Constraints on coupled dark energy using CMB data from WMAP and South Pole Telescope

We consider the case of a coupling in the dark cosmological sector, where a dark energy scalar field modifies the gravitational attraction between dark matter particles. We find that the strength of the coupling is constrained using current Cosmic Microwave Background data, including WMAP7 and the South Pole Telescope (SPT), to be less than 0. 063 (0. 11) at a 68% (95%) confidence level. Further, we consider the additional effects of the Cosmic Microwave Background lensing amplitude, curvature, effective number of relativistic species, and massive neutrinos and show that the bound from current data on is already strong enough to be rather stable with respect to any of these variables. The strongest effect is obtained when we allow for massive neutrinos, in which case the bound becomes slightly weaker, <0. 084 (0. 14). A larger value of the effective number of relativistic degrees of freedom favors larger couplings between dark matter and dark energy, as well as values of the spectral index closer to 1. Adding the present constraints on the Hubble constant, as well as those from baryon acoustic oscillations and Type Ia supernovae, we find <0. 050 (0. 074). In this case, we also find an interesting likelihood peak for =0. 041 (still compatible with 0 at 1). This peak comes mostly from a slight difference between the Hubble parameter measured by the Hubble Space Telescope and the WMAP7+SPT best fit. Finally, we show that forecasts of Planck+SPT mock data can pin down the coupling to a precision of better than 1% and detect whether the marginal peak we find at small nonzero coupling is a real effect.