Cosmic acceleration versus axion-photon mixing

Axion-photon mixing has been proposed as an alternative to acceleration as the explanation for supernovae dimming. We point out that the loss of photons due to this mixing will induce a strong asymmetry between the luminosity, d(L)(z), and angular diameter distance, d(A)(z), since the latter is unaffected by mixing. In a first search for such an asymmetry, we introduce a dimensionless mixing amplitude lambda such that lambda = 0 if no photons are lost and lambda approximate to 1 if axion-photon mixing occurs. The best fit to Type Ia supernovae and radio galaxy data is lambda = -0.3(-0.4)(+0.6) (95% confidence level), corresponding to an unphysical, negative mixing length. This same argument limits the attenuation of light from supernovae due to dust. We show that future d(L) and d(A) data from the Supernova/Acceleration Probe and galaxy surveys such as DEEP2 and KAOS will detect or rule out mixing at more than 5 sigma almost independently of the dark energy dynamics. Finally, we discuss the constraints from the near-maximal polarization of the gamma-ray burst (GRB) GRB 021206. Since mixing reduces the polarization of distant sources, future observations of high-redshift GRBs will provide orthogonal constraints on axion-photon mixing and related scenarios.