SCIENCE UPDATE

For the first time, physicists have witnessed two black holes merging to form a medium-sized black hole that weighs 100+ times more than the Sun.

Sharks, whales, black holes and dalliances with Neanderthals...

Astrophysicists have recently witnessed the biggest collision ever between two black holes, and the result is a large hole of a kind that scientists have never seen before. The two black holes individually weighed 66 and 85 times the mass of the Sun, and physicists’ calculations show that the merger resulted in a black hole of 142 solar masses.

This indicates that the collision discharged energy corresponding to nine solar masses. Most of it flowed through space in the shape of gravitational waves, which the LIGO and Virgo detectors on Earth can measure.

The merger of the two black holes is not only the biggest but also the most remote from which scientists have ever captured gravitational waves. It happened in a region that is now 17 billion light years away from us, although the collision did not happen 17 billion years ago, rather only seven billion years ago, when the universe was about half its present age. The region is now much further away because of the universe’s expansion since then.

The black hole of 142 solar masses is particularly important because it is the first discovery of a medium-sized black hole which, according to theory, originates when two or more black holes merge. Smaller holes are formed by collapsing stars that weigh less than 60 solar masses. If a star is heavier than this, it should blow itself to pieces instead of becoming a black hole. So scientists suspect that the two black holes of 66 and 85 solar masses might therefore have themselves formed in earlier mergers between smaller black holes.

Astronomers now lack evidence only for the smallest category of black hole. According to theory, these would have formed shortly after the Big Bang, and weigh only as much as a large mountain. We have no evidence of these – yet.