Relaxing CMB bounds on primordial black holes: The role of ionization fronts
Gaétan Facchinetti, Matteo Lucca, Sébastien Clesse
Abstract
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 ${10}^{4}$ ${M}_{\ensuremath{\bigodot}}$. 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.