Probing the interaction between dark energy and dark matter with the
  parametrized post-Friedmann approach
  There possibly exists some direct, non-gravitational coupling between dark
energy and dark matter. This possibility should be seriously tested by using
observations, which requires us to understand such a scenario from the aspects
of both expansion history and growth of structure. It is found that once
calculating the perturbations in the interacting dark energy (IDE) scenario,
for most cases the curvature perturbation on superhorizon scales is divergent,
which is a catastrophe for the IDE cosmology. We found a solution to this
issue, which is to establish an effective theory to treat the dark energy
perturbations totally based on the basic facts of dark energy. This scheme
generalizes the parametrized post-Friedmann framework of uncoupled dark energy
and can be used to cure the instability of the IDE cosmology. The whole
parameter space of IDE models can henceforward be explored by observational
data. The IDE scenario can thus be tested or falsified with current and future
observational data by using the PPF approach. We expect that the future highly
accurate observational data would offer the certain answer to the question
whether there is a direct coupling between dark energy and dark matter.
