IUCAA Preprints

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Author: search.filters.author.Sami, M.

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    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.
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    Relic gravity waves from Braneworld inflation
    (2001-10-02) Sahni, Varun; Sami, M.; Souradeep, Tarun
    We discuss a scenario in which extra dimensional effects allow a scalar field with a steep potential to play the dual role of the inflaton as well as dark energy (quintessence). The post-inflationary evo- lution of the universe in this scenario is generically characterised by a ‘kinetic regime’ during which the kinetic energy of the scalar field greatly exceeds its potential energy resulting in a ‘stiff’ equa- tion of state for scalar field matter Pφ ≃ ρφ. The kinetic regime precedes the radiation dominated epoch and introduces an important new feature into the spectrum of relic gravity waves created quantum mechanically during inflation. The amplitude of the gravity wave spectrum increases with wavenumber for wavelengths shorter than the comoving horizon scale at the commencement of the radiative regime. This ‘blue tilt’ is a generic feature of models with steep potentials and imposes strong constraints on a class of inflationary braneworld models. Prospects for detection of the grav- ity wave background by terrestrial and space-borne gravity wave observatories such as LIGO II and LISA are discussed.
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    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.
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    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
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    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.
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    Steep inflation followed by born-infeld reheating
    (2011-07-05) Sami, M.; Dadhich, Naresh
    We discuss a model in which high energy brane corrections allow a single scalar field to describe inflation at early epochs and quintessence at late times. The reheating mechanism in the model originates from Born-Infeld matter whose energy density mimics cosmological constant at very early times and manifests itself as radiation subsequently. For most of the inflationary evolution the Born- Infeld matter remains subdominant to the the scalar field. Shortly before the end of inflation driven by the scalar field, the energy density of Born-Infeld matter starts scaling as radiation and drops by several orders of magnitudes at the epoch inflation ends. The problem of over production of gravity wave background in scenarios based upon reheating through gravitational particle production is successfully resolved by suitably fixing the initial value of radiation energy density at the end of inflation. No additional fine tuning of the parameters is required for a viable evolution.
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    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.
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    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
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    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.
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    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.