidt will share half of the 10 million Swedish crowns ($1.5 million) prize money. Perlmutter won the rest.

CURIOSITY DRIVEN

Swedish Academy member Lars Brink told Reuters practical developments from the findings were not obvious: This is very curiosity driven research, he said. It tells us something about the basic laws of nature. We are putting together pieces of what is the basic laws of nature. This is one brick.

It is not that we are going to use it for new gadgets.

Peter Knight, president of Britain's Institute of Physics, said: These researchers have opened our eyes to the true nature of our universe.

Mark Sullivan, a physicist at the University of Oxford, said: Their ... discovery ... has rewritten textbooks, and was one of the landmark breakthroughs of 20th-century physics.

Among exciting possible developments from the study of dark energy would be a way to reconcile anomalies between laws of physics observed at the subatomic level -- quantum mechanics -- with those Albert Einstein described for the world we see.

Martin Rees, Britain's Astronomer Royal, praised the prizewinners but criticized the Nobel Committee's rules that a maximum of three people could share in an award: It would have been fairer, and would send a less distorted message about how this kind of science is actually done, if the award had been made collectively to all members of the two groups, he said.

There was no repeat of the drama in Stockholm on Monday, when the Nobel Committee, whose rules forbid posthumous awards, discovered it had just given a share of the prize for medicine to a man who had died three days earlier. In the end, the award was confirmed to Ralph Steinman, who used his own discoveries to treat his cancer but succumbed to the disease on Friday.

In keeping with many recent prizes, the Committee noted, the winners of the physics category were all relatively young.

At Johns Hopkins University in Baltimore, Riess, who was still in his 20s when the research was published, joked to a colleague that he had been quick to react to a pre-dawn call from Stockholm: When I picked up the phone early this morning and I heard Swedish voices, he said, I knew it wasn't IKEA.