Evidence Mounts for Hierarchical Black Hole Mergers

Evidence is mounting that some merging black hole binaries detected through gravitational waves involve hierarchical mergers, according to a viewpoint published July 6, 2026, in Physics. Hundreds of merging black hole binaries have been detected via gravitational waves. Different analyses of the masses and spins of these black holes are converging on the conclusion that a subpopulation of the observed mergers are likely hierarchical. In such cases, at least one black hole in the pair is the remnant of a previous coalescence rather than the direct product of a dead star. Hierarchical mergers typically occur in dense astrophysical environments. Two teams, one led by Cailin Plunkett at MIT and the other by Sharan Banagiri at Monash University in Australia, present evidence linking a subset of binary black hole observations to this origin. The convergence of these and other analyses, despite different assumptions, strengthens the case that hierarchical mergers constitute an important contribution to the binary black hole population, the article states.