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.
