Electromagnetic waves propagating in the string axiverse

TL;DR

This paper investigates how axion dark matter in string theory can induce electromagnetic phenomena such as parametric resonance and oscillations in the speed of light, offering potential observational constraints.

Contribution

It analyzes the effects of axion-photon coupling on electromagnetic wave resonance and light speed oscillations, providing new insights into axion properties and detection prospects.

Findings

Parametric resonance enhances electromagnetic wave amplitudes.

Circularly polarized monochromatic waves can be generated.

Light speed oscillates but is difficult to observe.

Abstract

It is widely believed that axions are ubiquitous in string theory and could be the dark matter. The peculiar features of the axion dark matter are coherent oscillations and a coupling to the electromagnetic field through the Chern-Simons term. In this paper, we study consequences of these two features of the axion with the mass in a range from $10^{-13}\,{\rm eV}$ to $10^{3}\,{\rm eV}$. First, we study the parametric resonance of electromagnetic waves induced by the coherent oscillation of the axion. As a result of the resonance, the amplitude of the electromagnetic waves is enhanced and the circularly polarized monochromatic waves will be generated. Second, we study the velocity of light in the background of the axion dark matter. In the presence of the Chern-Simons term, the dispersion relation is modified and the speed of light will oscillate in time. It turns out that the change of speed of light would be difficult to observe. We argue that the future radio wave observations of the resonance can give rise to a stronger constraint on the coupling constant and/or the density of the axion dark matter.