Stability of curvature perturbation with new covariant form for energy-momentum transfer in dark sector

It was found that the model with interaction between cold dark matter (CDM) and dark energy (DE) proportional to the energy density of CDM ρ m and constant equation of state of DE wd suffered from instabilities of the density perturbations on the super-Hubble scales. Here we suggest a new covariant model for the energy-momentum transfer between CDM and DE. Then using the covariant model, we analyze the evolution of density perturbations on the super-Hubble scale. We find that the instabilities can be avoided in the model with constant wd and interaction proportional to ρ m. Furthermore, we analyze the dominant non-adiabatic mode in the radiation era and find that the mode grows regularly. (orig.)