Since gravitational waves were first detected in 2015, instruments including LIGO, Virgo and KAGRA have picked up a steady stream of signals from colliding black holes, building a catalog that now numbers in the hundreds. Yet despite this wealth of data, a fundamental question has remained stubbornly unresolved: How do these black holes actually form?
Now, two independent research teams have used fresh theoretical approaches to comb through the data, and both arrived at a similar conclusion: Merging black holes don’t form a single uniform group, but instead separate into distinct subpopulations, each bearing the fingerprints of different formation mechanisms. Both studies have been published in Physical Review Letters.
Hidden information
Every time two black holes spiral together and merge, the resulting ripples in spacetime encode information about their masses and spins—the rate and direction at which each one rotates.
Astronomers can extract these properties from the gravitational-wave signal, but the picture is often incomplete: While some binaries formed from pairs of stars that lived and died together, others came together later, pulled into orbit by chance encounters in crowded stellar environments. Because these two pathways leave subtly different imprints on mass and spin, sifting through hundreds of detections to spot broader patterns has been a persistent challenge.
Uncovering subpopulations
The first team’s study, led by Cailin Plunkett at MIT, built a model that focuses on two well-measured spin parameters, capturing how a black hole’s spin aligns with its orbital motion. The second study, led by Sharan Banagiri at Monash University in Australia, took a more open-ended approach, letting the data itself dictate how many distinct groups were present without assuming a particular formation story in advance.
Despite their different starting points, both teams identified a population of unusually massive black holes that stood apart from the rest, each roughly 40 times the mass of the sun or heavier.
Plunkett’s team found that these heavyweights carry fast, randomly oriented spins consistent with black holes built from earlier mergers rather than stellar collapse. Banagiri’s team reached a similar mass threshold and also found high spins in this group—though without the same clear signature of a merger origin. This prompted some caution about the interpretation.
Clarifying boundaries
Together, these findings offer some of the strongest evidence to date that a portion of observed black hole mergers are “second-generation” events, born from black holes that had already merged once before—rather than from the collapse of massive stars.
This distinction could ultimately help explain how black holes end up in a mass range otherwise thought to be off-limits, and how the seeds of the supermassive black holes at galaxies’ centers might have grown. As gravitational-wave detectors grow more sensitive to these massive binaries, the boundaries between subpopulations could soon become clearer.
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Publication details
Cailin Plunkett et al, Signatures of a Subpopulation of Hierarchical Mergers in the GWTC-4 Gravitational-Wave Dataset, Physical Review Letters (2026). DOI: 10.1103/n6p4-ftgq
Sharan Banagiri et al, Evidence for Three Subpopulations of Merging Binary Black Holes at Different Primary Masses, Physical Review Letters (2026). DOI: 10.1103/blyb-lqv6
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Gravitational waves reveal hidden populations within black hole mergers (2026, July 12)
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