The Chirp-Mass Ladder: A New Rung Emerges

The population of binary black holes (BBHs) observed through gravitational waves (GWs) now includes around 250 events with the release of GWTC-5.0, enabling more detailed studies. The inferred chirp-mass distribution shows prominent peaks at approximately $7.5M_{\odot}$, $14M_{\odot}$, and $27M_{\odot}$, where the locations of subsequent peaks increase by approximately a factor of two. A parsimonious explanation for this structured distribution is a hierarchical merger scenario, in which the first peak arises from mergers of black holes of stellar origin, and higher-mass peaks arise from repeated mergers. Notably, with the addition of new observations, an intermediate peak near $19M_{\odot}$ emerges. This feature had been anticipated in earlier work as a consequence of intergenerational mergers involving second- and third-generation (G) black holes, thereby highlighting the predictive power of the hierarchical-merger interpretation. Furthermore, two groups of $1G+2G$ mergers recently reported in separate studies can be understood as distinct rungs -- $1G+2G$ and $3G+4G$ -- within this hierarchical chirp-mass ladder, a unification that describes both spin transitions with a single mechanism. Although expected correlations between mass ratios and spins are observed in multiple events across the mass range, the lack of clear signatures across all rungs invites investigation into the role of hierarchical mergers in shaping the \ac{BBH} population.