Comprehensively Constraining Ultra-Light Primordial Black Holes Through Relic Formation and Early Mergers

TL;DR

This paper explores constraints on ultra-light primordial black holes, considering relic formation and early mergers, which extend the limits on their abundance and impact on early universe phenomena.

Contribution

It provides a comprehensive analysis of how relics and early mergers affect constraints on ultra-light PBHs, extending previous bounds to smaller masses.

Findings

Relic formation significantly constrains PBH abundance.

Early mergers extend PBH lifetime and influence BBN constraints.

New constraint plots for ultra-light PBHs including relic effects.

Abstract

We investigate constraints on the least explored, smallest mass scales of primordial black holes (PBHs), which evaporate prior to Big Bang Nucleosynthesis (BBN). Our study examines the impact of Planck-mass relics on the allowed fraction of dark matter composed of PBHs ($f_{PBH}$), as well as on the resulting stochastic gravitational wave background and the formation of primordial binaries. We discuss how these binaries and early mergers lead to longer PBH lifetimes, extending the reach of the stringent BBN constraints to smaller masses than usually expected. We make comprehensive constraint plots on the collapse fraction $\beta$ and $f_{PBH}$ (including relics), focusing on ultra-light PBHs.