Abstract

Chameleon scalar fields have been proposed as candidates for dark energy. The chameleon property, 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. A direct coupling between the chameleon scalar field and the electromagentic field has been postulated, which causes mixing between photons and chameleons in a magnetic field. We examine the effects of this mixing on the CMB when it passes through the magnetic field of a galaxy cluster. The mixing affects both the intensity and polarisation of the incoming radiation. We predict the modification to the Stokes parameters of the CMB radiation over a range of typical galaxy clusters, and compare our predictions for the specific case of the Coma cluster to observations of the Sunyaev-Zel’dovich effect.