Do any observed black-hole mergers involve primordial black holes?
In plain words
Gravitational-wave detectors have recorded hundreds of colliding black holes. Some of them might have been born in the early universe instead of from dying stars.
Precise statement
Determine the fraction of the LIGO-Virgo-KAGRA binary black hole merger rate contributed by primordial black holes, or an upper bound on it, using mass, spin and redshift distributions. A sub-solar-mass black hole ($M < 1\,M_{\mathrm{sun}}$) in a merger would strongly favor primordial origin, since stellar collapse cannot make black holes below about $1\,M_{\mathrm{sun}}$ (the remaining alternative being neutron stars converted to black holes by captured dark matter); mergers at redshift $z > 30$ seen by third-generation detectors would also point to primordial origin.
What would settle it
Detection or exclusion of sub-solar-mass compact binary mergers, plus redshift-resolved merger rates from Einstein Telescope or Cosmic Explorer.
Status in the literature
Unverified note
Mroz et al. (2024) bound compact halo objects of $1.8e-4 \text{ to } 6.3\,M_{\mathrm{sun}}$ below 1 percent of dark matter; the O4 candidate S251112cm (2025), found by the sub-solar-mass search, has low significance (false-alarm rate about 1 per 6 yr), and no confident sub-solar-mass merger exists.