Primordial Black Holes from the QCD Axion.

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_{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, Ω_{PBH}≳10^{-6}Ω_{CDM} of heavy M∼10^{4}-10^{7}  M_{⊙} PBHs.