Cosmological effects of scalar-photon couplings: dark energy and varying-α Models

Abstract

We study cosmological models involving scalar fields coupled to radiation and discuss their effecton the redshift evolution of the cosmic microwave background temperature, focusing on links withvarying fundamental constants and dynamical dark energy. We quantify how allowing for the couplingof scalar fields to photons, and its important effect on luminosity distances, weakens current and futureconstraints on cosmological parameters. In particular, for evolving dark energy models, joint constraintson the dark energy equation of state combining BAO radial distance and SN luminosity distancedeterminations, will be strongly dominated by BAO. Thus, to fully exploit future SN data one must alsoindependently constrain photon number non-conservation arising from the possible coupling of SNphotons to the dark energy scalar field. We discuss how observational determinations of the backgroundtemperature at different redshifts can, in combination with distance measures data, set tight constraints oninteractions between scalar fields and photons, thus breaking this degeneracy. We also discuss prospectsfor future improvements, particularly in the context of Euclid and the E-ELT and show that Euclid can, evenon its own, provide useful dark energy constraints while allowing for photon number non-conservation.