Study finds evidence of black holes formed from previous mergers

Physicists have identified evidence that some of the black holes detected by gravitational‑wave observatories are themselves the remnants of earlier mergers. By analysing the waveforms recorded by LIGO and Virgo, researchers found signatures—such as unusually high masses and specific spin orientations—that match predictions for “hierarchical” mergers, where two black holes first collide, form a larger remnant, and then that remnant later merges with another black hole.

The study, published in *Physical Review Letters*, examined dozens of events from the third observing run and applied statistical models to distinguish between black holes formed in single stellar collapses and those that grew through successive collisions. The results indicate that a non‑negligible fraction of the observed black holes have masses and spins consistent with having undergone at least one prior merger, suggesting that dense stellar environments like globular clusters or galactic nuclei may host repeated black‑hole collisions.

These findings refine our understanding of black‑hole demographics and the environments that foster their growth. If hierarchical mergers are common, they could influence the rate of gravitational‑wave detections and provide a new probe of the dynamical processes shaping the most massive stellar remnants in the Universe.