DARK energy, the elusive force that seems to be pulling the Universe apart, may not exist after all. Two physicists say that rather than being caused by an extra force, the Universe’s speeding expansion could be explained by gravity leaking out into a huge extra dimension, weakening its effect over long distances.
Astronomers originally thought that since the early moments after the big bang, the rate of the Universe’s expansion must have been falling. But in 1998, they found that the light from distant supernovae was dimmer than expected, implying that space is accelerating outwards. The findings proved to be a headache for physicists, who have been struggling ever since to explain what force could be counteracting gravity and pushing the matter in the Universe apart.
One alternative explanation to dark energy might be that gravity is somehow weaker than expected over long distances. But until recently no one had been able to say how or why this might happen. Gia Dvali and colleagues of New York University say they have an answer – gravity is leaking out into extra dimensions. Small extra dimensions wouldn’t do the job, as they would only affect gravity over very short distances. To affect gravity over such huge scales, the extra dimensions would have to be infinite in volume. So the NYU group started looking at a class of theories called “brane-world” scenarios, which see the Universe as a membrane floating on one or more extra, infinite dimensions of space.
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Now Dvali and and Michael Turner of the University of Chicago in Illinois have taken Einstein’s equations of gravity, which tell us how the expansion rate of the Universe depends on the density of matter and energy within it, and tweaked them to account for the effects of these extra dimensions.
When they ran the equations for just one extra dimension, they found that the Universe the equations predicted accelerates just like our own (; ). “In our approach there is no dark energy, the effect of dark energy is completely imitated by gravity itself,” explains Dvali.
Although the brane-world scenario might seem far-fetched, the cause of the acceleration is likely to be something very exotic, says Turner, so it’s important to get different ideas on the table. “We need to think outside the box. This is thinking outside the Universe,” he says.
Other experts are also open to the idea. “We have no idea what’s going on at this point, so it’s definitely worth exploring the possibilities,” says Harvard University’s Lisa Randall, who specialises in theories that involve extra dimensions.
A proposed satellite called the SuperNova/Acceleration Probe (SNAP), which could launch as soon as 2009, will make more detailed observations of distant supernovae that could test the theory, says Dvali. Extremely sensitive laser measurements of the Earth-Moon distance might also be able detect whether the effects of gravity are modified in the way that Dvali and Turner predict.