GWTC-5.0 population study finds structured black-hole masses and a rapidly spinning subpopulation

Gravitational-wave astronomy is starting to look less like a string of one-off discoveries and more like a census. In a new population study built from 267 compact-binary mergers, the LIGO Scientific Collaboration, Virgo Collaboration and KAGRA Collaboration report their clearest statistical evidence yet that black holes do not come in a smooth, featureless spread of masses and spins. The analysis, published as the arXiv preprint “GWTC-5.0: Population Properties of Merging Compact Binaries,” revised as v2 on July 1 with a July 2 draft date on the PDF, also finds a rapidly spinning subpopulation that the authors say is consistent with hierarchical black-hole mergers.

That is the larger shift behind the paper. Rather than highlighting one especially unusual collision, the study uses a sample now large enough to infer how merging black holes are distributed across the population. The cumulative GWTC-5.0 catalog contains 390 candidate events overall, including 161 new events from the O4b observing run. But the population analysis uses the 267-event subset that passed the significance threshold for this study. Of those additions, the paper says 104 new binary-black-hole systems from O4b had enough significance to be included, along with two reanalyzed O4a events.

The dataset contains no new neutron-star sources, so the paper is mainly about binary black holes. For black-hole binaries with component masses between 2.5 and 200 times the mass of the sun, the collaboration estimates a merger rate of 27.5 to 49.4 per cubic gigaparsec per year at redshift 0.2. The paper reports those figures as 90% credible intervals.

One of the most striking results is evidence for a subpopulation in which at least one black hole is spinning rapidly, with a dimensionless spin of about 0.7. That is notable because black holes produced in an earlier merger and then merged again — so-called hierarchical mergers — are expected to sometimes show higher spins and, in some cases, higher masses. The paper does not claim proof for any specific event, but it says the population-level signal is consistent with that channel. The rapidly spinning subpopulation appears at two primary-mass scales: roughly 10 to 20 solar masses and above about 45 solar masses. Its estimated merger-rate density is 0.2 to 3.11 per cubic gigaparsec per year at redshift 0.2.

The mass distribution also shows structure. The study reports a peak around 10 solar masses and a change in slope around 35 solar masses. It also finds that above about 40 solar masses, the distribution of the less massive companion falls off more steeply than that of the primary black hole, a pattern the authors say is consistent with more unequal-mass binaries at high primary mass. Around 35 solar masses, by contrast, black holes tend to pair with companions of similar mass.

Another statistical clue comes from the effective inspiral spin, a simple measure of how much the black holes’ spins line up with the orbit as they spiral together. The paper finds that this distribution is asymmetric about zero, rather than balanced evenly between aligned and misaligned systems. From that, the collaboration infers that at least 9% of mergers occur through channels with some preference for spin-orbit alignment.

“Nearly 400 gravitational-wave events accumulated in our catalog have ushered us into a new era of statistical astronomy — where this growing collection of detected signals enables population studies and tests of general relativity with unprecedented precision,” Leo Tsukada of the University of Nevada, Las Vegas, said in the LIGO press release announcing GWTC-5.0 in May. This new paper is one of the clearest examples of that transition. But the authors also stress caution: some interpretations, especially the exact meaning of the feature near 35 solar masses, depend on the population models used, so the emerging map of black-hole demographics is sharper than before, not final.