What happens when two black holes collide?

20 November 2011

Question

What happens when two black holes collide?

Answer

We posed this question to cosmologist and astrophysicist Professor Martin Rees, University of Cambridge.......

Martin - One of the things we've learnt in the last 10 years or so is that in the centre of every galaxy, there lurks a black hole which is as massive as a million, or even several billion, suns. It's not quite clear how this black hole formed, but we believe that its formation happened at the same time as the galaxy formed and that the black hole grew as the galaxy grew. We also know that one way galaxies grow is by merging with each other - two small galaxies get close and eventually fall together and make one bigger galaxy. Now, if two galaxies merge and each has a black hole at its centre, then the black holes will of course power towards the centre of the common system and form a binary orbiting around each other and the question then is what happens? We think what happens is that various drag forces bring the holes close enough that so-called gravitational radiation then carries away more energy. Gravitational radiation is something predicted by Einstein's theory to happen whenever a gravitational field changes rapidly. So when two black holes get close enough, then gravitational radiation from them carries away a lot of energy and that brings them closer and closer until they eventually merge into a single black hole. And in the final coalescence, a huge burst of gravitational radiation is emitted and this is a challenge to detect. But the biggest events of all would be the gravitational bursts from these super massive black holes which merge and there are plans to have arrays in space which could detect the slight jitter in space that happens when these black holes merge.

Diana - The meeting of two black holes will produce a large one and it will also produce lots of energy in the form of gravity waves. But that happens when the two black holes are of different size.

Martin - So we believe that these black holes are merged and it's by this process that eventually, some of them accumulate masses as much as in some cases, 5 billion times the mass of the sun. There's another interesting consequence. It's being possible for the last few years to actually do computations of what would happen when two black holes merge and when the black holes are of an equal mass, then you get a rather interesting effects namely recoil. The final merged black hole doesn't necessarily sit in the centre of the merged galaxy. It's given a kick and sometimes this kick could be big enough to expel the merged black hole from the galaxy it's in. So, the final thought I leave you with is that it could be some of these huge black holes hurtling through intergalactic space, having been kicked out by the huge recoil speed from the galaxy in which they formed.

Diana - So beware of a super massive black hole flying through space like a very dark, very heavy, juggernaut of the skies.

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