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Sheykhi A.,Shiraz University | Sheykhi A.,Center for Excellence in Astronomy and Astrophysics
Physical Review D - Particles, Fields, Gravitation and Cosmology

Padmanabhan argues that the cosmic acceleration can be understood from the perspective that spacetime dynamics is an emergence phenomena. By calculating the difference between the surface degrees of freedom and the bulk degrees of freedom in a region of space, he also arrives at the Friedmann equation in a flat universe. In this paper, by modification of his proposal, we are able to derive the Friedmann equation of the Friedmann-Robertson-Walker universe with any spatial curvature. We also extend the study to higher-dimensional spacetime and derive successfully the Friedmann equations not only in Einstein gravity but also in Gauss-Bonnet and more general Lovelock gravity with any spatial curvature. This is the first derivation of Friedmann equations in these gravity theories in a nonflat Friedmann-Robertson-Walker universe by using the novel idea proposed by Padmanabhan. Our study indicates that the approach presented here is powerful enough and further supports the viability of Padmanabhan's perspective of emergence gravity. © 2013 American Physical Society. Source

Dehghani M.H.,Center for Excellence in Astronomy and Astrophysics | Dehghani M.H.,Shiraz University | Shakuri Ch.,Shiraz University | Vahidinia M.H.,Shiraz University
Physical Review D - Particles, Fields, Gravitation and Cosmology

In this paper, we investigate the thermodynamics of Lifshitz black branes in the presence of a nonlinear massless electromagnetic field. We begin by introducing the appropriate action in grand-canonical and canonical ensembles for a nonlinear electromagnetic field. The condition on the parameters of the metric for having black brane solutions will be presented. Since the field equations cannot be solved for an arbitrary value of the critical exponent z, we obtain a conserved quantity along the r coordinate that enables us to relate the parameters of the metric at the horizon and at infinity. Then, we calculate the energy density of the Lifshitz black brane through the use of the counterterm method generalized for the asymptotic Lifshitz spacetimes. Finally, we present a relation between the energy density and the thermodynamical quantities, electric potential, charge density, temperature, and entropy density. This relation is the generalization of the Smarr formula for anti-de Sitter black branes. © 2013 American Physical Society. Source

Sheykhi A.,Center for Excellence in Astronomy and Astrophysics | Sheykhi A.,Shiraz University
Physical Review D - Particles, Fields, Gravitation and Cosmology

We construct a new class of higher-dimensional black hole solutions of f(R) theory coupled to a nonlinear Maxwell field. In deriving these solutions the traceless property of the energy-momentum tensor of the matter filed plays a crucial role. In n-dimensional spacetime the energy-momentum tensor of conformally invariant Maxwell field is traceless provided we take n=4p, where p is the power of conformally invariant Maxwell Lagrangian. These black hole solutions are similar to higher-dimensional Reissner-Nordstrom anti-de Sitter black holes but only exist for dimensions which are multiples of four. We calculate the conserved and thermodynamic quantities of these black holes and check the validity of the first law of black hole thermodynamics by computing a Smarr-type formula for the total mass of the solutions. Finally, we study the local stability of the solutions and find that there is indeed a phase transition for higher-dimensional f(R) black holes with conformally invariant Maxwell source. © 2012 American Physical Society. Source

Mehdizadeh M.R.,Shiraz University | Riazi N.,Shiraz University | Riazi N.,Center for Excellence in Astronomy and Astrophysics
Physical Review D - Particles, Fields, Gravitation and Cosmology

We present dynamic wormhole solutions in the framework of Lovelock gravity with compact extra dimensions. Calculations are done in D≥7, although they can be extended to an arbitrary number of extra dimensions. It is shown that as the wormhole inflates with the three-dimensional space, the extra dimensions deflate to very small, yet nonvanishing scales. The weak energy condition is shown to hold for certain ranges of the free parameters, everywhere, down to the throat of the wormhole. © 2012 American Physical Society. Source

Nozari K.,Center for Excellence in Astronomy and Astrophysics | Kiani F.,University of Mazandaran | Rashidi N.,University of Mazandaran
Advances in High Energy Physics

We analyze the background cosmology for an extension of the DGP gravity with Gauss-Bonnet term in the bulk and f(R) gravity on the brane. We investigate implications of this setup on the late-time cosmic history. Within a dynamical system approach, we study cosmological dynamics of this setup focusing on the role played by curvature effects. Finally, we constrain the parameters of the model by confrontation with recent observational data. © 2013 Kourosh Nozari et al. Source

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