How do you cool a black hole? Just add a small moon.
Stephen Hawking predicted that black holes should emit a small amount of radiation, giving them a temperature that depends on their mass. But to find it, physicists also need to know the surface gravity, which so far cannot be calculated unless the black hole is sitting still.
To expand Hawking's theory to moving black holes, Samuel Gralla and Alexandre Le Tiec at the University of Maryland in College Park modelled a scenario in which a moon is orbiting a black hole at the same speed that the hole rotates, so that the moon seems to hover in place.
"From the point of view of the black hole, things are stationary," says Gralla. "It is a way of having your cake and eating it too."
The team could then use existing equations to work out its temperature. They also found that a partner would cause the black hole to wobble, lowering surface gravity and cooling it ? although not by much. A lone black hole about five times the mass of the sun and 30 kilometres wide would be about 10 billionths of a degree above absolute zero, Gralla calculates. Adding a moon with a mass of 1000 tonnes would reduce this by just 10-35 kelvin.
Although we are unlikely to spot such a minuscule effect in the real world, the underpinning maths might help us to develop the theory of gravitational waves, says Gary Gibbons at the University of Cambridge. Two black holes that orbit each other are thought to be strong sources of these space-time ripples. Astrophysicists hope to use the waves to test general relativity, and perhaps to probe black hole mergers.
Finding a way to include temperature among the properties of orbiting black holes may refine our understanding of such systems, says Gibbons.
Journal reference: arxiv.org/abs/1210.8444
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