New Limits on Coupled Dark Energy from Planck

Recently, the Planck collaboration has released the first cosmological papers providing the high resolution, full sky, maps of the cosmic microwave background (CMB) temperature anisotropies. It is crucial to understand that whether the accelerating expansion of our universe at present is driven by an unknown energy component (Dark Energy) or a modification to general relativity (Modified Gravity). In this paper we study the coupled dark energy models, in which the quintessence scalar field nontrivially couples to the cold dark matter, with the strength parameter of interaction $beta$. Using the Planck data alone, we obtain that the strength of interaction between dark sectors is constrained as $beta < 0.102$ at $95%$ confidence level, which is tighter than that from the WMAP9 data alone. Combining the Planck data with other probes, like the Baryon Acoustic Oscillation (BAO), Type-Ia supernovae ``Union2.1 compilation'' and the CMB lensing data from Planck measurement, we find the tight constraint on the strength of interaction $beta < 0.052$ ($95%$ C.L.). Interestingly, we also find a non-zero coupling $beta = 0.078 pm 0.022$ ($68%$ C.L.) when we use the Planck, the ``SNLS'' supernovae samples, and the prior on the Hubble constant from the Hubble Space Telescope (HST) together. This evidence for the coupled dark energy models mainly comes from a tension between constraints on the Hubble constant from the Planck measurement and the local direct $H_0$ probes from HST.