'Hundreds of black holes that can swallow whole planets and solar systems roam galaxy'
Last updated at 10:22 10 January 2008
The spiralling traps, created by the collapse of matter to an infinitely-dense state, are places where gravity is so strong it sucks in even light.
Now a new threat of 200 wandering "rogue" black holes has been predicted by supercomputer simulations of what happens when two black holes of different sizes or spin speeds collide.
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Scientists believe they would merge into a single black hole and be kicked into space at speeds as high as 2,500 miles per second.
Any star, planet or spacecraft that crossed the path of one of these black holes would be engulfed by its gravity, never to escape.
It would be difficult to see the danger coming, according to US astronomer Dr Kelly Holley-Bockelmann, from Vanderbilt University in Nashville.
"Rogue black holes like this would be very difficult to spot," she said.
"Unless it's swallowing a lot of gas, about the only way to detect the approach of such a black hole would be to observe the way in which its super-strength gravitational field bends the light that passes nearby.
"This produces an effect called gravitational lensing that would make background stars appear to shift and brighten momentarily."
Dr Holley-Bockelmann studied models of "intermediate mass" black holes that may reside in globular clusters - ancient collections of 100,000 to a million stars.
Black holes of this type, whose existence is controversial, would contain the equivalent of a few thousand Suns.
Dr Holley-Bockelmann's team investigated what would happen if an intermediate black hole collided with a smaller "stellar" black hole created by the explosion of a giant star at the end of its life.
The result is a "big kick" that would shoot the combined black hole out of the globular cluster at high velocity.
Astronomers have ample evidence for the existence of stellar black holes weighing less than 100 solar masses, which are thought to be plentiful in globular clusters.
A kick would also occur if black holes rotating at different speeds were to collide.
"We used different assumptions for the initial black hole mass, for the range of stellar black hole masses within a globular cluster, and assumed that the spins and spin orientations were distributed randomly," said Dr Holley-Bockelmann, who presented her findings today at a meeting of the American Astronomical Society in Austin, Texas.
"With our most conservative assumptions, we found that, even if every globular cluster started out with an intermediate-sized black hole, only about 30 per cent retain them through the merger epoch.
With our least conservative assumptions, less than 2 per cent of the globular clusters should contain intermediate mass black holes today."
Our galaxy, the Milky Way, contains roughly 200 globular clusters, leading Dr Holley-Bockelmann's team to conclude that hundreds of rogue black holes could be at large.
However the scientists say it is "extremely unlikely" that they will present any threat to us in the lifetime of the universe.
"Their danger zone, the Schwarzschild radius, is really tiny, only a few hundred kilometres," said Dr Holley-Bockelmann.
"There are far more dangerous things in our neighbourhood!"
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