When a star dies, a black hole is born. This has been the textbook origin story for most black holes. At the end of a massive star’s life, its outer layers blast away in a brilliant supernova, and its core collapses into a gravitationally tight and dense region, forming a black hole. Recent discoveries from gravitational-wave detectors have revealed hundreds of merging black holes across the universe. Many of them have been thought to come directly from exploding stars. But black holes can also come from other, smaller black holes. The products of previous black hole mergers can, in principle, merge again, creating a more massive black hole. This alternative, black-holes-birthing-black-holes pathway is known as “hierarchical merging.” Now MIT scientists are finding that a good number of merging black holes may have indeed merged before. They carried out a new analysis…

“This process can repeat potentially ad infinitum, by virtue of the fact that you have a ton of stars and black holes in this really dense environment.” One sign of a hierarchical merger is that one black hole in a pair of merging black holes has a much higher spin, and higher mass, than the other. Such a lopsided duo would signal that at least one of the black holes came from the collision of two previous black holes. In 2024, scientists detected two such lopsided mergers in signals recorded by the LIGO, Virgo, and KAGRA observatories. The observatories detect incoming gravitational waves — incredibly small wobbles in the fabric of space and time — that are the reverberations from distant cosmic phenomena, such as colliding black holes. The observatories detected two gravitational-wave signals, labeled GW241011 and GW241110, each of which likely contain a black hole…