Bending of light in axion backgrounds

TL;DR

This paper investigates how light bends in axion backgrounds, comparing full wave solutions with geometric optics, and finds conditions where wave effects are significant or suppressed, impacting lensing predictions.

Contribution

It provides a detailed analysis of light refraction in axion backgrounds, highlighting the importance of wave-optical effects and plasma conditions for accurate modeling.

Findings

Refraction agrees with geometric optics near the optical axis without plasma.

Wave effects dominate for rays far from the optical axis at small incidence angles.

Large plasma masses suppress wave effects, validating geometric optics across all rays.

Abstract

In this work we examine refraction of light by computing full solutions to axion electrodynamics. We also allow for the possibility of an additional plasma component. We then specialise to wavelengths which are small compared to background scales to determine if refraction can be described by geometric optics. We also allow for the possibility of an additional plasma component. In the absence of plasma, for small incidence angles relative to the optical axis, axion electrodynamics and geometric optics are in good agreement, with refraction occurring at $\mathcal{O}(g_{a \gamma \gamma}^2)$. However, for rays which lie far from the optical axis, the agreement with geometric optics breaks down and the dominant refraction requires a full wave-optical treatment, occurring at $\mathcal{O}(g_{a \gamma \gamma})$. In the presence of sufficiently large plasma masses, the wave-like nature of light becomes suppressed and geometric optics is in good agreement with the full theory for all rays. Our results therefore suggest the necessity of a more comprehensive study of lensing and ray-tracing in axion backgrounds, including a full account of the novel $\mathcal{O}(g_{a \gamma \gamma})$ wave-optical contribution to refraction.