Browsing by Author "Gumjudpai, Burin"

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    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
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    Quintessentially geometric model
    (2006-10-16) Gumjudpai, Burin; Naskar, Tapan; Ward, John
    We consider string inspired cosmology on a solitary D3 brane moving in the background of a ring of branes located on a circle of radius R. The motion of the D3 brane transverse to the plane of the ring gives rise to a radion field which can be mapped to a massive non-BPS Born-Infeld type field with a cosh potential. For certain bounds of the brane tension we find an inflationary phase is possible, with the string scale relatively close to the Planck scale. The relevant perturbations and spectral indices are all well within the expected observational bounds. The evolution of the universe eventually comes to be dominated by dark energy, which we show is a late time attractor of the model. However we also find that the equation of state is time dependent, and will lead to late time Quintessence.