Three US-born scientists have been awarded the Nobel Prize for Physics after discovering that the universe is expanding at an ever-increasing rate.
Saul Perlmutter of Berkeley National Laboratory, along with Brian Schmidt of Australian National University, and Adam Riess of Johns Hopkins University together share the $1.5m prize.
Their study of exploding stars revealed that after the Big Bang, the universe continued to expand for several billion years, but at a slower rate than today.
In 1998, Perlmutter, Schmidt and Adam Riess presented findings that overturned the conventional idea that the expansion was slowing down 13.7 bn years after the big bang.
Their discovery raised a fundamental question: What is pushing the universe apart? Scientists have labelled it "dark energy," but nobody knows what it is.
That mystery is "an enigma, perhaps the greatest in physics today," the Nobel committee said.
Perlmutter, 52, of the Lawrence Berkeley National Laboratory and University of California, Berkeley, will receive half the $1.5m prize.
The other half will go to Schmidt, 44, at the Australian National University in Weston Creek, Australia, and Riess, 41, an astronomy professor at Johns Hopkins University and Space Telescope Science Institute in Baltimore.
Working in two teams, with Perlmutter heading one, they had raced to measure the universe's expansion by analysing light from dozens of exploding stars called supernovas. They found the light was weaker than expected, signalling that the expansion of the universe was accelerating.
"(It was) one of the truly great discoveries in the history of science, and one whose implications are not fully understood," said Paul Steinhardt, a physics professor at Princeton University.
One consequence of the finding is that in a trillion years, galaxies will be spread apart from each other by more than the current size of the universe, he said. And the ever-greater expansion rate means the light from
one galaxy will no longer be visible from another as it is today, he said.
"It's like changing from New York City to suddenly where everyone is spread out across some huge desert and there's nothing around to view," Steinhardt said.
Discovery a 'mistake'
Riess initially thought the prize-winning discovery was a mistake.
"I assumed I had made some mistakes and spent a long time analysing that and could not find a mistake," Riess said during a teleconference with reporters, adding that he then asked Schmidt and finally his research team to also review his work.
"None of us could really find what was wrong and at some point we decided maybe this was the way the universe really was, it wasn't slowing down it was speeding up," he said.
But while Riess shook off the disbelief that surrounded the early days of his discovery, he has since been confronted with an even larger mystery that astronomers and physicists are not sure how to solve.
"Really, we created a bigger question than we answered," said Riess. "We discovered that the universe is accelerating and it is filled with dark energy, but the question we created is, 'What is dark energy?' We don't
understand the physics of it.
"It seems to live at the nexus between quantum mechanics and general relativity, two of our great theories of physics, but it lives just at that nexus where they don't work together."
Perlmutter told AFP he too is grappling with the enormity of what they found, and did not find.
"It's a mysterious force... it may be three-quarters of all the stuff in the universe is this form and we did not know that before."