Published online 1 February 2008 | Nature | doi:10.1038/news.2008.549

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Extra dimensions in spacetime could trigger blast.

The despite being too small to see, the little brother of conventional black holes such as this may be a source of cosmic explosions. NASA Goddard Space Flight Center

A group of radio astronomers has begun looking for signals from small, exploding black holes. The search is a long shot, but if it finds anything, it could be the best evidence yet for extra dimensions beyond the paltry four that we live in.

The Universe as we see it is made up of three dimensions of space and one of time, but some theories predict that many more dimensions exist beyond the ones we experience. In particular, string theory, which suggests that the Universe is composed of tiny, vibrating 'strings', suggests that several extra dimensions might exist beyond our own.

To date, observations have failed to turn up these dimensions, and most theorists suspect that if the dimensions do exist, they are 'curled up' in tiny closed loops around a billionth of a nanometer (10-18 m) in size. Some theorists believe that the Large Hadron Collider, a giant particle accelerator now nearing completion at CERN, Europe's particle-physics laboratory near Geneva in Switzerland, might be able to spot them.

But now John Simonetti and colleagues at Virginia Polytechnic Institute and State University in Blacksburg are investigating another way to look for these dimensions — in the explosions of 'primordial' black holes. The team outlines their plan in a paper preprinted at http://arxiv.org1.

Little holes

Forged in the earliest days of the Universe, primordial black holes are microscopic in size, despite having the same mass as a small mountain. Because of their small size, the holes are all but impossible to spot directly. Still, some experts believe that they can produce a unique radio signature.

Like all black holes, primordial black holes undergo 'evaporation', a process that causes them slowly to lose their mass. As they shrink, they evaporate more quickly, and theorists predict that their ever-accelerating evaporation might lead to a sudden explosion. To date, however, astronomers have yet to see signals from such a burst.

In a new twist, Simonetti and colleagues are betting that an extra spatial dimension might trigger a black hole to blow up. A primordial black hole would initially be bigger than the curled-up extra dimension, Simonetti says. But as it evaporates, it would get closer and closer to the scale of the dimension.

"Eventually," he says, "it will find itself in a very peculiar state." The black hole will begin to be stretched around the dimension, something like a rubber band. When it evaporated enough, the hole will suddenly 'snap' into the tiny dimension, triggering an explosion.

5-D explosions

The blast would be smaller than those predicted from normal evaporation, but Simonetti believes they still should be detectable as radio waves. Using an 8-metre antenna array in North Carolina and a second array under construction in Virginia, his group hopes to look for exploding primordial holes up to about 300 light years away.

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The search is a long shot, says Martin Rees, an astronomer at Cambridge University, UK, who made some of the early predictions of primordial black holes. The existence of the holes isn't assured, nor is the presence of extra dimensions. Nevertheless, Rees says, the survey will have merit: "It's worth looking for them."

Roger Blandford, a physicist at Stanford University in California who has also worked extensively on primordial holes, adds that the search will have other merits. Because it is sensitive to other short-lived sources, it might spot radiowaves from supernovae or colliding neutron stars. "I would be very supportive of the investigation, independent of the theory," he says.

"Who knows what we're going to see," Simonetti says. The group hopes to run the search for five years or more.