Time-domain properties of electromagnetic signals in a dynamical axion background

TL;DR

This paper investigates how electromagnetic signals behave in a dynamical axion background, revealing superluminal velocities, instabilities, and causal propagation features that differ from standard Maxwell theory.

Contribution

It derives retarded Green functions in axion electrodynamics and analyzes the unique time-domain properties of electromagnetic signals in this context.

Findings

Electromagnetic waves can have superluminal group velocities at high frequencies.

Instabilities cause exponential growth of energy density in certain modes.

Propagation occurs both inside and on the lightcone, unlike in standard electromagnetism.

Abstract

Electromagnetic waves in a dynamical axion background exhibit superluminal group velocities at high frequencies and instabilities at low frequencies, altering how photons propagate through space. Local disturbances propagate causally, but unlike in ordinary Maxwell theory, propagation occurs inside as well as on the lightcone. For the unstable modes, the energy density in the electromagnetic field grows exponentially along timelike displacements. In this paper we derive retarded Green functions in axion electrodynamics in various limits and study the time-domain properties of propagating signals.