Revisiting the Instability Problem of Interacting Dark Energy Model in
  the Parametrized Post-Friedmann Framework
  Dark energy might interact with dark matter in a direct, non-gravitational
way, which can help remedy several theoretical defects. In order to find out
the properties of interacting dark energy models, it is necessary to
investigate the cosmological perturbations in detail. However, due to the
improper use of pressure perturbation of dark energy, a large-scale instability
at the early stage occurs occasionally. In recent years, parametrized
post-Friedmann approach has been used to calculate the perturbation equations
in the interacting dark energy scenario. Under this framework, the dark energy
pressure perturbation was replaced by the relationship between the momentum
density of dark energy and other components on a large scale. However, this
paper shows that if the interaction terms are related to the velocity
perturbation of dark energy, the density perturbation of dark matter and the
matter power spectrum will diverge when the equation of state parameter of dark
energy $w$ is close to $-1$. A simple parameterization steering clear of this
problem is proposed in this paper which is a more general form and can be
applied to explore the interaction between dark matter and dark energy by using
various cosmological data.
