99 research outputs found

    Brans-Dicke Theory and Thermodynamical Laws on Apparent and Event Horizons

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    In this work, we have described the Brans-Dicke theory of gravity and given a particular solution by choosing a power law form of scalar field ϕ\phi and constant ω\omega. If we assume first law and entropy formula on apparent horizon then we recover Friedmann equations. Next, assuming first law of thermodynamics, the validity conditions of GSL on event horizon are presented. Also without use first law, if we impose the entropy relation on the horizon, then we also obtain the condition of validity of GSL on event horizon. The validity of GSL completely depends on the model of BD scalar field solutions. We have justified that on the apparent horizon the two process are equivalent, but on the event horizon they are not equivalent. If first law is valid on the event horizon then GSL may be satisfied in BD solution, but if first law is not satisfied then GSL is not satisfied in BD solution. So first law always favours GSL on event horizon. In our effective approach, the first law and GSL is always satisfied in apparent horizon, which do not depend on BD theory of gravity.Comment: 6 pages, 2 figure

    Cosmological Constant, Gauge Hierarchy and Warped Geometry

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    It is suggested that the mechanism responsible for the resolution of the gauge hierarchy problem within the warped geometry framework can be generalized to provide a new explanation of the extremely tiny vacuum energy density rho_V suggested by recent observations. We illustrate the mechanism with some 5D examples in which the true vacuum energy is assumed to vanish, and rho_V is associated with a false vacuum energy such that rho_V^{1/4} ~ TeV^2/M_{Pl} ~ 10^{-3} eV, where M_{Pl} denotes the reduced Planck mass. We also consider a quintessence-like solution to the dark energy problem.Comment: 10 pages, LaTeX, 2 figures, section on quantum corrections added, version to appear in Phys. Rev.

    A New Cosmological Model of Quintessence and Dark Matter

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    We propose a new class of quintessence models in which late times oscillations of a scalar field give rise to an effective equation of state which can be negative and hence drive the observed acceleration of the universe. Our ansatz provides a unified picture of quintessence and a new form of dark matter we call "Frustrated Cold Dark Matter" (FCDM). FCDM inhibits gravitational clustering on small scales and could provide a natural resolution to the core density problem for disc galaxy halos. Since the quintessence field rolls towards a small value, constraints on slow-roll quintessence models are safely circumvented in our model.Comment: Revised. Important new results added in response to referees comment

    Variable Modified Chaplygin Gas and Accelerating Universe

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    In this letter, I have proposed a model of variable modified Chaplygin gas and shown its role in accelerating phase of the universe. I have shown that the equation of state of this model is valid from the radiation era to quiessence model. The graphical representations of statefinder parameters characterize different phase of evolution of the universe. All results presented in the letter concerns the case k=0k=0.Comment: 7 Latex pages, 5 figures, revtex styl

    A Two-Field Quintessence Model

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    We study the dynamics of a quintessence model based on two interacting scalar fields. The model can account for the (recent) accelerated expansion of the Universe suggested by astronomical observations. Acceleration can be permanent or temporary and, for both scenarios, it is possible to obtain suitable values for the cosmological parameters while satisfying the nucleosynthesis constraint on the quintessence energy density. We argue that the model dynamics can be made consistent with a stable zero-energy relaxing supersymmetric vacuum.Comment: 4 pages, 3 eps figures, to be published in Phys. Rev.

    A single model of traversable wormholes supported by generalized phantom energy or Chaplygin gas

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    This paper discusses a new variable equation of state parameter leading to exact solutions of the Einstein field equations describing traversable wormholes. In addition to generalizing the notion of phantom energy, the equation of state generates a mathematical model that combines the generalized phantom energy and the generalized Chaplygin gas models.Comment: 6 pages, no figure

    Observational Constraints of Modified Chaplygin Gas in Loop Quantum Cosmology

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    We have considered the FRW universe in loop quantum cosmology (LQC) model filled with the dark matter (perfect fluid with negligible pressure) and the modified Chaplygin gas (MCG) type dark energy. We present the Hubble parameter in terms of the observable parameters Ωm0\Omega_{m0}, Ωx0\Omega_{x0} and H0H_{0} with the redshift zz and the other parameters like AA, BB, CC and α\alpha. From Stern data set (12 points), we have obtained the bounds of the arbitrary parameters by minimizing the χ2\chi^{2} test. The best-fit values of the parameters are obtained by 66%, 90% and 99% confidence levels. Next due to joint analysis with BAO and CMB observations, we have also obtained the bounds of the parameters (B,CB,C) by fixing some other parameters α\alpha and AA. From the best fit of distance modulus μ(z)\mu(z) for our theoretical MCG model in LQC, we concluded that our model is in agreement with the union2 sample data.Comment: 14 pages, 10 figures, Accepted in EPJC. arXiv admin note: text overlap with arXiv:astro-ph/0311622 by other author

    Role of Brans-Dicke Theory with or without self-interacting potential in cosmic acceleration

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    In this work we have studied the possibility of obtaining cosmic acceleration in Brans-Dicke theory with varying or constant ω\omega (Brans- Dicke parameter) and with or without self-interacting potential, the background fluid being barotropic fluid or Generalized Chaplygin Gas. Here we take the power law form of the scale factor and the scalar field. We show that accelerated expansion can also be achieved for high values of ω\omega for closed Universe.Comment: 12 Latex pages, 20 figures, RevTex styl

    Generalized Chaplygin Gas, Accelerated Expansion and Dark Energy-Matter Unification

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    We consider the scenario emerging from the dynamics of a generalized dd-brane in a (d+1,1)(d+1, 1) spacetime. The equation of state describing this system is given in terms of the energy density, ρ\rho, and pressure, pp, by the relationship p=A/ραp = - A/\rho^{\alpha}, where AA is a positive constant and 0<α10 < \alpha \le 1. We discuss the conditions under which homogeneity arises and show that this equation of state describes the evolution of a universe evolving from a phase dominated by non-relativistic matter to a phase dominated by a cosmological constant via an intermediate period where the effective equation of state is given by p=αρp = \alpha \rho.Comment: 5 pages, 4 figures, revte

    Can the Chaplygin gas be a plausible model for dark energy?

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    In this note two cosmological models representing the flat Friedmann Universe filled with a Chaplygin fluid, with or without dust, are analyzed in terms of the recently proposed "statefinder" parameters. Trajectories of both models in the parameter plane are shown to be significantly different w.r.t. "quiessence" and "tracker" models. The generalized Chaplygin gas model with an equation of state of the form p=A/ραp = -A/\rho^{\alpha} is also analyzed in terms of the statefinder parameters.Comment: 6 pages, 2 figure
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