Optical properties of dynamical axion backgrounds

We discuss spectral distortions, time delays, and refraction of light in an axion or axion-plasma background. This involves solving the full set of geodesic equations associated to the system of Hamiltonian optics, allowing us to self-consistently take into account the evolution of the frequency, momentum, and position of photons. We support our arguments with analytic approximations and full numerical solutions. We also describe both nonintegrated and integrated versions of these effects, the latter of which depends on the whole history of the photon trajectory through the axion background. Remarkably, the introduction of a plasma enhances the sensitivity to axion-induced optical phenomena, allowing chiral refraction, integrated time delays, and integrated frequency shifts to occur at first order in the axion-photon coupling. This suggests a general enhancement of many axion-induced dispersive effects when the background refractive index is different from 1.