Cosmological Effects of Scalar-Photon Couplings: Dark Energy and
  Varying-alpha Models
  We study cosmological models involving scalar fields coupled to radiation and
discuss their effect on the redshift evolution of the cosmic microwave
background temperature, focusing on links with varying fundamental constants
and dynamical dark energy. We quantify how allowing for the coupling of scalar
fields to photons, and its important effect on luminosity distances, weakens
current and future constraints on cosmological parameters. In particular, for
evolving dark energy models, joint constraints on the dark energy equation of
state combining BAO radial distance and SN luminosity distance determinations,
will be strongly dominated by BAO. Thus, to fully exploit future SN data one
must also independently constrain photon number non-conservation arising from
the possible coupling of SN photons to the dark energy scalar field. We discuss
how observational determinations of the background temperature at different
redshifts can, in combination with distance measures data, set tight
constraints on interactions between scalar fields and photons, thus breaking
this degeneracy. We also discuss prospects for future improvements,
particularly in the context of Euclid and the E-ELT and show that Euclid can,
even on its own, provide useful dark energy constraints while allowing for
photon number non-conservation.
