2005 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/443
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Item Phantom damping of matter perturbations(2005-06-01) Amendola, Luca; Tsujikawa, Shinji; Sami, M.Cosmological scaling solutions are particularly important in solving the coincidence problem of dark energy. We derive the equations of sub-Hubble linear matter perturbations for a general scalar-field Lagrangian–including quintessence, tachyon, dilatonic ghost condensate and k-essence–and solve them analytically for scaling solutions. We find that matter perturbations are always damped if a phantom field is coupled to dark matter and identify the cases in which the gravitational potential is constant. This provides an interesting possibility to place stringent observational constraints on scaling dark energy models.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 Dark energy and cosmological solutions in second-order string gravity(2005-09-01) Calcagni, Gianluca; Tsujikawa, Shinji; Sami, M.We study the cosmological evolution based upon a D-dimensional action in low-energy effective string theory in the presence of second-order curvature corrections and a modulus scalar field (dilaton or compactification modulus). Abarotropic perfect fluid coupled to the scalar field is also allowed. Phase space analysis and the stability of asymptotic solutions are performed for a number of models which include (i) fixed scalar field, (ii) linear dilaton in string frame, and (iii) logarithmic modulus in Einstein frame. We confront analytical solutions with observational constraints for deceleration parameter and show that Gauss-Bonnet gravity (with no matter fields) may not explain the current acceleration of the universe. We also study the future evolution of the universe using the GB parametrization and find that big rip singularities can be avoided even in the presence of a phantom fluid because of the balance between the fluid and curvature corrections. A non-minimal coupling between the fluid and the modulus field also opens up the interesting possibility to avoid big rip regardless of the details of the fluid equation of state.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