Electromagnetic Couplings of Dark Domain Walls

Abstract

We extend Maxwell electrodynamics with a Chern--Simons coupling to a dark sector top form sourced by domain walls. Cosmic birefringence can arise from a distinct mechanism in which photon polarization is rotated when crossing vacuum interfaces, rather than through adiabatic propagation in a background field. Ultrathin walls induce a finite, frequency-independent polarization rotation generated by an electromagnetic Chern-Simons term localized at the interface. The effect persists even in the absence of ultralight axions or other propagating scalar degrees of freedom. For phenomenologically viable parameters, such walls can generate cosmic microwave background polarization rotation at the level $Δ\vartheta \sim 10^{-3}$ rad, providing a signature of the topological structure of the dark-sector vacuum.