Prefab Black Holes: How Remnants of Past Mergers Collide
Two recent black hole mergers have proven unusual: one of the black holes in each pair may not have been born from a star but instead formed in an earlier collision — like a “granddaughter” in a cosmic pedigree. This suggests that black holes can grow by merging repeatedly in dense stellar “megacities.” Will we ever trace their full genealogy?
Recently, detectors of gravitational waves recorded two unusual mergers of black holes. Analysis showed that in each pair, one of the black holes wasn't a simple one, but a 'prefabricated' one — it itself formed from the collision of two other holes in the past. It's like playing with a construction set: instead of only picking individual bricks, you sometimes use already assembled blocks.
The idea of black holes goes back to the calculations of Karl Schwarzschild, and the first attempt to catch gravitational waves was made by Joseph Weber half a century ago. Today we know that galaxies, whose classification was pioneered by Edwin Hubble, often harbor supermassive black holes and gas disks at their centers — ideal places for the birth of 'prefabricated' black holes.
Gravitational waves carry information through space at the speed of light, and we've learned to read it. To see such mergers also in ordinary light, we'll need spectroscopy — a method that allows us to split light into colors and find lines of hydrogen from heated gas. This will be helped by the Hubble telescope and future observatories. Thus, a new chapter in astronomy will open: unified vision in gravity and light.
🎯 Offspring black holes from mergers always spin at roughly the same speed — about 70% of the maximum possible. This makes them easily identifiable in gravitational wave data.