A cold relic axion condensate resulting from vacuum misalignment in the early universe oscillates in time 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. In this talk I will discuss three different effects: axion-induced bremstrahlung from high-energy cosmic rays, some photon wave-vectors are forbidden in the universe due to the axion oscillations, and possible (and extremely difficult) measurements of the axion density in laboratory experiments.