How a cold axion background influences photons.

Domenec Espriu & Albert Renau
A cold relic axion condensate resulting from vacuum misalignment in the early universe oscillates with a frequency ~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.