Detuning primordial black hole dark matter with early matter domination and axion monodromy

TL;DR

This paper proposes a string-inspired inflationary scenario with early matter domination that reduces tuning in primordial black hole dark matter models, allowing PBHs in the observationally viable mass range to account for all dark matter while addressing evaporation constraints.

Contribution

It introduces a novel early matter domination framework with axion-like inflaton potential features, significantly reducing parameter tuning for PBH dark matter formation.

Findings

Primordial black holes in the mass range $10^{-13}M_{igodot}$ to $10^{-16}M_{igodot}$ can constitute all dark matter.

The scenario reduces the tuning of inflationary parameters compared to radiation domination models.

Potential conflicts with evaporation bounds can be mitigated with mild assumptions on collapse energy density.

Abstract

We present a scenario that ameliorates the tuning problems present in models of primordial black hole dark matter from inflation. Our setup employs the advantages of gravitational collapse in a long epoch of early matter domination with reheating temperature $\lesssim 10^{6}~\text{GeV}$. Furthermore, we make use of a string-inspired class of models where the inflaton is identified with a non-compact axion field. In this framework, the presence of multiple local minima in the inflaton potential can be traced back to an approximate discrete shift symmetry. This scenario allows the formation of primordial black holes in the observationally viable range of masses ($M_{\text{PBH}}\sim 10^{-13}M_{\odot}-10^{-16}M_{\odot}$) accounting for all dark matter, and in excellent agreement with the CMB. Crucially, we find a significant reduction in the required tuning of the parameters of the inflationary potential, in contrast to the standard case of primordial black hole formation during radiation domination. However, abundant formation of light PBHs during an early phase of matter domination can be more easily in conflict with evaporation bounds. We discuss how these can be avoided under mild assumptions on the collapsing energy density fraction.