Space

EVER since Arthur Eddington travelled to the island of Príncipe off Africa to measure starlight bending around the sun during a 1919 eclipse, evidence for Einstein's theory of general relativity has only become stronger. Could it now be that starlight from distant galaxies is illuminating cracks in the theory's foundation?

Everything from the concept of the black hole to GPS timing owes a debt to the theory of general relativity, which describes how gravity arises from the geometry of space and time. The sun's gravitational field, for instance, bends starlight passing nearby because its mass is warping the surrounding space-time. This theory has held up to precision tests in the solar system and beyond, and has explained everything from the odd orbit of Mercury to the way pairs of neutron stars perform their pas de deux.

Yet it is still not clear how well general relativity holds up over cosmic scales, at distances much larger than the span of single galaxies. Now the first, tentative hint of a deviation from general relativity has been found. While the evidence is far from watertight, if confirmed by bigger surveys, it may indicate either that Einstein's theory is incomplete, or else that dark energy, the stuff thought to be accelerating the expansion of the universe, is much weirder than we thought (see "Not dark energy, dark fluid").

The analysis of starlight data by cosmologist Rachel Bean of Cornell University in Ithaca, New York, has generated quite a stir. Shortly after the paper was published on the pre-print physics archive, prominent physicist Sean Carroll of the California Institute of Technology in Pasadena praised Bean's research. "This is serious work by a respected cosmologist," he wrote on his blog Cosmic Variance. "Either the result is wrong, and we should be working hard to find out why, or it's right, and we're on the cusp of a revolution."

"It has caused quite a furore in astronomy circles," says Richard Massey of the Royal Observatory Edinburgh in the UK. "This paper has generated a lot of interest."

Bean found her evidence lurking in existing data collected by the Cosmic Evolution Survey, a multi-telescope imaging project that includes the longest survey yet by the Hubble Space Telescope. COSMOS, which detected more than 2 million galaxies over a small patch of sky, takes advantage of gravity's ability to bend light. Massive objects like galaxy clusters bend the light of more distant objects so that it is directed towards or away from Earth. This effect, called gravitational lensing, is at its most dramatic when it creates kaleidoscopic effects like luminous rings or the appearance of multiple copies of a galaxy.

The sky is also dominated by the distorting effects of "weak lensing", in which intervening matter bends light to subtly alter the shapes and orientations of more distant galaxies, creating an effect similar to that of looking through old window glass. Since galaxies come in all shapes and sizes, it is difficult to know whether the light from an individual galaxy has been distorted, because there is nothing to compare it with. But by looking for common factors in the distortion of many galaxies, it is possible to build up a map of both the visible and even unseen matter that bend their light.

The weak lensing technique can also be used to measure two different effects of gravity. General relativity calls for gravity's curvature of space to be equivalent to its curvature of time. Light should be influenced in equal amounts by both.

When the COSMOS data was released in 2007, the team - led by Massey - assumed these two factors were equivalent. Their analysis revealed that gravitational tugs on light were stronger than anticipated, but they put this down to a slightly higher concentration of ordinary and dark matter in the survey's patch of sky than had been predicted.

To look for potential deviations from general relativity, Bean reanalysed the data and dropped the requirement that these two components of gravity had to be equal. Instead the ratio of the two was allowed to change in value. She found that between 8 and 11 billion years ago gravity's distortion of time appeared to be three times as strong as its ability to curve space. An observer around at the time wouldn't have noticed the effect because it only applies over large distances. Nonetheless, "there is a preference for a significant deviation from general relativity", says Bean (www.arxiv.org/abs/0909.3853).

At this stage, it's hard to say what would happen if the deviation from general relativity was confirmed. Cosmologists have already considered some modifications to general relativity that could explain the universe's acceleration (see "Not dark energy, dark fluid").

Yet finding a deviation when the universe was less than half its current age is odd - if general relativity had broken down at some level, the signs should be most dramatic more recently, long after the repulsive effect of dark energy overwhelmed the attractive powers of gravity some 6 billion years ago.

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