Relaxing CMB bounds on primordial black holes: The role of ionization fronts

The accretion of matter onto primordial black holes (PBHs) during the dark ages and the subsequent energy injection in the medium should have left imprints on the cosmic microwave background (CMB) anisotropies. Recent works have claimed stringent CMB limits on the PBH abundance, hardly compatible with a PBH interpretation of the gravitational-wave observations of binary black hole (BH) mergers. By using a more realistic accretion model based on hydrodynamical simulations and conservative assumptions for the emission efficiency, we show that CMB limits on the PBH abundance are up to 2 orders of magnitude less stringent than previously estimated between 10 and 104 M circle dot. This reopens the possibility that PBHs might explain at the same time (at least a fraction of) the dark matter, some of the LIGO-Virgo-KAGRA binary BH mergers, and the existence of supermassive BHs. More generally, we emphasize that PBH accretion can be a rather complex physical process with velocity dependences that are hard to assess, which introduces large uncertainties in accretion-based limits on the PBH abundance.

Automated summary

We find that the constraints on primordial black hole (PBH) abundance from cosmic microwave background (CMB) radiation are less strict than previously estimated by up to two orders of magnitude, based on a more realistic accretion model and conservative assumptions. This reopens the possibility that PBHs could explain the existence of dark matter, some LIGO-Virgo-KAGRA binary black hole mergers, and supermassive black holes.