New light shines on dark mystery
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The science world’s highest international honour was awarded in October to the leaders of two research teams that identified a mysterious and potentially profound phenomenon known as dark energy, an unseen force driving the universe apart at an ever-increasing rate.

The Nobel Prize in Physics 2011 was divided, with one half awarded to Saul Perlmutter and the other half jointly to Brian P Schmidt and Adam G Riess, for the discovery of the accelerating expansion of the universe through observations of distant supernovae.

The force driving this acceleration was dubbed ‘dark energy’ because it sheds no visible light that can be seen by telescopes, and was only able to be inferred from its influence on galaxies and galactic clusters. Its discovery, in 1998, ended the previously accepted principle that the universe was slowing in its expansion due to the mutual gravitational attraction of the matter within it, as might be the case if it was driven by a single explosive event (the Big Bang) and restrained by gravity.

Dr Chris Blake, of Swinburne’s Centre for Astrophysics and Supercomputing (CAS), is part of an international team that has produced new results in agreement with the Nobel-prize-winning theory.

“The discovery of dark energy was a shock, because it clearly said we didn’t understand as much about the universe and physics as we had thought,” Dr Blake says.

A galaxy of scientists

Just as the original breakthrough called upon multiple minds, the latest dark energy work is a rich international collaboration, involving 26 researchers from Australia and overseas led by Dr Chris Blake, Professor Warrick Couch and Professor Karl Glazebrook, from Swinburne, and Professor Michael Drinkwater from the University of Queensland.

Four years ago the team embarked on the Wiggle Z Dark Energy Survey using a powerful spectrograph attached to the Anglo–Australian Observatory (AAO) at Siding Spring, NSW. With this they captured light from 392 distant galaxies every hour – a total of 200,000 across seven billion light years, stretching halfway to the limits of the known universe.

For Professor Couch, director of CAS, the survey was a continuation of work he began in Australia in 1987 as a member of Professor Perlmutter’s team, searching for distant supernovae.

The resulting accelerating universe discovery was made using supernovae as ‘standard candles’ (objects of known absolute brightness) to measure distances within the universe.

“It was important to check the supernova results for two reasons: first, because the conclusion was so surprising, and second, because we wanted to confirm that supernovae are truly standard candles across cosmic time – perhaps their nature is changing with time, or we are being confounded by cosmic dust,” Dr Blake says.

While its presence is confirmed, dark energy remains a fundamental mystery.

“It appears to be the dominant force on our universe – yet we have absolutely no idea what causes it. It appears to be evenly distributed, everywhere,” Dr Blake says.

“Our survey shows this dark energy acting in two distinct ways. First, we observed how dark energy opposes gravity by speeding up the overall rate of expansion of the universe. Second, we observed how dark energy opposes gravity by slowing down the growth of clusters and super-clusters of galaxies with time.”

Getting the universe’s measure

To assess the distances between galaxies as they move further apart, the team took advantage of the known ‘preference’ of galaxies to operate in pairs about 490 million light years distant from one another. This provides a useful ruler for comparing other cosmic distances.

To measure all this required the creation of a vast three-dimensional map of the universe out to seven billion light years – more than halfway to its known edge and twice the scale of any previous map.

“There are only two ways to explain the accelerating expansion of the universe,” Professor Karl Glazebrook says.
“One involves a fundamental rethink of the theory of relativity and the second is that there is a new, smooth material unlike any other we know, pushing it apart. Our observations strongly support the latter view.”

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