Interaction between dark energy and dark matter: observational
  constraints from OHD, BAO, CMB and SNe Ia
  In order to test if there is energy transfer between dark energy and dark
matter, we investigate cosmological constraints on two forms of nontrivial
interaction between the dark matter sector and the sector responsible for the
acceleration of the universe, in light of the newly revised observations
including OHD, CMB, BAO and SNe Ia. More precisely, we find the same tendencies
for both phenomenological forms of the interaction term $Q=3\gamma H\rho$,
i.e., the parameter $\gamma$ to be a small number, $|\gamma|\approx 10^{-2}$.
However, concerning the sign of the interaction parameter, we observe that
$\gamma>0$ when the interaction between dark sectors is proportional to the
energy density of dust matter, whereas the negative coupling ($\gamma<0$) is
preferred by observations when the interaction term is proportional to dark
energy density. We further discuss two possible explanations to this
incompatibility and apply a quantitative criteria to judge the severity of the
coincidence problem. Results suggest that the $\gamma_m$IDE model with a
positive coupling may alleviate the coincidence problem, since its coincidence
index $C$ is smaller than that for the $\gamma_d$IDE model, the interacting
quintessence and phantom models by four orders of magnitude.
