Gravitational Origin of Phantom Dark Energy and Late Cosmic Acceleration
  In this letter, dark energy is obtained using dual roles of the Ricci scalar (as a physical field as well as geometry). Dark energy density, obtained here, mimics phantom and the derived Friedmann equation contains a term $\rho^2_{\rm de}/2 \lambda$ with $\rho_{\rm de}$ being the dark energy density and $\lambda$ being the cosmic tension. It is like brane-gravity inspired Friedmann equation, which arises without using the brane-gravity theory. It is found that acceleration is transient for $\lambda < 0$, but for $\lambda > 0$, expansion is found to encounter big-rip problem. It is shown that this problem can be avoided if dark energy behaves as barotropic fluid and generalized Chaplygin gas simultaneously. Moreover, time for transition (from deceleration to acceleration of the universe) is derived as a function of equation of state parameter ${\rm w}_{\rm de} = p_[rm de}/\rho_{\rm de}$ with $p_{\rm de}$ being the pressure for dark energy fluid.. 
S.K.Srivastava