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'Dark energy' – the constant we can't live without
21 May 2007
From issue 2604 of New Scientist magazine
by Stuart Clark

It's like the gatecrasher who won't leave the party. The cosmological constant, the simplest explanation of why the expansion of the universe is accelerating, just won't go away, much as astronomers would like to see the back of it.

The cosmological constant originated as a "fudge factor" that Einstein added to general relativity to square its predictions with a universe that he thought was static. He later disowned it, when it was realised that the universe is expanding, but with the discovery in the late 1990s that this expansion is accelerating, the cosmological constant came back into favour.

In essence, the cosmological constant represents the "dark energy" of space-time that repulses gravity. There is one huge problem with it, though: the value for the cosmological constant predicted by particle physics is about 120 orders of magnitude greater than is consistent with the observed expansion of the universe.

So could there be another explanation for the universe's behaviour? Some have suggested that modifying general relativity to weaken gravity could explain the accelerated expansion, without the need for dark energy.

To check whether modified gravity is at work, Shen Wang of Brookhaven National Laboratory in Upton, New York, and colleagues from Columbia University, New York, looked at the large-scale structure of the universe today. Dark energy influences the growth of clusters of galaxies, so cluster formation can be used to estimate how much dark energy there is.

The researchers found that the cluster growth they observed needs about 70 per cent of the universe to be dark energy of the kind represented by the cosmological constant. This agrees very well with estimates of dark energy derived from supernova studies (www.arxiv.org/abs/0705.0165). Their conclusion: no modified gravity is necessary. So the cosmological constant wins out over modified gravity, this time round at least.

It's not yet a knock-out, though. Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena, thinks a definitive statement on whether modified gravity is needed to explain the accelerating universe will have to wait until cosmologists have collected more accurate data. "This is a good analysis, showing the kind of things that we can do with current data, but the story isn't over," he says. "They certainly haven't ruled out modified gravity as an alternative to dark energy."

The options have, however, become much more limited. Wang and his team say they have put tight constraints on a new theory of gravity: any tweak to the strength of gravity can only deviate from general relativity by less than 1 per cent.

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