# A strange LIGO signal could point to black holes born before the first stars

> A study in The Astrophysical Journal has placed one unusual number at the center of a cosmic mystery. Researchers at the University of Miami argue that a gravitational-wave candidate reported by the LIGO-Virgo-KAGRA collaboration may involve a black hole lighter than the...

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Byline: University of Miami
Published: 2026-07-18T17:45:02+00:00
Categories: News, Space

![Artist's rendering of a binary black hole merger](https://www.argo.net/wp-content/uploads/2026/07/A_strange_LIGO_signal_could_point_to_black_holes_born_before_the_first_stars.jpg)

A [study](https://doi.org/10.3847/1538-4357/ae48f9) in The Astrophysical Journal has placed one unusual number at the center of a cosmic mystery. Researchers at the **University of Miami** argue that a gravitational-wave candidate reported by the LIGO-Virgo-KAGRA collaboration may involve a black hole lighter than the Sun.

That would be extraordinary. Known black holes usually form when massive stars die, which gives them masses several times larger than the Sun. A **subsolar black hole** would point toward a much older origin, perhaps the first fraction of a second after the Big Bang.

The idea remains tentative. The signal could still turn out to be an instrumental fluctuation or a statistical oddity. Even so, Nico Cappelluti and Alberto Magaraggia say the event fits a long-discussed possibility, that some dark matter may be made of primordial black holes.

"We believe our study will aid in confirming that they actually do exist," said **Nico Cappelluti**, an associate professor in the University of Miami's Department of Physics.

## A subsolar signal from LIGO

The unusual event traces back to a compact binary merger candidate reported by the **LIGO-Virgo-KAGRA collaboration** on November 12, 2025. The candidate, known as S251112cm, appeared in gravitational-wave data from the global detector network.

Gravitational waves are ripples in spacetime produced when massive objects accelerate. LIGO first detected them in 2015, opening a new way to observe black holes through their motion rather than their light.

In this case, the signal was unusual because at least one object appeared to fall below one solar mass. That mass range is difficult to explain with ordinary stellar evolution. Stars that collapse into black holes usually leave behind objects heavier than the Sun.

The candidate also lacked an obvious electromagnetic counterpart. In plain terms, telescopes did not report a clear flash of light linked to the same event. That leaves the gravitational-wave data as the central clue.

## Why the mass looks so unusual

Black holes come in several broad families. Stellar-mass black holes form from massive stars and can weigh a few to many times the mass of the Sun. Supermassive black holes sit in galactic centers and can reach millions or billions of solar masses.

A black hole lighter than the Sun sits in a stranger category. Current stellar physics does not provide an easy route for a dead star to make one. That is why a subsolar mass candidate immediately attracts attention.

Cappelluti explained the usual picture plainly. "The most common black holes form as the result of a supernova, the death of a massive star. So, their masses can range from a few times the Sun's mass to billions of solar masses," he said.

The Miami team focuses on the possibility that the smaller object formed before stars existed. Such a black hole would come from extreme density variations in the newborn universe. In that scenario, gravity could have crushed pockets of matter directly into black holes.

This is where the study becomes especially provocative. A single strange mass measurement can't settle the case, but it gives theorists a measurable target. If LIGO sees more events like this, the pattern could become much harder to dismiss.

## Primordial black holes and dark matter

**Primordial black holes** have been discussed for decades. Yakov Zeldovich, Igor Novikov and later Stephen Hawking helped develop the idea that black holes could have formed in the early universe, before galaxies and stars took shape.

Their possible sizes could span a wide range. Some models allow objects far smaller than stars, while others allow much heavier bodies. That flexibility makes them interesting and also hard to pin down.

Dark matter adds another layer to the story. Astronomers infer dark matter from its gravitational pull on galaxies and galaxy clusters. It appears to make up about 85 percent of all matter, yet it does not reveal itself through ordinary light.

Black holes share one key dark matter trait. They can exert gravity while remaining difficult to see directly. A population of ancient black holes could therefore contribute to the unseen mass that shapes galaxies.

The Miami study treats the LIGO signal as a possible test of that idea. If a subsolar black hole exists, primordial formation becomes one of the most natural explanations. If many such objects exist, they could account for a significant share of dark matter.

## What the University of Miami team calculated

Magaraggia and Cappelluti asked a practical question. If primordial black holes make up dark matter, how often should LIGO detect mergers involving subsolar objects?

"We attempted to estimate how many primordial black holes may exist in the universe and how many of them LIGO should be able to detect," said **Alberto Magaraggia**, a Ph.D. student at the University of Miami.

Their calculation connects cosmic abundance with detector sensitivity. A large hidden population would produce some mergers, but the rate must also match the scarcity of observed subsolar candidates. Too many predicted detections would weaken the model.

According to the researchers, the numbers line up in an intriguing way. Subsolar black holes like the one LIGO may have observed should be rare. That rarity matches the limited number of such signals reported so far.

"Our results are encouraging," Magaraggia said. The team argues that the candidate is most consistent with a primordial black hole, given the lack of a conventional astrophysical explanation.

## Why one signal isn't enough

One gravitational-wave candidate leaves room for caution. LIGO's instruments are exquisitely sensitive and they must separate cosmic signals from local disturbances. A candidate event can look promising before later analysis changes its status.

The researchers acknowledge that more detections are needed. A second similar event would strengthen the case. Several events with matching properties would give scientists a population to analyze.

"But we'll need to detect another such signal or even several others to get the smoking-gun confirmation that they are real," Cappelluti said.

The reason is statistical as much as astrophysical. A single event can be rare noise, an unusual merger, or a genuine sign of new physics. A repeated pattern lets researchers measure rates, masses and distances with far more confidence.

For now, S251112cm is a clue. It invites a sharper search for black holes below one solar mass. It also gives dark matter researchers a concrete observational pathway.

## The next detectors in the hunt

LIGO operates two large detectors in the United States, one in Hanford, Washington and one in Livingston, Louisiana. Together with Virgo in Italy and KAGRA in Japan, the network listens for tiny distortions in spacetime from distant mergers.

Upgrades should improve sensitivity. Better detectors can observe weaker signals and more distant events. That increases the chance of catching rare mergers involving **subsolar mass black holes**.

Future observatories could extend the search even further. ESA's planned **LISA mission** is designed to detect gravitational waves from space. Its target launch is 2035 and it will probe a different range of gravitational-wave signals than ground-based detectors.

Another proposed facility, **Cosmic Explorer**, is being designed in the United States. Researchers expect it to be far more sensitive than current LIGO detectors. That could allow it to find black hole and neutron star mergers across a much larger stretch of cosmic history.

The appeal of the Miami result is that it turns an old idea into an observational challenge. Primordial black holes have lived for decades in theory. A repeating gravitational-wave signature would bring them much closer to the world of measured astronomy.

If future detections confirm the pattern, the implications would reach well beyond black hole physics. They could reveal a population of objects born before the first stars and offer a new route into the dark matter problem.