Page last updated at 12:42 GMT, Wednesday, 17 December 2008

Light on the dark energy mystery

Galaxy cluster Abell 85 was one of 86 observed by the Chandra telescope

Astronomers have observed and analysed the effect of dark energy, the exotic form of energy thought to dominate the Universe, on galaxy clusters.

Using Nasa's Chandra x-ray space telescope, researchers watched the growth of dozens of clusters.

They say dark energy appears to be retarding the clusters' development.

Dark energy is not well understood, but scientists believe it is a repulsive force close to the "cosmological constant" proposed by Albert Einstein.

Another way of looking at it is that nothing - the vacuum of space - contains repulsive energy.

Observations over the last decade or so suggest this is accelerating expansion of the Universe.

"Putting all of this data together gives us the strongest evidence yet that dark energy is the cosmological constant, or in other words, that 'nothing weighs something'," said Alexey Vikhlinin, who led the research from the Smithsonian Astrophysical Observatory in Cambridge, US.

Whatever is forcing the expansion of the Universe to speed up is also forcing its development to slow down
Alexey Vikhlinin

"A lot more testing is needed, but so far Einstein's theory is looking as good as ever."

It appears that dark energy makes up about 70% of the energy in the Universe, with dark matter accounting for a further 25% and "normal" matter just 5%.

The fact that observations appear to tally with the notion of a cosmological constant is further confirmation that Einstein's general theory of relativity applies at large scales, the astronomers say.

"For years, scientists have wanted to start testing how gravity works on large scales," said William Forman, also from the Smithsonian observatory.

"And now, we finally have. This is a test that general relativity could have failed."

Arrested development

The Chandra telescope is one of four original space-based observatories envisaged by Nasa that would, together, cover the electromagnetic spectrum from the infra-red through visible light into x-rays and gamma-rays.

Launched in 1999, it is named after the Indian physicist Subrahmanyan Chandrasekhar who gained a Nobel prize for his work on the evolution of stars.

The current research project, conducted over many years, involved observing hot gas inside galaxy clusters. Some of those observed are relatively close in space, others are halfway across the Universe.

The clusters grew more slowly than would have been indicated by the visible galaxies. The explanation is that dark energy is stretching space.

"This result could be described as 'arrested development of the Universe'," said Dr Vikhlinin.

"Whatever is forcing the expansion of the Universe to speed up is also forcing its development to slow down."

The new findings can now be combined with other evidence on dark energy coming from study of supernovae, the cosmic microwave background and the way galaxies are distributed across the cosmos.



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