# Constraining the primordial black hole abundance with 21cm cosmology

## TL;DR

This paper explores how 21cm cosmology observations during reionization and cosmic dawn can tightly constrain the abundance of primordial black holes, which could significantly influence early universe properties and dark matter composition.

## Contribution

It introduces a novel approach to constrain solar mass primordial black holes using 21cm observations, considering multiple effects on the intergalactic medium.

## Key findings

- 21cm observations can improve constraints on PBHs by over an order of magnitude.
- PBHs cause distinct effects on the intergalactic medium, detectable via 21cm signals.
- Upcoming experiments like SKA and HERA are key to these constraints.

## Abstract

The discoveries of a number of binary black hole mergers by LIGO and VIRGO has reinvigorated the interest that primordial black holes (PBHs) of tens of solar masses could contribute non-negligibly to the dark matter energy density. Should even a small population of PBHs with masses $\gtrsim \mathcal{O}(M_\odot)$ exist, they could profoundly impact the properties of the intergalactic medium and provide insight into novel processes at work in the early Universe. We demonstrate here that observations of the 21cm transition in neutral hydrogen during the epochs of reionization and cosmic dawn will likely provide one of the most stringent tests of solar mass PBHs. In the context of 21cm cosmology, PBHs give rise to three distinct observable effects: ${\textit{(i)}}$ the modification to the primordial power spectrum (and thus also the halo mass function) induced by Poisson noise, ${\textit{(ii)}}$ a uniform heating and ionization of the intergalactic medium via X-rays produced during accretion, and ${\textit{(iii)}}$ a local modification to the temperature and density of the ambient medium surrounding isolated PBHs. Using a four-parameter astrophysical model, we show that experiments like SKA and HERA could potentially improve upon existing constraints derived using observations of the cosmic microwave background by more than one order of magnitude.