Uses of the axion-photon coupling

• Georges Obied (Oxford)

In this talk, I will discuss two applications of the Chern-Simons term coupling the axion to gauge fields (such as in axion electrodynamics). First, I will describe how this coupling can facilitate the non-perturbative decay of parallel electric and magnetic fields via a process analogous to Schwinger pair production. In this case, instead of producing particles, an axion domain wall is nucleated with induced charges and currents on the wall arising from the Witten and Sikivie (anomalous Hall) effects, respectively. This process leads to elongated bubble configurations with potentially non-trivial wall substructure. Next, I will examine the friction experienced by axion domain walls due to their interaction with a relativistic plasma through the axion-photon coupling. This discussion will outline how the friction varies with the thickness of the axion domain wall compared to various plasma scales. Finally, I will briefly discuss the potential applications of these phenomena to early universe cosmology.