Gravitational wave signals of early universe axion dynamics

Synopsis

The detection of gravitational waves from the merger of black  holes of several tens of solar masses has reignited the  interest of  primordial black holes (PBHs) as the source of part of the dark matter  in the universe. A small relic abundance of heavy PBHs may play an  important role in the generation of cosmological structures, and we  will discuss how such a PBH population can be generated by the collapse of axionic topological defects created during the QCD phase transition. Furthermore, some axion-like particle (ALP) models feature a strong first order phase transition at a high energy scale, ~f_a. We will show  how future gravitational-wave experiments can probe the ALP parameter  space in a way that is complementary to current laboratory and  astrophysical searches