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

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.

8 pages, 5 figures. v2: small correction in figure 1 (right panel)

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• Observational Evidence for Primordial Black Holes: A Positivist Perspective
• Curbing PBHs with PTAs
• Constraints on primordial black holes from LIGO-Virgo-KAGRA O3 events
• Primordial black holes: constraints, potential evidence and prospects
• Primordial Black-Hole-Based Pathways to Little Red Dots
• Astrophysical uncertainties challenge 21-cm forecasts: A primordial black hole case study
• Investigating Primordial Black Hole Accretion through Cosmic Optical Depth