IUCAA Preprints
Permanent URI for this communityhttp://localhost:4000/handle/11007/149
Browse
11 results
Search Results
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 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 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.Item String-inspired cosmology: Late time transition from scaling matter era to dark energy universe caused by a gauss-bonnet coupling(2006-12-18) Tsujikawa, Shinji; Sami, M.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 Dynamics of dark energy(2006-06-16) Copeland, E. J.; Sami, M.; Tsujikawa, ShinjiIn this paper we review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating Universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark energy models in addition to the conventional cosmological constant, paying particular attention to scalar field models such as quintessence, K-essence, tachyon, phantom and dilatonic models. The importance of cosmological scaling solutions is emphasized when studying the dynamical system of scalar fields including coupled dark energy. We study the evolution of cosmological perturbations allowing us to confront them with the observation of the Cosmic Microwave Background and Large Scale Structure and demonstrate how it is possible in principle to reconstruct the equation of state of dark energy by also using Supernovae Ia observational data. We also discuss in detail the nature of tracking solutions in cosmology, particle physics and braneworld models of dark energy, the nature of possible future singularities, the effect of higher order curvature terms to avoid a Big Rip singularity, and approaches to modifying gravity which leads to a late-time accelerated expansion without recourse to a new form of dark energy.