Primordial Black Holes from the QCD axion

TL;DR

This paper introduces a new mechanism for primordial black hole formation linked to QCD axion models, occurring below the QCD phase transition through string-domain wall collapse, with implications for dark matter and cosmology.

Contribution

It presents a novel, inflation-independent PBH formation scenario in QCD axion models with domain wall number greater than one, connecting axion physics to primordial black holes.

Findings

PBHs with masses $10^4$-$10^7 M_igodot$ can form from string-domain wall collapse.

Dark matter is primarily axions with a small PBH component ($ ext{Ω}_ ext{PBH} extgreater{} 10^{-6} ext{Ω}_ ext{CDM}$).

The scenario offers observable signatures in axion searches and gravitational wave detectors.

Abstract

We propose a mechanism to generate Primordial Black Holes (PBHs) which is independent of cosmological inflation and occurs slightly below the QCD phase transition. Our setup relies on the collapse of long-lived string-domain wall networks and is naturally realized in QCD axion models with domain wall number $N_{DW}>1$ and Peccei-Quinn symmetry broken after inflation. In our framework, dark matter is mostly composed of axions in the meV mass range along with a small fraction, $\Omega_{\text{PBH}} \gtrsim 10^{-6} \Omega_{\text{CDM}} $ of heavy $M \sim 10^4-10^7 M_\odot$ PBHs. The latter could play a role in alleviating some of the shortcomings of the $\Lambda$CDM model on sub-galactic scales. The scenario has distinct signatures in ongoing axion searches as well as gravitational wave observatories.