Primordial Black Holes as Dark Matter: Almost All or Almost Nothing

TL;DR

This paper investigates the potential of primordial black holes as dark matter candidates by analyzing the luminous signals from ultracompact minihalos formed around them, setting new constraints on their abundance.

Contribution

It introduces a novel method to constrain primordial black hole abundance using gamma-ray and neutrino flux limits from annihilation in UCMHs, improving previous bounds significantly.

Findings

Omega_PBH <~ 10^-4 for m_DM c^2 ~ 100 GeV

Strong flux constraints limit PBH abundance

UCMH luminosity evolution affects limits

Abstract

Primordial black holes (PBHs) are expected to accrete particle dark matter around them to form ultracompact minihalos (UCMHs), if the PBHs themselves are not most of the dark matter. We show that if most dark matter is a thermal relic, then the inner regions of UCMHs around PBHs are highly luminous sources of annihilation products. Flux constraints on gamma rays and neutrinos set strong abundance limits, improving previous limits by orders of magnitude. Assuming enough particle dark matter exists to form UCMHs, we find that Omega_PBH <~ 10^-4 (for m_DM c^2 ~ 100 GeV) for a vast range in PBH mass. We briefly discuss the uncertainties on our limits, including those due to the evolution of the UCMH luminosity as it annihilates.