  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.
