Original Source: Anglo-Australian Observatory

March 21, 2002

New evidence that the expansion of the Universe is accelerating

A team of UK and Australian astronomers has discovered new, independent evidence that the expansion of the universe is accelerating, using results from a giant galaxy survey done with the 3.9-m Anglo-Australian Telescope near Coonabarabran in eastern Australia.

Credit: AAO/2dF

Their finding has been published in the Monthly Notices of the Royal Astronomical Society. The paper's lead author is Professor George Eftstathiou of the University of Cambridge. "The finding is significant because we've used a completely different line of investigation from the original, controversial result," said Dr Matthew Colless of the Australian National University, a co-leader of the 2dF Galaxy Redshift Survey that provided the new evidence.

Astronomers have known for decades that the Universe is expanding. Until 1998 they believed this expansion was slowing down, with the gravitational attraction of the Universe's matter gradually putting the brakes on.

But then two research teams found that instead the Universe is accelerating like a runaway car, expanding faster and faster as time goes on. This shock finding was based on the brightnesses of supernovae (exploding stars) in extremely distant galaxies.

To explain the result, cosmologists revived a concept first proposed by Einstein - 'dark energy' or the 'cosmological constant'. "The cosmological constant is really the springiness of spacetime," explained Dr Colless. "Spacetime wants to unfurl itself. The cosmological constant is a measure of how hard it's pushing." Einstein himself discarded the notion of a cosmological constant because it spoilt the simplicity and elegance of his General Theory of Relativity. Even in the wake of the supernovae teams' findings, some theoretical physicists were reluctant to revive the idea.

Now Professor George Efstathiou of the University of Cambridge, a member of the 27-strong galaxy survey team, has come up with a completely different line of evidence that supports the supernovae finding.

He and his colleagues looked at the clustering pattern of 220,000 galaxies in a large volume of the universe surveyed with the Anglo-Australian Telescope at Siding Spring Observatory in New South Wales, Australia. This pattern represents the 'structure' in the Universe now, some 15 billion years after the Big Bang. The researchers then compared this structure with the clumpiness in the cosmic microwave background radiation, which shows the structure the Universe had when it was only 150,000 years old.

"By looking at how the early structure evolves into the structure we see today, we can calculate the cosmological constant," Dr Colless said.

"It seems that Einstein did not made a blunder after all - dark energy appears to exist and to dominate over more conventional types of matter," said Professor Efstathiou. "An explanation of the dark energy may involve String Theory, extra dimensions or even what happened before the Big Bang. At present nobody knows. The ball is now firmly in the theorists' court."

Credit: AAO/2dF

Designed and built by the Anglo-Australian Observatory, the 2dF (two-degree field) instrument is one of the world's most complex astronomical instruments, able to capture 400 spectra simultaneously. A robot arm positions up to 400 optical fibres on a field plate, each to within an accuracy of 20 micrometres. Light from up to 400 objects is collected and fed into two spectrographs for analysis. The expansion of the Universe shifts galaxy spectra to longer wavelengths. By measuring this 'redshift' in a galaxy's spectrum, the galaxy's distance can be determined.

The 2dF survey covers a total area of about 2 000 square degrees, selected from both northern and southern skies.

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see also:

• 2dF Galaxy Redshift Survey