Relaxing CMB bounds on Primordial Black Holes: the role of ionization fronts

TL;DR

This paper revises constraints on primordial black holes from CMB data by using more realistic accretion models, suggesting PBHs could still account for dark matter, some gravitational wave sources, and supermassive black holes.

Contribution

It introduces a hydrodynamical simulation-based accretion model that significantly relaxes previous CMB bounds on PBH abundance, highlighting uncertainties in accretion physics.

Findings

CMB limits on PBH abundance are up to 100 times less stringent.

PBHs could still explain dark matter and some gravitational wave events.

Accretion physics introduces large uncertainties in PBH constraints.

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 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 two orders of magnitude less stringent than previously estimated between $10$ and $10^4$ M$_\odot$. 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 super-massive 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.