2004 (IPP)
Permanent URI for this collectionhttp://localhost:4000/handle/11007/625
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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 Unifying Brane World Inflation with Quintessence(2011-07-06) Sami, M.; Dadhich, NareshWe review the recent attempts of unifying inflation with quintessence. It appears natural to join the two ends in the framework of brane world cosmology. The models of quintessential inflation belong to the class of non-oscillatory models for which the mechanism of conventional reheating does not work. Reheating through gravitational particle production is inefficient and leads to the excessive production of relic gravity waves which results in the violation of nucleosynthesis constraint. The mechanism of instant preheating is quite efficient and is suitable for brane world quintessential inflation. The model is shown to be free from the problem of excessive production of gravity waves. The prospects of Gauss-Bonnet brane world inflation are also briefly indicated.Item Unified approach to scaling solutions in a general cosmological background(2011-07-06) Tsujikawa, Shinji; Sami, M.Our ignorance about the source of cosmic acceleration has stimulated study of a wide range of models and modifications to gravity. Cosmological scaling solutions in any of these theories are privileged because they represent natural backgrounds relevant to dark energy. We study scaling solutions in a generalized background H2 ∝ ρn T in the presence of a scalar field ϕ and a barotropic perfect fluid, where H is a Hubble rate and ρT is a total energy density. The condition for the existence of scaling solutions restricts the form of Lagrangian to be p = X1/ng(Xenλϕ), where X = −gµν ∂µϕ∂νϕ/2 and g is an arbitrary function. This is very useful to find out scaling solutions and corresponding scalar-field potentials in a broad class of dark energy models including (coupled)- quintessence, ghost-type scalar field, tachyon and k-essence. We analytically derive the scalar-field equation of state wϕ and the fractional density Ωϕ and apply it to a number of dark energy models.Item Quintessential inflation on the brane and the Relic Gravity wave background(2011-07-06) Sami, M.; Sahni, VarunPune 411 007, India. Quintessential inflation describes a scenario in which both inflation and dark energy (quintessence) are described by the same scalar field. In conventional braneworld models of quintessential inflation gravitational particle production is used to reheat the universe. This reheating mechanism is very inefficient and results in an excessive production of gravity waves which violate nucleosynthesis constraints and invalidate the model. We describe a new method of realizing quintessential inflation on the brane in which inflation is followed by ‘instant preheating’ (Felder, Kofman & Linde 1999). The larger reheating temperature in this model results in a smaller amplitude of relic gravity waves which is consistent with nucleosynthesis bounds. The relic gravity wave background has a ‘blue’ spectrum at high frequencies and is a generic byproduct of successful quintessential inflation on the brane.Item Phantom field and the fate of the Universe(2011-07-06) Sami, M.; Toporensky, A.In this paper we analyze the cosmological dynamics of phantom field in a variety of potentials unbounded from above. We demonstrate that the nature of future evolution generically depends upon the steepness of the phantom potential and discuss the fate of Universe accordingly.Item Observational constraints on braneworld inflation: The effect of a Gauss-Bonnet term(2011-07-06) Tsujikawa, Shinji; Sami, M.; Roy, MaartensHigh-energy modifications to general relativity introduce changes to the perturbations generated during inflation, and the latest high-precision cosmological data can be used to place constraints on such modified inflation models. Recently it was shown that Randall-Sundrum type braneworld inflation leads to tighter constraints on quadratic and quartic potentials than in general relativity. We investigate how this changes with a Gauss-Bonnet correction term, which can be motivated by string theory. Randall-Sundrum models preserve the standard consistency relation between the tensor spectral index and the tensor-to-scalar ratio. The Gauss-Bonnet term breaks this relation, and also modifies the dynamics and perturbation amplitudes at high energies. We find that the Gauss- Bonnet term tends to soften the Randall-Sundrum constraints. The observational compatibility of the quadratic potential is strongly improved. For a broad range of energy scales, the quartic potential is rescued from marginal rejection. Steep inflation driven by an exponential potential is excluded in the Randall-Sundrum case, but the Gauss-Bonnet term leads to marginal compatibility for sufficient e-folds.Item Note on inflation the Tachyon rolling on the Gauss-Bonnet Brane(2011-07-06) Paul, B. C.; Sami, M.In this paper we study the tachyonic inflation in brane world cosmology with Gauss-Bonnet term in the bulk. We obtain the exact solution of slow roll equations in case of exponential potential. We attempt to implement the proposal of Lidsey and Nunes[1] for the tachyon condensate rolling on the Gauss-Bonnet brane and discuss the difficulties associated with the proposal.Item Generation of electromagnetic fields in string cosmology with a massive scalar field on the anti D-brane(2012-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, ShinjiWe study the generation of electromagnetic fields in a string-inspired scenario associated with a rolling massive scalar field φ on the anti-D3 branes of KKLT de Sitter vacua. The 4-dimensional DBI type effective action naturally gives rise to the coupling between the gauge fields and the inflaton φ, which leads to the production of cosmological magnetic fields during inflation due to the breaking of conformal invariance. We find that the amplitude of magnetic fields at decoupling epoch can be larger than the limiting seed value required for the galactic dynamo. We also discuss the mechanism of reheating in our scenario and show that gauge fields are sufficiently enhanced for the modes deep inside the Hubble radius with an energy density greater than that of the inflaton.Item Cosmology from rolling massive scalar field on the anti-D3 Brane of de Sitter Vacua(2012-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, ShinjiWe investigate a string-inspired scenario associated with a rolling massive scalar field on D-branes and discuss its cosmological implications. In particular, we discuss cosmological evolution of the massive scalar field on the ant-D3 brane of KKLT vacua. Unlike the case of tachyon field, because of the warp factor of the anti-D3 brane, it is possible to obtain the required level of the amplitude of density perturbations. We study the spectra of scalar and tensor perturbations generated during the rolling scalar inflation and show that our scenario satisfies the observational constraint coming from the Cosmic Microwave Background anisotropies and other observational data. We also implement the negative cosmological constant arising from the stabilization of the modulus fields in the KKLT vacua and find that this leads to a successful reheating in which the energy density of the scalar field effectively scales as a pressureless dust. The present dark energy can be also explained in our scenario provided that the potential energy of the massive rolling scalar does not exactly cancel with the amplitude of the negative cosmological constant at the potential minimum.Item Constraints on Dirac-Born-Infeld type dark energy models from varying alpha(2011-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, ShinjiWe study the variation of the effective fine structure constant alpha for Dirac-Born-Infeld (DBI) type dark energy models. The DBI action based on string theory naturally gives rise to a coupling between gauge fields and a scalar field responsible for accelerated expansion of the universe. This leads to the change of alpha due to a dynamical evolution of the scalar field, which can be compatible with the recently observed cosmological data around the redshift ˜ z < ∼ 3. We place constraints on several different DBI models including exponential, inverse power-law and rolling massive scalar potentials. We find that these models can satisfy the varying alpha constraint provided that mass scales of the potentials are fine-tuned. When we adopt the mass scales which are motivated by string theory, both exponential and inverse power-law potentials give unacceptably large change of alpha, thus ruled out from observations. On the other hand the rolling massive scalar potential is compatible with the observationally allowed variation of alpha. Therefore the information of varying alpha provides a powerful way to distinguish between a number of string-inspired DBI dark energy models.