Testing modified gravity with Planck: the case of coupled dark energy
  The Planck collaboration has recently published maps of the Cosmic Microwave
Background (CMB) radiation, in good agreement with a LCDM model, a fit
especially valid for multipoles l > 40. We explore here the possibility that
dark energy is dynamical and gravitational attraction between dark matter
particles is effectively different from the standard one in General Relativity:
this is the case of coupled dark energy models, where dark matter particles
feel the presence of a fifth force, larger than gravity by a factor beta^2. We
investigate constraints on the strength of the coupling beta in view of Planck
data. Interestingly, we show that a non-zero coupling is compatible with data
and find a likelihood peak at beta = 0.036 \pm 0.016 (Planck + WP + BAO)
(compatible with zero at 2sigma). The significance of the peak increases to
beta = 0.066 \pm 0.018 (Planck + WP + HST) (around 3.6sigma) when Planck is
combined to Hubble Space Telescope data. This peak comes mostly from the small
difference between the Hubble parameter determined with CMB measurements and
the one coming from astrophysics measurements. In this sense, future
observations and further tests of current observations are needed to determine
whether the discrepancy is due to systematics in any of the datasets. Our aim
here is not to claim new physics but rather to show how Planck data can be used
to provide information on dynamical dark energy and modified gravity, allowing
us to test the strength of an effective fifth force between dark matter
particles with precision smaller than 2%.
