For the first time, astronomers have created a data-driven estimate for how many black holes are in our Universe: more than anyone expected.
Black holes are wondrous objects, but how many are out there?
Most black holes form when high-mass stars end their lives.
Those stars die in core-collapse supernova events.
Some leave neutron stars behind, but the more massive ones leave remnant black holes.
Neutron star mergers supplement the black hole population.
Occasionally, stars also directly collapse: (probably) leaving black holes behind.
Although we’ve quantified star-formation throughout cosmic history, the black hole “fraction” remained uncertain.
All of this changed, however, since the dawn of gravitational wave astronomy.