If black holes grow by merging and consuming stellar matter, there should be evidence of black holes of all sizes. But "intermediate mass" black holes and black holes of a few dozen solar masses are astonishingly rare, throwing some black hole evolution theories into doubt.
VIDEO: Gravitational Waves Confirmed: A Historic Discovery
With the detection of gravitational waves on Sept. 14 came the realization that a black hole binary merger caused it. Two black holes, "weighing in" at 29 and 36 solar masses, collided and merged as one, generating a very clear gravitational wave signal. But, as pointed out by Hawking, how black holes of this specific mass came to being could provide some clues as to how black holes grow.
One thing is clear, however: This is the first time that we've acquired direct evidence of a black hole merger - a key mechanism that underlies black hole evolution theories - so it's good to know we're on the right track.
Watch the full BBC Stephen Hawking interview here: