Black holes — bodies crushed after a star’s death — sometimes approach and merge, emitting gravitational waves (ripples in spacetime). The LIGO, Virgo, and KAGRA observatories recorded 259 such events. Like an archaeologist sifting through layers of an ancient city to find traces of different eras, astronomers discovered four layers among these mergers.
The lowest and thickest layer is light holes of about 10 solar masses. They make up roughly 70% of all mergers. Both partners are nearly identical, spinning in sync with their orbit. Above that lies a dip: holes meet 26 times less often than expected. Then come two layers: one of unequal pairs (small hole of 10 masses, larger up to 40), and another of nearly equal pairs peaking around 31 solar masses. And at the very top, the heaviest — from 47 solar masses and up, with chaotic spins. These are holes grown from many collisions.
Now every registered burst becomes archaeological evidence. Upcoming observing runs will gather even more data — and the picture will become even more detailed.
Four prototype events — GW170608, GW190412, GW150914, and GW170729—were already recorded in the first two observing catalogs. Only now, with more data, has it become clear that each likely represents a distinct evolutionary era.
Hierarchical mergers, in which black holes grow generation after generation, are reminiscent of the “hyperspatial” structures in Greg Egan’s novel “Schild’s Ladder,” where objects are built through successive amalgamations in boundless space.