By Anna Hart
Seven billion years ago, two black holes collided 150 billion trillion km away from Earth. The immense gravitational waves created by this event were detected by astronomers in May last year, making it the biggest black hole collision ever discovered.
Early this September, astronomers announced their observations of this deep space event in Physical Review Letters, revealing that they had detected the collision of two black holes 66 and 85 times more massive than the Sun, creating a black hole of 142 solar masses.
The black hole created by this merger is an ‘intermediate-sized’ black hole – those in the range of 100-1000 solar masses – and the first ever discovered within this intermediate range. Astronomers have found black holes weighing in at tens of solar masses and supermassive black holes billions of times more massive than the sun, but the discovery of a 142-solar-mass black hole is scientifically significant.
The energy of eight Suns was released instantaneously as these two black holes collided, producing as much energy as if a million billion atomic bombs were set off every second for the entire age of the observable universe: 13.8 billion years. This energy was released in the form of gravitational waves, which are space-time disturbances created by massive cosmic events, and picked up by the international LIGO-VIRGO collaboration: an American-European system of gravitational wave detectors.
Caltech astronomer, Matthew Graham, told National Geographic that this event is:
“probably the largest explosion we’ve ever known in the universe.”
In a companion paper in The Astrophysical Journal Letters, the LIGO-VIRGO scientists discuss the properties and implications of this merger – known as GW190521 – hypothesising about how it was formed.
Despite the immensity of GW190521, some physicists are more excited about one of the two original black holes than the resulting 142-solar-mass one. That’s because a black hole of 85 solar masses should theoretically not exist, because stars in the range of 65 to 120 solar masses end their lives by exploding as supernovae according to current theory. This is due to the energy created by nuclear fusion in the star’s core outweighing the gravitational forces holding the star together; the imbalance causes a super-powerful and bright supernova explosion, destroying the star so it cannot turn into a black hole.
The main theory as to how this unusual 85-solar-mass black hole could have formed is via a previous merger between two smaller black holes. This could give way to subsequent black hole collisions, as observed in GW190521, eventually creating the supermassive black holes we currently believe exist at the centre of galaxies.
As astronomers work on the theory to explain this exciting new observation, we take another step towards understanding our universe and the physical laws that govern it.