IUCAA Preprints
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Item What is needed of a tachyon if it is to be the dark energy?(2011-07-06) Copeland, E. J.; Garousi, M. R.; Sami, M.; et al.We study a dark energy scenario in the presence of a tachyon field φ with potential V (φ) and a barotropic perfect fluid. The cosmological dynamics crucially depends on the asymptotic behavior of the quantity λ = −MpVφ/V 3/2 . If λ is a constant, which corresponds to an inverse square potential V (φ) ∝ φ−2 , there exists one stable critical point that gives an acceleration of the universe at late times. When λ → 0 asymptotically, we can have a viable dark energy scenario in which the system approaches an “instantaneous” critical point that dynamically changes with λ. If |λ| approaches infinity asymptotically, the universe does not exhibit an acceleration at late times. In this case, however, we find an interesting possibility that a transient acceleration occurs in a regime where |λ| is smaller than of order unity.Item Fate of (phantom) dark energy universe with string curvature corrections(2005-01-01) Sami, M.; Tsujikawa, Shinji; Tretjakov, P. V.; et al.We study the evolution of (phantom) dark energy universe by taking into account the higher-order string corrections to Einstein-Hilbert action with fixed dilaton and modulus fields. While the presence of a cosmological constant gives stable de-Sitter fixed points in the cases of heterotic and bosonic strings, no stable de-Sitter solutions exist when a phantom fluid is present. We find that the universe can exhibit a Big Crunch singularity with a finite time for type II string, whereas it reaches a Big Rip singularity for heterotic and bosonic strings. Thus the fate of dark energy universe crucially depends upon the type of string theory under consideration.Item Coupled dark energy: Towards a general description of the dynamics(2005-01-01) Gumjudpai, Burin; Sami, M.; Naskar, Tapan; et al.In dark energy models of scalar-field coupled to a barotropic perfect fluid, the existence of cosmological scaling solutions restricts the Lagrangian of the field ϕ to p = Xg(Xeλϕ), where X = −g ν ∂ ϕ∂νϕ/2, λ is a constant and g is an arbitrary function. We derive general evolution equations in an autonomous form for this Lagrangian and investigate the stability of fixed points for several different dark energy models–(i) ordinary (phantom) field, (ii) dilatonic ghost condensate, and (iii) (phantom) tachyon. We find the existence of scalar-field dominant fixed points (Ωϕ = 1) with an accelerated expansion in all models irrespective of the presence of the coupling Q between dark energy and dark matter. These fixed points are always classically stable for a phantom field, implying that the universe is eventually dominated by the energy density of a scalar field if phantom is responsible for dark energy. When the equation of state wϕ for the field ϕ is larger than −1, we find that scaling solutions are stable if the scalar-field dominant solution is unstable, and vice versa. Therefore in this case the final attractor is either a scaling solution with constant Ωϕ satisfying 0 < Ωϕ < 1 or a scalar-field dominant solution with Ωϕ = 1