Abstract

The chameleon scalar field has been proposed as a possible candidate for dark energy. The chameleon mechanism, whereby the mass of the scalar depends on the surrounding matter density, allows the scalar field to have a strong coupling to normal matter and yet still evade fifth-force constraints. An extension of the original chameleon model includes a direct coupling between the chameleon and electromagnetic field. This generates mixing between chameleons and photons in the presence of a background magnetic field. Here we consider the modification to the average intensity of the cosmic microwave background as a result of chameleon-photon mixing in a primordial magnetic field. We find that comparison of these predictions to precision measurements of the CMB black-body spectrum places strong constraints on the combined magnitude of the primordial magnetic field and chameleon-photon coupling strength.