A University of Miami team argues a single subsolar mass LIGO merger has no clean stellar explanation. The next decade of gravitational wave data will decide whether that means primordial black holes are real.
A lone subsolar-mass gravitational-wave blip in LIGO data has revived a half-century-old question: could primordial black holes, born in the universe's first fraction of a second, account for the invisible bulk of its mass?
In November 2025, the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) fired off an automated alert for a merger in which at least one of the components weighed less than one solar mass. That is anomalously light. Black holes born from collapsing stars typically weigh at least a few times the mass of the Sun; sub-stellar-mass mergers have no clean conventional explanation inside the standard astrophysical catalog. According to a new paper in The Astrophysical Journal by University of Miami physicists Nico Cappelluti and his Ph.D. student Alberto Magaraggia, the signal is "best explained" by a primordial black hole, a class of object first proposed by Yakov Zeldovich and Igor Novikov in the 1960s and expanded by Stephen Hawking in the early 1970s as a candidate component of dark matter.
The reasoning rests on a specific mass window. Stellar collapse has a lower limit: a star has to be heavy enough to burn through its fuel and then implode, leaving a black hole above roughly the Chandrasekhar mass. Anything lighter would have to come from somewhere else. In the early universe, density fluctuations a fraction of a second after the Big Bang could have seeded black holes of essentially any mass, including ones smaller than the Sun. If those primordial black holes still exist today, and if some of them pair up and merge, the gravitational-wave signal they produce would carry the same frequency profile as any other merger but with a telltale component mass: too low to be a stellar remnant. That is exactly what the November 2025 alert showed.
The signal itself is a single event. One event is not a population, and the authors are explicit about that. Their model predicts that subsolar primordial black hole mergers should be rare, which is consistent with LIGO having seen only one such candidate so far. As SciTechDaily reported, Cappelluti argues the LIGO signal "lacks any conventional astrophysical explanation," and the pair suggest that primordial black holes "could account for a significant portion, if not all, of dark matter." That last claim is a model output of their analysis, not an established fact. About 85% of the universe's matter is dark matter, a figure that has stood for decades because no known particle accounts for it. If primordial black holes really do compose that missing mass, the payoff would be enormous: a single, clean astrophysical object doing the work that generations of underground detectors and collider experiments have failed to do.
But the case is far from closed. The same SciTechDaily piece notes that "whether the signal represents a major scientific discovery or simply noise within LIGO's detectors remains a subject of debate among astrophysicists." Gravitational-wave pipelines are not infallible. Detector glitches, terrestrial vibrations, and template-matching errors all produce false alerts, and the subsolar-mass regime is especially fragile because the waveform models used to identify it were built primarily for stellar-mass binaries. Cappelluti himself acknowledges that definitive proof could still take years. LIGO made its first direct detection, a roughly 36-solar-mass merger, on 14 September 2015; the intervening decade of detections built confidence through replication, not single events. The PBH case will have to clear the same bar.
That uncertainty is the real story. A confirmed subsolar-mass population of black holes would not only validate a fifty-year-old theoretical idea. It would also give physicists a candidate dark-matter component that does not require new particles, would be detectable by existing instruments, and would carry a distinct gravitational-wave fingerprint. None of those payoffs are guaranteed by a single candidate event, and a noise misinterpretation would set the field back by years of follow-up chasing an artifact.
What would actually move the needle? A pattern. The LIGO-Virgo-KAGRA network, which operates detectors in Hanford, Washington; Livingston, Louisiana; Cascina, Italy; and Kamioka, Japan, would need to see a statistically meaningful rate of subsolar-mass mergers across its observing runs, not just one isolated alert. Independent analyses of the November 2025 candidate by groups outside the Miami team would need to confirm the signal survives reweighted detector noise and updated waveform models. And the candidate's mass and spin would have to fall in the range primordial-black-hole formation predicts, and outside the range any stellar-collapse pathway could plausibly produce. The Miami paper, formally titled "Implications for Primordial Black Hole Dark Matter from a Single Subsolar Mass Gravitational-wave Detection in LVK O1–O4," is now the reference point for that test.
Two future observatories will sharpen the question. The European Space Agency's LISA mission, a space-based gravitational-wave antenna scheduled to launch in 2035, will listen for the low-frequency rumble of supermassive black hole mergers and, if primordial black holes exist in the right mass range, their distinct inspiral signals. Cosmic Explorer, a U.S. ground-based detector now in design, aims for roughly ten times LIGO's current sensitivity and would extend the catalog of detectable mergers by orders of magnitude. Either instrument, or both, could settle whether the November 2025 candidate is the first of a population or the last of a single suspicious event.
Until then, the alert sits in a particular kind of limbo: not confirmed, not refuted, but useful. It tells researchers where to look, what mass range to target, and which noise margins to harden. Half a century after Hawking sketched the case, primordial black holes have moved from a speculative idea to a hypothesis with a defined observational test. The next few years of LIGO data, and eventually LISA, will determine whether that test is passed.