Saul Perlmutter Credit: Redux/eyevine

Three astrophysicists have been awarded a Nobel prize for planting a perplexing puzzle at the heart of cosmology.

Half of the Nobel Prize in Physics goes to Saul Perlmutter of Lawrence Berkeley National Laboratory in California for leading a team that discovered that the Universe is expanding at an ever-increasing rate (S. Perlmutter et al. Astrophys. J. 517, 565–586; 1999). Brian Schmidt of the Australian National University in Weston Creek and Adam Riess of the Space Telescope Science Institute in Baltimore, Maryland, share the other half of the prize for independent measurements of the cosmic acceleration (A. G. Riess et al. Astron. J. 116, 1009–1038; 1998), which researchers have struggled to explain ever since.

Brian Schmidt Credit: AAS/ID photographics/AAP Image

"I feel kind of weak in the knees," Schmidt told reporters in Sweden via telephone. "It sort of feels like when my children were born."

All three scientists reached their conclusions on the basis of measurements of distant Type Ia supernovae. These occur in very specific types of binary star system, in which a white dwarf star tears matter away from its partner until it gains enough mass to explode. At their peak, Type Ia supernovae always emit roughly the same amount of light, making them useful as 'standard candles' by which to measure vast distances across the cosmos.

Adam Riess Credit: Johns Hopkins Univ.

In the late 1980s and early 1990s, the prizewinners precisely measured the brightness of these supernovae using newly developed digital sensors. They then compared the brightness to the redshift — the change in colour of the light that results from the motion of the supernovae away from us. Both teams found that the supernovae were dimmer than expected at the measured redshift. The inescapable conclusion was that the Universe was not only expanding — which astronomers first realized in the 1920s — but expanding faster and faster.

Schmidt says that the finding was initially "pretty perplexing". Most astronomers had expected that the Universe's rapid growth following the Big Bang would gradually slow down as gravity pulled distant galaxies towards each other. Yet the discovery was accepted almost immediately by the astronomical community — in part because the idea of a cosmic pressure pushing the Universe outwards had already been mooted by Albert Einstein.

When Einstein applied his general theory of relativity to the Universe as a whole in 1917, his equations included a 'cosmological constant' which described just such an outward force. Over the past decade, observations of the large-scale structure of the Universe, together with the cosmic microwave background radiation — the faint afterglow of the Big Bang — have also indicated that the majority of the Universe's energy remains undetected. Today, the astronomical community accepts that about 73% of the Universe's energy is invested in this cosmic acceleration. Known as dark energy, it remains a largely mysterious force. "Nobody really knows what it is that has been discovered," says Peter Coles, an astrophysicist at Cardiff University, UK.

The predominant view is that dark energy results from quantum fluctuations in the vacuum of space, but efforts to use quantum theory to describe it have so far failed. Other theories, including modifications of gravity, have gained little acceptance.

"It could be none of the above," Coles says. But "we wouldn't be on the trail of this 'none-of-the-above' if it hadn't been for these experiments".