A note on holographic dark energy with varying $c^2$ term
  We reconsider the holographic dark energy (HDE) model with a slowly time varying $ c^2(z)$ parameter in the energy density, namely $\rho_D=3M_p^2 c^2(z)/L^2$, where $L$ is the IR cutoff and $z$ is the redshift parameter. As the system's IR cutoff we choose the Hubble radius and the Granda-Oliveros (GO) cutoffs. The latter inspired by the Ricci scalar curvature. We derive the evolution of the cosmological parameters such as the equation of state and the deceleration parameters as the explicit functions of the redshift parameter $z$. Then, we plot the evolutions of these cosmological parameters in terms of the redshift parameter during the history of the universe. Interestingly enough, we observe that by choosing $L=H^{-1}$ as the IR cutoff for the HDE with time varying $ c ^2(z) $ term, the present acceleration of the universe expansion can be achieved, even in the absence of interaction between dark energy and dark matter. This is in contrast to the usual HDE model with constant $c^2$ term, which leads to a wrong equation of state, namely that for dust $w_D=0$, when the IR cutoff is chosen the Hubble radius. 
A. Sheykhi, S. Ghaffari and N. Roshanshah
physics.gen-ph hep-th