Browsing by Author "Sami, M."

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Aspects of scalar field dynamics in Gauss-Bonnet Brane Worlds
(2011-07-06) Sami, M.; Savchenko, N.; Toporensky, A.
The Einstein-Gauss-Bonnet equations projected from the bulk to brane lead to a complicated Friedmann equation which simpliﬁes to H2 ∼ ρq in the asymptotic regimes. The Randall-Sundrum (RS) scenario corresponds to q = 2 whereas q = 2/3 & q = 1 give rise to high energy Gauss-Bonnet (GB) regime and the standard GR respectively. Amazingly, while evolving from RS regime to high energy GB limit, one passes through a GR like region which has important implications for brane world inﬂation. For tachyon GB inﬂation with potentials V (φ) ∼ φp investigated in this paper, the scalar to tensor ratio of perturbations R is maximum around the RS region and is generally suppressed in the high energy regime for the positive values of p. The ratio is very low for p > 0 at all energy scales relative to GB inﬂation with ordinary scalar ﬁeld. The models based upon tachyon inﬂation with polynomial type of potentials with generic positive values of p turn out to be in the 1σ observational contour bound at all energy scales varying from GR to high energy GB limit. The spectral index nS improves for the lower values of p and approaches its scale invariant limit for p = −2 in the high energy GB regime. The ratio R also remains small for large negative values of p, however, diﬀerence arises for models close to scale invariance limit. In this case, the tensor to scale ratio is large in the GB regime whereas it is suppressed in the intermediate region between RS and GB. Within the frame work of patch cosmologies governed by H2 ∼ ρq , the behavior of ordinary scalar ﬁeld near cosmological singularity and the nature of scaling solutions are distinguished for the values of q < 1 and q > 1. The tachyon dynamics, on the other hand, exhibits stable scaling solutions ∀q if the adiabatic
Aspects of Tachyonic inflation with exponential potential
(2011-07-06) Sami, M.; Chingangbam, Pravabati; Qureshi, Tabish
We consider issues related to tachyonic inﬂation with exponential potential. We ﬁnd exact solution of evolution equations in the slow roll limit in FRW cosmology. We also carry out similar analysis in case of Brane assisted tachyonic inﬂation. We investigate the phase space behavior of the system and show that the dust like solution is a late time attractor. The diﬃculties associated with reheating in the tachyonic model are also indicated.
Avoidance of future singularities in loop quantum cosmology
(2006-05-21) Sami, M.; Singh, Parampreet; Tsujikawa, Shinji
We consider the fate of future singularities in the eﬀective dynamics of loop quantum cosmology. Non-perturbative quantum geometric eﬀects which lead to ρ2 modiﬁcation of the Friedmann equation at high energies result in generic resolution of singularities whenever energy density ρ diverges at future singularities of Friedmann dynamics. Such quantum eﬀects lead to the avoidance of a Big Rip, which is followed by a recollapsing universe stable against perturbations. Resolution of sudden singularity, the case when pressure diverges but energy density approaches a ﬁnite value depends on the ratio of the latter to a critical energy density of the order of Planck. If the value of this ratio is greater than unity, the universe escapes the sudden future singularity and becomes oscillatory.
Constraints on Dirac-Born-Infeld type dark energy models from varying alpha
(2011-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We 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.
Cosmological aspects of rolling tachyon
(2011-07-06) Sami, M.
We examine the possibility of rolling tachyon to play the dual roll of inﬂaton at early epochs and dark matter at late times. We argue that enough inﬂation can be generated with the rolling tachyon either by invoking the large number of branes or brane world assisted inﬂation. However, reheating is problematic in this model.
Cosmological Dynamics of Phantom Field
(2011-07-05) Singh, Parampreet; Sami, M.; Dadhich, Naresh
We study the general features of the dynamics of the phantom ﬁeld in the cosmological context. In the case of inverse coshyperbolic potential, we demonstrate that the phantom ﬁeld can successfully drive the observed current accelerated expansion of the universe with the equation of state parameter wφ < −1. The de-Sitter universe turns out to be the late time attractor of the model. The main features of the dynamics are independent of the initial conditions and the parameters of the model. The model ﬁts the supernova data very well, allowing for −2.4 < wφ < −1 at 95 % conﬁdence level
Cosmology from rolling massive scalar field on the anti-D3 Brane of de Sitter Vacua
(2012-07-06) Garousi, M. R.; Sami, M.; Tsujikawa, Shinji
We 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.
