How a cold axion background influences photons

TL;DR

This paper explores how a cold axion background from the early universe affects photon propagation, highlighting subtle yet potentially significant modifications despite weak coupling.

Contribution

It provides a summary of the photon property modifications caused by a cold axion condensate, emphasizing the physical implications of these effects.

Findings

Photon propagation is altered in a cold axion background.

Effects are small but can have notable consequences.

Potential implications for astrophysical observations.

Abstract

A cold relic axion condensate resulting from vacuum misalignment in the early universe oscillates with a frequency \sim m_a, where m_a is the axion mass. We summarize how the properties of photons propagating in such a medium are modified. Although the effects are small due to the magnitude of the axion-photon coupling, some consequences are striking.