The three are Saul Perlmutter and Adam Riess of the United States and US-Australian Brian Schmidt, who were honoured for findings that were -- to their own admission -- both a complete surprise and a little scary.[break]
The young trio looked at so-called type 1a supernovae to set down a benchmark for the movement of light on a cosmological scale.
This kind of supernova, also called a white dwarf, can in the matter of a few weeks emit as much light as an entire galaxy.
But to their astonishment, the laureates found through observations of more than 50 distant supernovae that light from the dying stars was weaker than expected, meaning they were further away than anticipated.
They concluded that, instead of slowing down as previously believed, the expansion of the Universe that began after it was created by the Big Bang some 14 billion years ago was accelerating.
"The discovery that this expansion is accelerating is astounding," the Nobel jury said, adding that if the acceleration continues "the Universe will end in ice."
Asked about the discovery, Riess, who at 41 is the youngest of the laureates, recalled first thinking he´d gotten the calculation wrong.
"I remember thinking ah, I made a terrible mistake and then spent weeks looking for it," he told the Nobel Prize website after being informed of his award.
It took a long time before he would "allow the possibility that the sign could be real and that the Universe could be accelerating," he said.
Schmidt, 44, also described to nobelprize.org how "we were frantically trying to sort out where we had gone wrong."
"It seemed too crazy to be right. We were a little scared," he acknowledged.
What may drive the acceleration is an enigmatic force called dark energy, thought to constitute about three-quarters of the Universe.
The jury compared the discovery to "throwing a ball up in the air, and instead of having it come back down, watching as it disappears more and more rapidly into the sky, as if gravity could not manage to reverse the ball´s trajectory."
"Something similar seemed to be happening across the Universe," it said.
Experts hailed the decision.
"Before the discovery ... astronomers had been attempting to measure its deceleration," Roger Davies, the president of the Royal Astronomical Society at Oxford University, and astrophysics professor Philip Wetton pointed out to AFP in an email.
"The discovery led to the realisation that empty space exerts a pressure that pushes the galaxies apart, something that demands new physics and a new understanding of space-time," they said.
Perlmutter, born in 1959, who heads the Supernova Cosmology Project at the Berkeley National Laboratory in California, won half of the 10 million Swedish kronor ($1.48 million, 1.08 million euros) prize.
Schmidt, who heads the High-z Supernova Search Team at the Australian National University (ANU), shared the other half with Riess, a professor of Astronomy and Physics at the Johns Hopkins University in Baltimore, Maryland.
When hearing the news, Schmidt told Swedish public broadcaster SVT winning "sort of feels like when my children were born."
Reached by AFP at his home in Canberra, the US-born physicist said the win "certainly was a big surprise," adding his success could largely be attributed to ANU and Australia, which allowed him at tha age of 27 to lead an international research team, and "all the support I got here as a very young person."
While Schmidt and Riess are considered young to have won a Nobel, they are far from being the youngest. That honour went to Laurence Bragg who won the 1915 prize for Physics at the age of just 25.
Perlmutter told Swedish public radio by phone the news of the prize was "wonderful."
"It´s also wonderful to hear that it´s shared with the other team, since it seems like such a community activity that we all did together," he added.
All three laureates shared the Shaw Prize in Astronomy (worth $1 million) in 2006. Riess and Perlmutter also won the Albert Einstein Medal in February this year.
The trio will receive their Nobel Prize at a formal ceremony in Stockholm on December 10, the anniversary of prize founder Alfred Nobel´s death in 1896.
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