On November 12, 2025, the LIGO-Virgo-KAGRA collaboration issued an automatic alert about an unusual black hole merger, the event S251112cm, which may become the first direct hint of primordial black holes (PBH) formed in the first seconds after the Big Bang. One of the objects had a mass significantly below that of the Sun, which is far too small for a stellar black hole or a neutron star but fits perfectly with the theoretical PBH range. If confirmed, this discovery would overturn our understanding of dark matter and the early Universe, though the probability of a false signal remains high.

What primordial black holes are and why they matter

Ordinary black holes, whether stellar (5 to 100 solar masses) or supermassive (millions to billions of solar masses), are born from the collapse of stars or from mergers. Primordial black holes are completely different: they could have formed from extremely dense fluctuations in the hot plasma of the early Universe only microseconds after the Big Bang. Their masses range from microscopic (the size of an atom) to planetary, and they are not linked to stars.

PBH are one of the main candidates for dark matter, which makes up 85 percent of all matter in the Universe. Unlike exotic particles beyond the Standard Model, PBH fit the current cosmological framework without additional hypotheses. However, they have not yet been directly detected. Lightweight ones evaporate through Hawking radiation in seconds, and heavy ones are too rare. If real, the S251112cm signal could be a merger of two such PBH with subsolar masses.

The S251112cm signal: what the detectors saw

Since 2015, LIGO and Virgo have detected gravitational waves from black hole and neutron star mergers, with more than 300 events recorded. But S251112cm stands out because the mass of one object is below one solar mass, which rules out stellar remnants, as the minimum mass for such objects is at least one solar mass. The total mass of the system after the merger is below the mass of the Sun, and the wave pattern before the merger indicates very compact objects.

LIGO gravitational astronomer Christopher Berry wrote on Bluesky: “Interesting candidate #S251112cm potentially from a subsolar mass source.”

Theoretical physicist Juna Kroon from Durham University, who was not involved in the observation, noted: “If this turns out to be real, it is huge. It is an event that we cannot explain with standard astrophysical processes.”

The signal has been localized to a region of the sky six thousand times larger than the Moon. Searches for an electromagnetic counterpart in optical or gamma wavelengths are ongoing, but chances are low.

False alarm probability: one signal is not proof

The false alarm rate for such rare events is about once every four years, compared with once every ten to twenty years for typical black hole mergers. Detector noise such as vibrations or seismic activity can mimic a signal. Full confirmation requires:

  • deeper analysis of O4 (the fourth observing run of LIGO-Virgo-KAGRA)
    • searching for similar events in future runs, with O5 starting in 2026
    • modeling: if PBH are the source, their density in the subsolar mass range must be between 10^{-3} and 10^{-2} of the total dark matter

Kroon emphasizes: “It is unlikely that we will ever know for sure whether this alert was real or not.”

Summary

The S251112cm signal detected by LIGO-Virgo-KAGRA on November 12, 2025, may be the first hint of a merger of primordial black holes with subsolar masses, possible candidates for dark matter born in the Big Bang. The masses of the objects are too small for stellar remnants, but the probability of a false signal is approximately one in four years. Confirmation requires new data, and if real, the finding would be a revolution in cosmology.