LONDON: A major survey of more than 200,000 galaxies has provided the strongest evidence so far for the existence of mysterious 'dark energy', according to Australian astronomers.

Proof that this unknown quantity is real would revolutionise our understanding of the laws of physics, explaining why the universe appears to be expanding at an increasing rate and will tell us if, when, and how it might end. It could also resolve a century old 'mistake' in Einstein's Theory of Relativity.

The latest results from the WiggleZ Dark Energy Survey were derived from data collected by the Anglo-Australian Telescope in central New South Wales and are to be published in the Monthly Notices of the Royal Astronomical Society.

"WiggleZ says dark energy is real," said lead author Chris Blake, from Swinburne University's Centre for Astrophysics and Supercomputing in Melbourne. "Einstein remains untoppled."

Hunting in the dark

The search for dark energy began in the late 1990s when scientists studying exploding stars discovered the expansion of the universe was speeding up. They proposed that a force attributable to 'dark energy' was causing this acceleration by overcoming the pull of gravity between galaxies.

But although this material is now thought to be the main component of our universe, scientists still have no physical understanding of its existence or magnitude.

The WiggleZ survey examined a vast number of galaxies in unprecedented detail, looking back in time 7 billion years to provide new and compelling evidence that dark energy exists as an anti-gravitational agent.

392 galaxies per hour

"We have been able to track the effects of dark energy further back into the history of the Universe than ever before," explained Blake.

A team of 26 astronomers used the Anglo-Australian telescope located in Epping, Sydney, to study nearly 240,000 galaxies over hundreds of nights of observation between 2006 and 2010.

The success of the project depended on the use of cutting edge technology, in the form of a powerful spectrograph able to image 392 galaxies an hour.

A cosmological constant

By collecting two sets of data on the pattern of galaxy distribution in space and the formation of clusters of galaxies over time, the group found proof of dark energy speeding up the expansion of the universe and slowing down the attraction between matter.

"According to the standard model of the universe described by Einstein's equations, our data demonstrates the existence of dark energy as a real material using these twin methods," said Blake. "Unlike any known form of matter or energy, it counters the force of gravity via its uncontrollable desire to expand."

The results serve as independent confirmation of the previous observations from exploding stars.

They also provide an explanation for a constant in Einstein's Theory of Relativity that the genius himself discarded as a mistake. "Dark energy is a real 'cosmological constant' filling the universe, rather than a failure of the laws of gravity," explained Blake.

Study "does not prove dark energy exists"

"This is a remarkable survey, and a convincing fit to the standard hypothesis of a homogeneous cosmology that includes Einstein's cosmological constant," said Thomas Buchert, a professor of cosmology at Claude Bernard University, France.

"However, it does not rule out alternatives to the standard model, and it does not prove that dark energy 'exists'."

This view is also taken by David Wiltshire, a theoretical cosmologist from the University of Canterbury in New Zealand. "If one makes the assumption that the universe expands as a uniform fluid, then these measurements are independent evidence for dark energy," he said.

"However, I see no reason for making the standard assumptions." Wiltshire himself prefers an alternative model of the universe that has no need for dark energy.

A tough test ahead

This does not, however, reduce the importance of WiggleZ. The vast amount of data from the survey could be extremely valuable for testing competing theories, including those proposed by Buchert and Wiltshire.

This would require revisiting decades of work, but it is a challenge considered worthy by Blake.

"I completely agree that other interpretations of our data are possible, if we abandon the usual assumptions. Our data is perfect for testing these models," he said. "I think it will be a tough test but we need to make those comparisons."