A screen projection shows 2011 Nobel Prize for physics laureates Saul Perlmutter, Brian Schmidt and Adam Riess during the announcement of the prize at the Royal Swedish Academy of Sciences in Stockholm October 4, 2011.  [Photo/Agencies]

STOCKHOLM - Three scientists shared the 2011 Nobel Prize for physics for the stunning discovery that the expansion of the universe is speeding up, meaning it may one day turn to ice, the prize committee said on Tuesday.

Scientists have known since the 1920s that the universe is expanding, as a result of the Big Bang some 14 billion years ago, but the discovery that this process is accelerating - and not slowing as many thought - rocked the research community.

"If the expansion will continue to speed up, the universe will end in ice," the Nobel committee said in a statement.

Half of the 10 million Swedish crown ($1.5 million) prize money went to American Saul Perlmutter and the rest to two members of a second team which conducted similar work - US-born Brian Schmidt, who is based in Australia, and American Adam Riess.

"We ended up telling the world we have this crazy result, the universe is speeding up," Schmidt told a news conference by telephone after the award was announced in Stockholm.

"It seemed too crazy to be right and I think we were a little scared," he added.

Nobel Committee for Physics at the Royal Swedish Academy of Sciences said in its statement that the discovery was made by looking at distant, exploding stars.

Instead of their light becoming brighter, it was fading.

"The surprising conclusion was that the expansion of the universe is not slowing down. Quite to the contrary, it is accelerating," the committee said.

The acceleration is thought to be driven by dark energy, although cosmologists have little idea what that is.

They estimate that dark energy - a kind of inverse gravity, repelling matter that comes close to it - accounts for around three quarters of the universe.  ($1 = 6.879 Swedish Crowns)