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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 | Year: 2013

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.


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 | Year: 2013

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.


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 | Year: 2012

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.


Bazrafshan A.,Jahrom University | Dehghani M.H.,Center for Excellence in Astronomy and Astrophysics | Dehghani M.H.,Shiraz University | Ghanaatian M.,Payame Noor University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

As in the case of Einstein or Lovelock gravity, the action of quartic quasitopological gravity has not a well-defined variational principle. In this paper, we first introduce a surface term which makes the variation of quartic quasitopological gravity well-defined. Second, we present the static charged solutions of quartic quasitopological gravity in the presence of a nonlinear electromagnetic field. One of the branches of these solutions presents a black brane with one or two horizons or a naked singularity depending on the charge and mass of the solution. The thermodynamics of these black branes are investigated through the use of the Gibbs free energy. In order to do this, we calculate the finite action by use of the counterterm method inspired by AdS/CFT correspondence. Introducing a Smarr-type formula, we also show that the conserved and thermodynamics quantities of these solutions satisfy the first law of thermodynamics. Finally, we present the charged rotating black branes in n+1 dimensions with k≤[n/2] rotation parameters and investigate their thermodynamics. © 2012 American Physical Society.


Sheykhi A.,Center for Excellence in Astronomy and Astrophysics | Sheykhi A.,Shiraz University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

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.


Sheykhi A.,Center for Excellence in Astronomy and Astrophysics | Sheykhi A.,Shiraz University
General Relativity and Gravitation | Year: 2012

We construct a new class of spinning magnetic string solutions in f(R) gravity with constant scalar curvature. These solutions which produce a longitudinal magnetic field have no curvature singularity and no horizon, but have a conic geometry with a deficit angle. We also generalize this class of solutions to the case of spinning magnetic solutions with one rotation parameter. We find that the spinning string has a net electric charge which is proportional to the rotation parameter. With choosing a suitable counterterm, we remove the divergences of the action. The conserved quantities of the solutions are also calculated by using the counterterm method. © 2012 Springer Science+Business Media, LLC.


Tavabi E.,Payame Noor University | Tavabi E.,CNRS Paris Institute of Astrophysics | Tavabi E.,Center for Excellence in Astronomy and Astrophysics | Koutchmy S.,CNRS Paris Institute of Astrophysics | Ajabshirizadeh A.,Center for Excellence in Astronomy and Astrophysics
Solar Physics | Year: 2013

We present an image processing technique, the improved OMC filter (also called Madmax), which selects maxima of convexities of intensity modulations of an image. The filter computes second derivatives of the image in multiple directions around each pixel. It is shown to be efficient for pattern recognition, and bright hair-like or small anisotropic features can be enhanced. The filter is tested on artificially generated images, and the effect of a different number of directions in which the second derivatives are calculated is evaluated. Ca ii H line images of spicules and prominences obtained with Hinode/SOT are also used to illustrate its efficiency on real data. It is shown how to make the choice of the sensitive parameters to be used in improving the image visibility. © 2012 Springer Science+Business Media B.V.


Rezazadeh K.,University of Kurdistan | Karami K.,University of Kurdistan | Karimi P.,Center for Excellence in Astronomy and Astrophysics
Journal of Cosmology and Astroparticle Physics | Year: 2015

We study the intermediate inflation in a non-canonical scalar field framework with a power-like Lagrangian. We show that in contrast with the standard canonical intermediate inflation, our non-canonical model is compatible with the observational results of Planck 2015. Also, we estimate the equilateral non-Gaussianity parameter which is in well agreement with the prediction of Planck 2015. Then, we obtain an approximation for the energy scale at the initial time of inflation and show that it can be of order of the Planck energy scale, i.e. MP ∼ 1018GeV. We will see that after a short period of time, inflation enters in the slow-roll regime that its energy scale is of order MP/100 ∼ 1016GeV and the horizon exit takes place in this energy scale. We also examine an idea in our non-canonical model to overcome the central drawback of intermediate inflation which is the fact that inflation never ends. We solve this problem without disturbing significantly the nature of the intermediate inflation until the time of horizon exit. © 2015 IOP Publishing Ltd and Sissa Medialab srl .


Nozari K.,Center for Excellence in Astronomy and Astrophysics | Kiani F.,University of Mazandaran | Rashidi N.,University of Mazandaran
Advances in High Energy Physics | Year: 2013

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.


Karami K.,University of Kurdistan | Karami K.,Center for Excellence in Astronomy and Astrophysics | Abdolmaleki A.,University of Kurdistan
Journal of Cosmology and Astroparticle Physics | Year: 2012

We investigate the validity of the generalized second law (GSL) of gravitational thermodynamics in the framework of f(T) modified teleparallel gravity. We consider a spatially flat FRW universe containing only the pressureless matter. The boundary of the universe is assumed to be enclosed by the Hubble horizon. For two viable f(T) models containing f(T) = T+μ 1{(-T)} n and f(T) = T-μ 2T(1-e βT0/T), we first calculate the effective equation of state and deceleration parameters. Then, {we investigate the null and strong energy conditions and conclude that a sudden future singularity appears in both models. Furthermore, using a cosmographic analysis we check the viability of two models. Finally, we examine the validity of the GSL and find that for both models it} is satisfied from the early times to the present epoch. But in the future, the GSL is violated for the special ranges of the torsion scalar T. © 2012 IOP Publishing Ltd and Sissa Medialab srl.

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