LIGO Subsolar-Mass Alert Points to Primordial Black Hole Candidate

LIGO issued the alert during its fourth observing run for an event whose component masses fall below the range expected from stellar collapse. Cappelluti and Magaraggia modeled the expected cosmic abundance of primordial black holes formed in the radiation-dominated era and calculated the rate at which LIGO should register subsolar mergers if these objects constitute the dark-matter density. Their calculation reproduces the observed scarcity of such events while remaining consistent with existing upper limits from microlensing and cosmic microwave background data. The result hinges on the absence of any electromagnetic counterpart or host galaxy that would be required for a conventional stellar-origin black hole. Because the inferred mass lies below the pair-instability supernova gap and above the maximum mass of a stable neutron star, standard astrophysical channels are effectively ruled out. The analysis therefore isolates primordial formation as the only remaining channel that simultaneously satisfies both the gravitational-wave data and the cosmological dark-matter budget. Confirmation will require additional subsolar events whose statistical properties match the predicted mass function and redshift distribution. Future LIGO-Virgo-KAGRA runs, together with third-generation detectors, can test whether the event rate rises at higher redshift as expected for a primordial population formed before reionization. A null result after several more years of observation would falsify the claim that these objects dominate dark matter.