Hunting Primordial Black Hole Dark Matter in Lyman-$\alpha$ Forest

TL;DR

This paper uses Lyman-alpha forest observations to set new constraints on primordial black holes as dark matter candidates by analyzing their Hawking radiation effects on the intergalactic medium's temperature.

Contribution

It introduces a novel method to constrain PBH dark matter using IGM temperature evolution from Lyman-alpha data, considering both astrophysical heating and non-heating scenarios.

Findings

Constraints on PBH fraction are below 0.1% for 10^{16} g mass.

The method provides competitive and complementary bounds compared to other observations.

Systematics differ from other probes, offering a new avenue for dark matter research.

Abstract

A very pressing question in contemporary physics is the identity of Dark Matter (DM). Primordial Black Holes (PBHs) are one of the most well-motivated DM candidates. Light PBHs have been constrained by either the non-detection of their Hawking radiation itself, or by the non-observation of any measurable effects of this radiation on astrophysical and cosmological observables. We constrain the PBH contribution to the DM density by non-detection of their Hawking radiation's effect on the intergalactic medium (IGM) temperature evolution. We use the latest deductions of IGM temperature from Lyman-$\alpha$ forest observations. We put constraints on the fraction of DM as PBHs with masses $5 \times 10^{15}$ g - $10^{17}$ g, separately for spinning and non-spinning BHs. We derive constraints by dealing with the heating effects of the astrophysical reionization sources on the IGM in two ways. In one way, we completely neglect this heating due to astrophysical sources, thus giving us weaker constraints, but completely robust to the reionization history of the universe. In the second way, we utilise some modelling of the ionization and temperature history, and use it to derive more stringent constraints. We find that for non-spinning PBHs of mass $10^{16}$ g, the current measurements can constrain the PBH-density to be $\lesssim$ 0.1\% of the total DM. We find that these constraints are competitive, and hence provide a new observable to probe the nature of PBH DM. The systematics affecting Lyman-$\alpha$ forest measurements are different from other constraining observations, and thus this is a complementary probe.