© X-ray: NASA/CXC/MIT/E.-H Peng et al; Optical: NASA/STScIStarlight behaving oddly
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 below).
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.
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.
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.
Most astronomers, including Bean, are cautious about the results. "Nobody is yet betting money that the effect is real," says cosmologist Dragan Huterer of the University of Michigan in Ann Arbor. Various other explanations, like a bias in the technique used to estimate the distances to galaxies, now need to be ruled out.
Although COSMOS photographed a deep patch of sky, it was fairly small by the standards of modern surveys. This opens up the possibility that this region might be anomalous, notes Asantha Cooray, an astrophysicist at the University of California, Irvine. "You could have a massive galaxy cluster that could boost your weak lensing signal up. Or by random chance you could have more dark matter," says Cooray, part of a team that analysed other survey data taken with the Canada-France-Hawaii Telescope in Hawaii and found no hint of a departure from general relativity. "The only way to take that into account is to look at data in a larger field."
Future projects will scan the sky over much wider areas and collect images of many more lensed galaxies. For example, the Dark Energy Survey is poised to start surveying the sky from 2011 and will build up an even more precise picture of how light has been bent over the course of the universe's history.
Whether these surveys find the effect or not, Bean hopes that her paper will generate more interest in the idea of using weak lensing to test general relativity. "I'm not putting my flag out there and saying this is a real thing," Bean says. "We need to look at more data sets. This is really just the first stage for trying to test gravity in this way."
Massey agrees: "At the moment we're in the mode of just trying to hack into general relativity to find the chinks in its armour, to find any places where it might not be working."
Not dark energy, dark fluid
Dark energy could be weirder than we thought. Evidence that over large distances gravity exerts a greater pull on time than on space might not necessarily suggest that the theory of general relativity is wrong. It could instead be a sign that the universe's acceleration may require a more exotic explanation.
The simplest way of explaining the universe's acceleration is to invoke a cosmological constant, originally proposed by Einstein to allow the universe to remain the same size in the presence of matter. This describes a universe filled with uniform, outward-pushing energy. But there are other possible explanations for acceleration.
One idea is that the entire universe exists on a membrane, or brane, floating inside an extra dimension. While matter will be confined to three dimensions, gravity could be leaking into this extra dimension. When the universe becomes large enough, this gravity could interact with matter in the brane, to produce acceleration on large scales.
A deviation could also be a sign that dark energy is a more complex "fluid" that exerts varying pressures in different directions. The snag is that telling the difference between a more exotic form of dark energy and a modification to our understanding of gravity could be tricky.
"If we were to detect a departure," says cosmologist Alessandra Silvestri of the Massachusetts Institute of Technology, we might not be able to tell whether there is a flaw in general relativity or just evidence that dark energy is "some sort of fancy fluid".
"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."
The theory of relativity puts time and space together into space-time, and considers all frames of reference as equivalent. This is contrary to the evidence that tells us that there are distinguished frames of reference. Relativists would argue that "all frames are equivalent in an empty space". Yet how can there be an empty space? This is contrary to the spirit of relativity itself that tells us that it is the matter in the universe that is responsible for time and space properties.
Of course relativists will try to twist it with their twists, yet they do not really believe in what they say. They know something is very wrong, but can't agree what exactly.