abstract:

The QCD axion may solve the strong CP problem and constitute the dark matter (DM) abundance in our Universe. I will discuss how the cosmology of string theory axions is fundamentally different from that of Peccei-Quinn (PQ) field theory axions. In particular, while field theory axions may form axion strings if the PQ phase transition occurs after inflation, string theory axions do not generically form strings. However, they may form in special inflationary paradigms such as brane inflation – in such cases I will discuss what to expect for the QCD axion DM abundance and gravitational wave signals. 
Lastly, I will briefly cover recent advances in the computation of the QCD axion mass from the most precise and accurate lattice simulations to-date of axion-string networks.