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Gravitational Origin of Phantom Dark Energy and Late Cosmic Acceleration.pdf

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Gravitational Origin of Phantom Dark Energy and Late Cosmic Acceleration

a r X i v : h e p - t h / 0 6 0 5 0 1 0 v 3 3 0 M a r 2 0 0 7 GRAVITATIONAL ORIGIN OF PHANTOM DARK ENERGY AND LATE COSMIC ACCELERATION S.K.Srivastava Department of Mathematics, North Eastern Hill University, NEHU Campus,Shillong - 793022 ( INDIA ) e-mail:srivastava@nehu.ac.in ; sushil@iucaa.ernet.in Abstract 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 ρ2de/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 prob- lem. It is shown that this problem can be avoided if the dark energy behaves as barotropic fluid and generalized Chaplygin gas simultane- ously. 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. Keywords : Dual role of the Ricci scalar; phantom dark energy;transient acceleration; Chaplygin gas and avoidance of big-rip singularity. PACS no. 98.80.Cq 1 2 S.K.SRIVASTAVA 1. Introduction Astronomical observations, during the last few years, provide com- pelling evidences in favor of accelerating universe at present, which is caused by dominance of dark energy (DE) [1, 2, 3, 4, 5, 6, 7, 8]. Obser- vation of 16 Type Ia supernovae (SNe Ia) by Hubble Space Telescope further modifies these results and shows evidence for cosmic deceler- ation preceding acceleration in the late universe [9]. So, DE has a significant role in cosmic dynamics of the current universe. The sim- plest candidate for DE is supposed to be the cosmological constant Λ which is very high in the early uni