Cosmic Acceleration versus Axion‐Photon Mixing

Axion-photon mixing has been proposed as an alternative to acceleration asthe explanation for supernovae dimming. We point out that the loss of photonsdue 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 unaffectedby mixing. In a first search for such an asymmetry we introduce a dimensionlessmixing amplitude lambda so that lambda=0 if no photons are lost and lambda = 1if axion-photon mixing occurs. The best-fit to SNIa and radio galaxy data islambda = -0.3^{+0.6}_{-0.4} (95% CL), corresponding to an unphysical, negative,mixing length. This same argument limits the attenuation of light fromsupernovae due to dust. We show that future d_L and d_A data from SNAP andgalaxy surveys such as DEEP2 and KAOS will detect or rule out mixing at morethan 5-sigma, almost independently of the dark energy dynamics. Finally wediscuss the constraints from the near maximal polarisation of the gamma-rayburst (GRB) GRB021206. Since mixing reduces the polarisation of distantsources, future observations of high redshift GRBs may provide orthogonalconstraints on axion-photon mixing and related scenarios.Comment: 4 pages, 4 colour figures. Matches version to appear in ApJ. Significantly stronger constraints arising from new data: universe still accelerating at 3 sigm