Distinguishing interacting dark energy from wCDM with CMB, lensing, and baryon acoustic oscillation data

We employ the Planck 2013 CMB temperature anisotropy and lensing data, and baryon acoustic oscillation (BAO) data to constrain a phenomenological $w$CDM model, where dark matter and dark energy interact. We assume time-dependent equation of state parameter for dark energy, and treat dark matter and dark energy as fluids whose energy-exchange rate is proportional to the dark-matter density. The CMB data alone leave a strong degeneracy between the interaction rate and the physical CDM density parameter today, $\omega_c$, allowing a large interaction rate $|\Gamma| \sim H_0$. However, as has been known for a while, the BAO data break this degeneracy. Moreover, we exploit the CMB lensing potential likelihood, which probes the matter perturbations at redshift $z \sim 2$ and is very sensitive to the growth of structure, and hence one of the tools for discerning between the $\Lambda$CDM model and its alternatives. However, we find that in the non-phantom models ($w_{\mathrm{de}}>-1$), the constraints remain unchanged by the inclusion of the lensing data and consistent with zero interaction, $-0.14 < \Gamma/H_0 < 0.02$ at 95\% CL. On the contrary, in the phantom models ($w_{\mathrm{de}}<-1$), energy transfer from dark energy to dark matter is moderately favoured over the non-interacting model; $-0.57 < \Gamma/H_0 < -0.10$ at 95\% CL with CMB+BAO, while addition of the lensing data shifts this to $-0.46 < \Gamma/H_0 < -0.01$.