Photon-axion oscillations and Type Ia supernovae

We compute the probability of photon-axion oscillations in the presence of both intergalactic magnetic fields and an electron plasma and investigate the effect on Type Ia supernovae observations. The conversion probability is calculated using a density matrix formalism by following light-paths through simulated universes in a Monte-Carlo fashion. We find, that even though the effect is highly frequency dependent, one needs to analyze relatively narrow spectral features of high redshift objects in order to discern between the dimming effect from oscillations and a cosmological constant, in contrast to earlier claims that broad-band photometry is sufficient.