Primordial Black Holes from Axion Domain Wall Collapse

Besides potentially solving the strong CP problem and being the dark matter of our universe, the QCD axion possesses a very rich cosmology. At early times, topological defects known as axion strings and domain walls can populate the universe. While most of these defects decay into axion dark matter, a small subset of domain walls may collapse in on themselves, potentially forming primordial black holes (PBHs). These PBHs - naturally a subfraction of the present day dark matter abundance - can possibly be observed by future gravitational lensing surveys. In this talk, I will discuss the abundance of domain walls at early times, the dynamics dictating how they form PBHs, and the PBH relic abundance they produce.