In 1998, two teams of astronomers —one led by the Australian National University’s Brian Schmidt—independently reached the same conclusion: the expansion of the Universe is not slowing down or petering out, as most people had assumed, it is accelerating.

The discovery has triggered a flurry of activity to understand more about dark energy, the hypothetical driving force pushing the Universe apart counteracting gravity. It has also brought the Nobel Prize for Physics to Schmidt, Adam Riess, and Saul Perlmutter.

Our expanding Universe

“It initially seemed a crazy result,” says Brian Schmidt, the leader of one of the two teams who made the discovery that the expansion of the Universe is accelerating. “But we were confident we’d eliminated any errors.”

To calculate the Universe’s rate of expansion, both teams were studying Type Ia supernovae— distant stellar explosions that all appear to have the same intrinsic brightness—as a means of measuring distances across the cosmos. The further away the star, the fainter the stellar explosion appears to us. By combining those distances with the supernovae’s redshifts, where light from receding stars is shifted towards the red end of the spectrum, the astronomers could gauge how fast the Universe was expanding at different stages of its life. Nearby objects, whose light has only been travelling through the Universe for millions of years, were compared to distant objects, the light of which had traversed the Universe for billions of years.

The researchers found that the expansion of the Universe is speeding up. But why? The answer could be ‘dark energy’, a hypothetical energy that fills space and opposes gravity. But its nature remains a mystery.

“We don’t know what it is,” says Brian. “And there are still lingering questions of whether it could be that Einstein’s general relativity equations are wrong in some weird way. One of the things we will be doing over the next 5-10 years here in Australia is testing how gravity works over long distances. If gravity works a little differently to how we think it does, we should get a different answer.”

For their theory-shattering result, Schmidt and his team, along with the second team lead by Saul Perlmutter at the University of California, Berkeley, have not only been recognised by the Nobel committee. They have also won the US$500,000 Gruber Cosmology Prize in 2007 and the US$1 million Shaw Prize in 2006 amongst many honours.

PHOTO 1: TYCHO’S SUPERNOVA REMNANT. CREDIT: NASA/MPIA/CALAR ALTO OBSERVATORY, OLIVER KRAUSE ET AL.

PHOTO 2: PRIME MINISTER JULIA GILLARD CONGRATULATES BRIAN SCHMIDT ON HIS 2011 NOBEL PRIZE FOR PHYSICS. CREDIT: PRIME MINISTER’S SCIENCE PRIZES/IRENE DOWDY.

Research School of Astronomy and Astrophysics, Australian National University, Canberra
Professor Brian Schmidt, Tel: +61 (2) 6125 8042 brian@mso.anu.edu.au, www.mso.anu.edu.au

Also in this section:

Galaxies point the way to dark energy
Spinning galaxies reveal missing matter
Ten times more galaxies
Measuring the Universe from start to finish