The universe’s most massive black holes may not be born directly from dying stars. Some of the heaviest ones detected through gravitational waves appear to be built, collision by collision, inside the densest stellar neighborhoods in the cosmos.
That conclusion comes from a new analysis of gravitational wave data from the LIGO-Virgo-KAGRA network. The research separates the population of detected black holes into two distinct classes — and in doing so, addresses a long-standing question about how the heaviest objects heard by gravitational wave detectors come to exist.
Two populations, two origin stories
The first population looks familiar. These are lower-mass black holes, spinning slowly, with masses that line up neatly with what theorists expect from the collapse of massive stars at the end of their lives.
The second population is stranger. These higher-mass black holes spin rapidly, and their spin axes point in seemingly random directions. That randomness is the giveaway.
The analysis shows that unlike the lower-mass systems, which are generally slowly spinning, the higher-mass systems are consistent with having more rapid spins, oriented in seemingly random directions. This is the signature expected if black holes were repeatedly merging in dense star clusters.
When two black holes spiral together and merge, the resulting object inherits the angular momentum of the collision itself. If that merged black hole then encounters another and merges again, the spin axis of the new object bears no simple relation to the spins of any original star. Run that process several times in a crowded environment and you produce what the data points toward: heavy black holes spinning fast, in many directions.
The 45-solar-mass wall
The split between the two populations falls near a specific number — about 45 times the mass of the Sun. Theorists have predicted a related boundary for decades, calling it the pair-instability mass gap.
The physics behind the gap is unforgiving. Stars massive enough to leave behind a black hole heavier than roughly 45 solar masses may instead enter a regime where gamma rays start producing electron-positron pairs. That process robs the core of the radiation pressure helping to hold the star up. The collapse can then run away into a thermonuclear explosion that destroys the star, leaving no black-hole remnant.
That theoretical prediction had never been cleanly confirmed in observation. The new gravitational wave data provides supporting evidence. Above about 45 solar masses, the spin distribution changes in a way that is hard to explain with normal stellar binaries alone but is naturally explained if these black holes have already been through earlier mergers in dense clusters.
Independent analysis of gravitational wave catalogs has reached a parallel finding: the smaller members of detected black-hole pairs stop appearing above roughly 45 solar masses, sharpening what had long been a statistical hint into stronger evidence.
Globular clusters as black hole factories
If many of the heaviest stellar-mass black holes cannot form directly from single stars, where do they come from? The data points to globular clusters — ancient, gravitationally bound balls of stars that orbit galaxies like the Milky Way. These environments can be vastly denser than the Sun’s relatively empty cosmic neighborhood.
That density matters. In a typical region of a galactic disk, two black holes will essentially never meet. In a globular cluster, gravitational interactions can drive stellar remnants toward the dense core, where they pair up, merge, and the resulting object can encounter another partner before being kicked out of the cluster entirely.
The result is a kind of assembly line. Small black holes combine into medium ones. Medium ones combine into heavy ones. The 45-solar-mass wall, which stops some single stars from producing heavy black holes directly, is no obstacle for an object that grew through a sequence of mergers.
Why this matters beyond black hole demographics
Gravitational wave astronomy began as a confirmation exercise. The first detection in 2015 proved that Albert Einstein’s prediction of ripples in spacetime was right. Each subsequent event added another data point.
The field has now crossed into something different. With expanded gravitational wave catalogs providing an increasing sample of compact-object collisions, statistics have replaced anecdote. Researchers can now ask population-level questions: How do black holes grow? Where do they live? What does the distribution of their masses say about the lives and deaths of stars across cosmic history?
Gravitational-wave astronomy is now doing more than counting black hole mergers. 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.
That shift mirrors the trajectory of every observational science. Optical astronomy moved from cataloging individual stars to mapping galaxies to charting the structure of the universe itself. Gravitational wave astronomy is taking the same path on a compressed timeline.
The supermassive question still open
One puzzle the new data does not solve: the origin of supermassive black holes — the millions-to-billions-of-solar-mass giants at the centers of galaxies. Hierarchical mergers in globular clusters can produce black holes of a few hundred solar masses, perhaps a few thousand. Getting from there to the billion-solar-mass quasars observed in the early universe remains an open problem.
That gap has motivated alternative theories, including proposals involving dark stars rather than ordinary stellar evolution. Other models invoke direct collapse of primordial gas clouds, or seeds inherited from the dense conditions of the very early universe, where cosmic structure formation produced extreme density contrasts.
The gravitational wave analysis does not claim to answer that question. But it does provide a tested mechanism for building black holes well above the pair-instability gap, which is a first step. If hierarchical mergers can reliably produce 100-solar-mass objects in clusters, those objects become plausible seeds for further growth through gas accretion in galactic centers.
What comes next
The detector network keeps improving. Sensitivity upgrades to LIGO and Virgo, combined with KAGRA’s growing contribution and planned future observatories, will push the detection rate higher. At larger volumes, the two populations identified in this work may fracture into finer subpopulations — black holes from different cluster types, different cosmic epochs, different metallicity environments.
The methodology is also expanding. Other gravitational-wave catalog work has found evidence for at least three subpopulations of merging binary black holes, hinting that the formation story may involve more than two clean channels.
For now, the headline finding is clear. The biggest black holes the gravitational wave network can hear appear to have been built, not simply born. They are the end products of crowded stellar neighborhoods running collision physics for billions of years, and the spacetime ripples they emit when they finally meet are letting astronomers reconstruct that history one merger at a time.
Photo by Zelch Csaba on Pexels
