Primordial Black Holes from the QCD axion

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.

Comments: 7 pages, 2 figures. v2: version accepted for publication in PRL