Feedback in the dark: a critical examination of CMB bounds on primordial black holes

TL;DR

This paper critically reexamines CMB-based constraints on primordial black holes by considering local ionization effects, which can weaken previous bounds on their abundance in the universe.

Contribution

It introduces a revised analysis of CMB constraints on PBHs, emphasizing the impact of local ionization and temperature effects on accretion and mini-halo enhancement.

Findings

Local ionization can weaken CMB constraints on PBHs.

Revised conservative limits on PBH abundance are derived.

Mini-halo effects may be less significant than previously thought.

Abstract

If present in the early universe, primordial black holes (PBHs) will accrete matter and emit high-energy photons, altering the statistical properties of the {Cosmic Microwave Background (CMB)}. This mechanism has been used to constrain the fraction of dark matter that is in the form of PBHs to be much smaller than unity for PBH masses well above one solar mass. Moreover, the presence of dense dark matter mini-halos around the PBHs has been used to set even more stringent constraints, as these would boost the accretion rates. In this work, we critically revisit CMB constraints on PBHs taking into account the role of the local ionization of the gas around them. We discuss how the local increase in temperature around PBHs can prevent the dark matter mini-halos from strongly enhancing the accretion process, in some cases significantly weakening previously derived CMB constraints. We explore in detail the key ingredients of the CMB bound and derive a conservative limit on the cosmological abundance of massive PBHs.