The most massive black holes ever detected through gravitational waves were not born from the direct collapse of massive stars. Instead, they grew through a series of repeated, catastrophically violent collisions inside densely packed star clusters — and the ripples they sent through spacetime have finally betrayed their origin. That is the central conclusion of a new study led by researchers at Cardiff University, published in Nature Astronomy.
Probing the heaviest mergers in the catalog
The team analyzed version 4.0 of the Gravitational-Wave Transient Catalog (GWTC-4), assembled from data collected by the LIGO–Virgo–KAGRA detector network. The catalog contains 153 sufficiently confident black hole merger detections — the largest and most statistically robust sample of its kind to date.
The core question driving the research was whether the heaviest black holes in the catalog were truly first-generation objects, formed when a single massive star runs out of nuclear fuel and collapses, or whether they were second-generation products assembled from earlier mergers. The answer, according to the data, points strongly toward the latter.
«Gravitational-wave astronomy is now doing more than counting black hole mergers,» said lead author Dr. Fabio Antonini from Cardiff University’s School of Physics and Astronomy. «It is starting to reveal how black holes grow, where they grow, and what that tells us about the lives and deaths of massive stars.»
Two populations, two distinct histories
One of the most striking findings in the study is the identification of two statistically distinct populations of black holes within the gravitational-wave data.
The first population consists of lower-mass black holes whose properties are consistent with what astrophysicists expect from ordinary stellar collapse: relatively slow spins and a mass distribution that follows naturally from the deaths of massive stars.
The second population tells a different story. These are the heaviest black holes in the sample, and their spin properties diverge sharply from the first group. Rather than being slowly spinning and aligned, they exhibit rapid spins oriented in seemingly random directions — precisely the signature that hierarchical mergers in dense star clusters would leave behind.
«What surprised us most was how clearly the high-mass black holes stand out as a separate population,» said co-author Dr. Isobel Romero-Shaw, Ernest Rutherford Fellow at Cardiff University. «Unlike the lower-mass systems, the higher-mass systems are consistent with having more rapid spins, oriented in seemingly random directions. This is the exact signature you would expect if black holes were repeatedly merging in dense star clusters. That makes the cluster origin much more compelling than it was with earlier catalogs.»
The dense cores of star clusters, where this process plays out, can pack stars up to a million times more tightly than the solar neighborhood. In such environments, black holes that form from stellar collapse can sink toward the center, encounter one another, merge, and then merge again — building progressively more massive objects with each collision.
The strongest evidence yet for the pair-instability mass gap
Beyond the question of formation channels, the study provides what the authors describe as the strongest evidence yet for the pair-instability mass gap — a theoretically predicted forbidden zone in the black hole mass spectrum.
The theory holds that when a star becomes extremely massive, the photons produced in its core gain enough energy to spontaneously convert into electron-positron pairs. This process removes radiation pressure, triggering a runaway collapse followed by a catastrophic thermonuclear explosion that completely destroys the star. No black hole is left behind. The result is a mass range from which stellar-origin black holes simply cannot emerge.
The Cardiff team pinpoints this forbidden zone at around 45 solar masses. Above that threshold, the spin distribution in the catalog changes in a way that is very difficult to explain through normal stellar binary evolution, but falls naturally into place if the black holes involved are the products of earlier mergers.
«In our study we find evidence for the long-predicted pair-instability mass gap — a range of masses where stars are not expected to leave behind black holes at all,» said Dr. Antonini. «Gravitational-wave detectors have successfully found black holes that appear to sit in or near that gap, which we identify at around 45 solar masses. The biggest black holes in the current sample seem to be telling us about cluster dynamics, not just stellar evolution.»
A window into nuclear physics
The implications extend beyond astrophysics into fundamental nuclear physics. The location of the pair-instability mass gap is not arbitrary — it is set by the details of nuclear reactions that govern helium burning in the cores of massive stars. If the mass boundary can be measured precisely enough through gravitational-wave data, it becomes possible to probe those reactions from a completely independent direction.
«In the future, gravitational-wave data may help scientists study nuclear physics, because the mass limit set by pair instability depends on the nuclear reactions taking place in the cores of massive stars,» said co-author Dr. Fani Dosopoulou, a research associate at Cardiff University.
The study represents a significant step in the transformation of gravitational-wave astronomy from a field that detects events to one that reconstructs the full evolutionary histories of the objects involved. As catalogs grow and detector sensitivity improves, the violence written into each ripple in spacetime will become an increasingly detailed record of how the universe builds its largest and most extreme objects.
The research was published as: Antonini et al. (2026), Nature Astronomy. DOI: 10.1038/s41550-026-02847-0
© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Total or partial reproduction prohibited without express authorization. Original source: Phys.org / Cardiff University / Nature Astronomy DOI: 10.1038/s41550-026-02847-0
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