# Primordial Black Holes from the QCD Axion

Ferrer, Francesc (Department of Physics, McDonnell Center for the Space Sciences, Washington University, St. Louis, Missouri 63130, USA) (IFAE and BIST, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain) ; Masso, Eduard (IFAE and BIST, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain) (Departament de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain) ; Panico, Giuliano (IFAE and BIST, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain) (Laboratory of High Energy and Computational Physics, National Institute of Chemical Physics and Biophysics, Rävala pst. 10, 10143 Tallinn, Estonia) ; Pujolas, Oriol (IFAE and BIST, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain) ; Rompineve, Fabrizio (IFAE and BIST, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Barcelona, Spain)

13 March 2019

Abstract: We propose a mechanism to generate primordial black holes (PBHs) that 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 ${N}_{\mathrm{DW}}>1$ 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, ${\Omega }_{\mathrm{PBH}}\gtrsim {10}^{-6}{\Omega }_{\mathrm{CDM}}$ of heavy $M\sim {10}^{4}-{10}^{7}\text{}\text{}{M}_{\odot }$ PBHs. The latter could play a role in alleviating some of the shortcomings of the standard cosmological model on subgalactic scales. The scenario might have distinct signatures in ongoing axion searches as well as gravitational wave observatories.

Published in: Physical Review Letters 122 (2019)