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
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.
Dark energy cosmology from higher-order, string-inspired gravity, and its reconstruction
(2006-04-10) Nojiri, Shin'ichi; Odintsov, Sergi D.; Sami, M.
In this paper we investigate the cosmological effects of modiﬁed gravity with string curvature corrections added to the Einstein-Hilbert action in the presence of a dynamically evolving scalar ﬁeld coupled to Riemann invariants. The scenario exhibits several features of cosmological interest for the late universe. We show that higher-order stringy corrections can lead to a class of dark energy models consistent with recent observations. The models can give rise to quintessence without recourse to a scalar ﬁeld potential. The detailed treatment of the reconstruction program for general scalar-Gauss-Bonnet gravity is presented for any given cosmology. The explicit examples of reconstructed scalar potentials are given for an accelerated (quintessence, cosmological constant, or phantom) universe. Finally, the relation with modiﬁed F G gravity is established at the classical level and is extended to include third order terms on the curvature.
Dynamics of dark energy
(2006-06-16) Copeland, E. J.; Sami, M.; Tsujikawa, Shinji
In 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.
Exact inflationary solution on the brane
(2001-11-01) Sami, M.
We study the evolution of universe with a single scalar ﬁeld of constant potential minimally coupled to gravity in the brane world cosmology.We ﬁnd an exact inﬂationary solution which is not in slow roll.We discuss the limiting cases of the solution.We show that at late times the solution is asymptotic to the de Sitter solution independently of the brane tension. For t → 0 the solution leads to singularity but the nature of the approach to singularity depends upon the brane tension.
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.
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, Shinji
We 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.
Implementing power law inflation with rolling techyon on the brane
(2002-07-09) Sami, M.
We study a minimally coupled tachyon ﬁeld rolling down to its ground state on the FRW brane. We construct tacyonic potential which can implements power law inﬂation in the brane world cos- mology. The potential turns out to be V0φ−1 on the brane and reduces to inverse square potential at ate times when brane corrections to the Friedmann equation become negligible. We also do similar exercise with a normal scalar ﬁeld and discover that the inverse square potential on the brane leads to power law inﬂation
Inflation with oscillations
(2001-02-02) Sami, M.
In this paper we investigate the general features of ”Oscillatory Inﬂation”. In adiabatic approxi- mation , we derive a general formula for the number of e-foldings ˜ N which reduces to the standard expression in case of the slow role approximation and leads to the Damour-Mukhanov type expres- sion for the slowly varying adiabatic index. We apply our result to the logarithmic potential and arrive at a simple and elegant formula for the number of e-foldings.We evolve the ﬁeld equations numerically and observe a remarkable agreement with the analytical result.
Manybody treatment of white dwarf and neutron stars on the brane
(2005-07-01) Azam, Mofazzal; Sami, M.
Brane-World models suggest modiﬁcation of Newton’s law of gravity on the 3-brane at submillimeter scales. The brane-world induced corrections are in higher powers of inverse distance and appear as additional terms with the Newtonian potential. The average inter-particle distance in white dwarf and neutron stars are 10−10 cms and 10−13 cms respectively, and therefore, the eﬀect of submillimeter corrections needs to be investigated. We show, by carrying out simple manybody calculations, that the mass and mass-radius relationship of the white dwarf and neutron stars are not eﬀected by submillimeter corrections. However, our analysis shows that the correction terms in the eﬀective theory give rise to force akin to surface tension in normal liquids.
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.
Note on the cosmological dynamics in finite-range gravity
(2011-07-05) Sami, M.
In this note we consider the homogeneous and isotropic cosmology in the ﬁnite-range gravity theory recently proposed by Babak and Grishchuk. In this scenario the universe undergoes late time accelerated expansion if both the massive gravitons present in the model are tachyons. We carry out the phase space analysis of the system and show that the late-time acceleration is an attractor of the model.
Observational constraints on braneworld inflation: The effect of a Gauss-Bonnet term
(2011-07-06) Tsujikawa, Shinji; Sami, M.; Roy, Maartens
High-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.