Optical properties of dynamical axion backgrounds

TL;DR

This paper investigates how axion and axion-plasma backgrounds affect light propagation, revealing that plasma enhances axion-induced birefringence and allows detection at linear order in the coupling.

Contribution

It provides a comprehensive analysis of spectral distortions, time delays, and refraction effects of light in axion backgrounds using Hamiltonian optics, including analytic and numerical methods.

Findings

Plasma enhances sensitivity to axion-induced birefringence.

Effects can occur at linear order in axion-photon coupling.

Refractive index differences amplify birefringence effects.

Abstract

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 momentum, frequency and position of photons. We support our arguments with analytic approximations and full numerical solutions. Remarkably, the introduction of a plasma enhances the sensitivity to axion-induced birefringence, allowing these effects to occur at linear order in the axion-photon coupling even when the axion background is not present at either the emission or detection points. This suggests a general enhancement of axion-induced birefringence when the background refractive index is different from one.