Cosmic microwave background constraints on coupled dark matter

We study CMB constraints on a scenario where a fraction of dark matter is non-minimally coupled to a massless scalar field and dark energy is in the form of a cosmological constant. In this case, there is an extra gravity-like fifth force which can affect the evolution of the Universe enough to have a discernible effect on measurements of cosmological parameters. Using Planck and WMAP polarisation data, we find that up to half of the dark matter can be coupled. The coupling can also be several times larger than in models with a single species of cold dark matter coupled to a quintessence scalar field, as the scalar field does not play the role of dark energy and is therefore less constrained by the data.