GRAVITATIONAL ORIGIN OF PHANTOM DARK ENERGY AND LATE COSMIC ACCELERATION S.K.Srivastava

Here, dark energy is obtained using dual roles of the Ricci scalar (as a physical field as well as geometry). Dark energy density, obtained in this model, mimics phantom and the derived Friedmann equation contains a term ρde/2λ with ρde being the dark energy density and λ, called as cosmic tension. It is like brane-gravity inspired Friedmann equation, which arises here without using the brane-gravity theory. It is found that acceleration is a transient phenomenon for λ < 0, but for λ > 0 accelerated expansion is found to encounter the big-rip problem. It is shown that this problem can be avoided if the 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 wde = pde/ρde with pde being the pressure for dark energy fluid. PACS no. 98.80.Cq