Image: 2dFGRS.

A team of UK and Australian astronomers has discovered new, independent evidence that the expansion of the universe is accelerating. Their findings have just appeared in the Monthly Notices of the Royal Astronomical Society.

Three years ago, two teams of astronomers rocked the scientific world by finding evidence that the expansion of the universe is accelerating rather than slowing down as had generally been expected because of the gravitational attraction between the matter within it. According to these groups, the brightnesses of supernovae (massive exploding stars) they observed in remote galaxies require a universe filled with a strange kind of dark energy that causes it to accelerate increasingly faster into the infinite future.

The concept of dark energy was first postulated by Einstein (who called it 'the cosmological constant'). But Einstein later referred to this idea as his greater scientific blunder since it spoilt the simplicity and elegance of his General Theory of Relativity. Since then, the cosmological constant has had a controversial history. The great Cambridge astronomer Sir Arthur Stanley Eddington was convinced of its existence, arguing that the cosmological constant distinguished between the vast size of the observable universe and the tiny scales of subatomic particles. But to most theoretical physicists the cosmological constant has seemed utterly mysterious and unnecessary, and many have been reluctant to accept the results of the supernovae teams.

Now, a team of 27 astronomers led by Professor George Efstathiou of the University of Cambridge has published strong evidence for the existence of dark energy using an entirely different technique. They used the clustering pattern of 250,000 galaxies in a large volume of the universe surveyed with the Anglo-Australian Telescope at Siding Spring in New South Wales, Australia. By comparing the structure in the universe now, some 15 billion years after the Big Bang, with structure observed in the cosmic microwave background radiation, which preserved information about what the universe was like when it was only 300,000 years old, the Anglo-Australian team could apply a simple geometrical test to elucidate the composition of the universe.

Their results show that the universe is full of dark energy, completely consistent with the earlier supernovae results. "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" says 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."