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 λ such that λ=0 if no photons are lost and λ~1 if axion-photon mixing occurs. The best fit to Type Ia supernovae and radio galaxy data is λ=-0.3^+0.6_-0.4 (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 σ 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.