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Still-mysterious dark energy takes physics Nobel

Three cosmologists have shared a Nobel prize in physics for their discovery that the expansion of the universe is accelerating.

"The observation has changed our understanding of the universe," said physicist Olga Botner for the Nobel prize committee at a press conference in Stockholm, Sweden. "This discovery is fundamental and a milestone for cosmology."

The accelerated expansion has been attributed to the energy of space-time itself, dubbed dark energy. Dark energy creates a repulsive force that counters gravity and is now tearing apart space-time. It's "dark" because physicists don't know its exact nature.

Half of this year's prize goes to Saul Perlmutter of the Lawrence Berkeley National Laboratory in California, and the other half will be shared by Brian Schmidt of the Australian National University in Weston Creek, Australian Capital Territory, and Adam Riess of Johns Hopkins University in Baltimore, Maryland.

Too crazy to be right

In the late 1990s, Perlmutter's team and another team led by Schmidt and Riess were independently studying distant exploding stars known as type-Ia supernovae. They were using these supernovae to measure how the universe's expansion is changing with time. The expectation was that, billions of years after the big bang, it would be slowing down – but they found the opposite.

"It seemed too crazy to be right," said Schmidt, speaking to the Nobel committee press conference from his home in Canberra, Australia.

But it was right. The presence of dark energy has been independently backed up by measurements of the cosmic microwave background, the radiation left over from the big bang.

Dark choices

When asked about how it felt to receive a Nobel prize, Schmidt said, "It sort of feels like [when] my children were born, weak at the knees, amazed."

Given that the Nobel committee have awarded a prize for an observation concerning something we still don't quite understand, they were asked by the press why they didn't consider awarding the prize for the equally mysterious dark matter, the substance needed to explain why spinning galaxies don't fly apart.

In the early 1970s, Vera Rubin made observations of the velocities of stars that imply dark matter exists.

"With regards to Vera Rubin," said Botner, "yes, she was one of the people who is said to have discovered dark matter. She's not the only one. And there's a history behind it which still has to be investigated. It's not as clear-cut as today's prize."

Martin Rees, a cosmologist at the University of Cambridge, commended the choice of dark energy for the Nobel. "This award recognises an important and surprising discovery," he says. "Even empty space contains energy and exerts a kind of 'antigravity' which causes cosmic expansion to accelerate. It will be a long time before theorists understand this force – it is part of the bedrock nature of space and time."

He did lament, however, that the prize can't be shared between more than three individuals.


Saul Perlmutter, exploring the darkness <i>(Image: Lawrence Berkeley National Laboratory)</i>

Saul Perlmutter, exploring the darkness (Image: Lawrence Berkeley National Laboratory)


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