Showing papers by "Mubasher Jamil published in 2012"
TL;DR: In this paper, the authors reconstruct cosmological models in the framework of f(R,T) gravity, where R is the Ricci scalar and T is the trace of the stress-energy tensor.
Abstract: In this paper, we reconstruct cosmological models in the framework of f(R,T) gravity, where R is the Ricci scalar and T is the trace of the stress-energy tensor. We show that the dust fluid reproduces ΛCDM, phantom–non-phantom era and phantom cosmology. Further, we reconstruct different cosmological models, including the Chaplygin gas, and scalar field with some specific forms of f(R,T). Our numerical simulation for the Hubble parameter shows good agreement with the BAO observational data for low redshifts, z<2.
316 citations
TL;DR: In this paper, the cosmological implications of interacting dark energy model in a torsion-based gravity namely f(T) were explored, assuming that dark energy interacts with dark matter and radiation components.
Abstract: In this paper, we explore the cosmological implications of interacting dark energy model in a torsion-based gravity namely f(T). Assuming that dark energy interacts with dark matter and radiation components, we examine the stability of this model by choosing different forms of interaction terms. We consider three different forms of dark energy: cosmological constant, quintessence and phantom energy. We then obtain several attractor solutions for each dark energy model interacting with the other components. This model successfully explains the coincidence problem via the interacting dark energy scenario.
145 citations
TL;DR: In this paper, the first law of black hole thermodynamics is violated for f(R,T) gravity, but there might exist some special cases in which the first-law for gravity is recovered.
Abstract: We derive the first law of thermodynamics using the method proposed by Wald. Treating the entropy as Noether charge and comparing with the usual first law of thermodynamics, we obtain explicitly the expression of entropy which contains infinitely many non-local terms (i.e. the integral terms). We have proved, in general, that the first law of black hole thermodynamics is violated for f(R,T) gravity. However, there might exist some special cases in which the first law for f(R,T) gravity is recovered.
128 citations
TL;DR: This paper derived the first law of thermodynamics using the method proposed by Wald using the Noether charge and compared with the usual first law in terms of entropy explicitly which contains infinitely many non-local terms (i.e. the integral terms).
Abstract: In this Letter, we derived the first law of thermodynamics using the method proposed by Wald. Treating the entropy as Noether charge and comparing with the usual first law of thermodynamics, we obtained the expression of entropy explicitly which contains infinitely many non-local terms (i.e. the integral terms). We have proved, in general, that the first law of black bole thermodynamics is violated for $f(R,T)$ gravity. But there might exist some special cases in which the first law for $f(R,T)$ gravity is recovered.
118 citations
TL;DR: In this article, the authors examine the interacting dark energy model in f(T) cosmology and show that there is one attractor solution to the dynamical equation of f (T) Friedmann equations.
Abstract: In this paper, we examine the interacting dark energy model in f(T) cosmology. We assume dark energy as a perfect fluid and choose a specific cosmologically viable form f(T) = β√T. We show that there is one attractor solution to the dynamical equation of f(T) Friedmann equations. Further we investigate the stability in phase space for a general f(T) model with two interacting fluids. By studying the local stability near the critical points, we show that the critical points lie on the sheet u* = (c − 1)v* in the phase space, spanned by coordinates (u, v, Ω, T). From this critical sheet, we conclude that the coupling between the dark energy and matter c ∈ (−2, 0).
92 citations
TL;DR: In this paper, a non-minimally conformally coupled scalar field and dark matter in F(T) cosmology is introduced and the authors investigate the stability and phase space behavior of the parameters of the scalar fields by choosing an exponential potential and cosmologically viable form of F (T).
Abstract: In this paper, we introduce a non-minimally conformally coupled scalar field and dark matter in F(T) cosmology and study their dynamics. We investigate the stability and phase space behavior of the parameters of the scalar field by choosing an exponential potential and cosmologically viable form of F(T). We found that the dynamical system of equations admits two unstable critical points; thus no attractor solutions exist in this cosmology. Furthermore, taking into account the scalar field mimicking quintessence and phantom energy, we discuss the corresponding cosmic evolution for both small and large times. We investigate the cosmological implications of the model via the equation of state and deceleration parameters of our model and show that the late-time Universe will be dominated by phantom energy and, moreover, phantom crossing is possible. Our results do not lead to explicit predictions for inflation and the early Universe era.
92 citations
TL;DR: In this article, the generalized Saez-Ballester scalar-tensor theory of gravity via Noether gauge symmetry (NGS) in the background of Bianchi type I cosmological spacetime is investigated.
Abstract: In this paper, we investigate the generalized Saez–Ballester scalar–tensor theory of gravity via Noether gauge symmetry (NGS) in the background of Bianchi type I cosmological spacetime. We start with the Lagrangian of our model and calculate its gauge symmetries and corresponding invariant quantities. We obtain the potential function for the scalar field in the exponential form. For all the symmetries obtained, we determine the gauge functions corresponding to each gauge symmetry which include constant and dynamic gauge. We discuss cosmological implications of our model and show that it is compatible with the observational data.
91 citations
TL;DR: In this article, the authors investigated the Noether symmetries of F(T) cosmology involving matter and dark energy, and obtained the scalar potential V(ϕ)∼ϕ姫 2 for both models of dark energy and discuss quantum implications of this model.
Abstract: In this paper, we investigate the Noether symmetries of F(T) cosmology involving matter and dark energy. In this model, the dark energy is represented by a canonical scalar field with a potential. Two special cases for dark energy are considered, including phantom energy and quintessence. We obtain F(T)∼T
3/4, and the scalar potential V(ϕ)∼ϕ
2 for both models of dark energy and discuss quantum picture of this model. Some astrophysical implications are also discussed.
89 citations
TL;DR: In this paper, a torsion-based toy model for dark matter is proposed to resolve the dark matter problem in f(T) gravity, and the model successfully obtained the flat rotation curves of galaxies containing dark matter.
Abstract: In this paper, we attempt to resolve the dark matter problem in f(T) gravity. Specifically, from our model we successfully obtain the flat rotation curves of galaxies containing dark matter. Further, we obtain the density profile of dark matter in galaxies. Comparison of our analytical results shows that our torsion-based toy model for dark matter is in good agreement with empirical data-based models. It shows that we can address the dark matter as an effect of torsion of the space.
85 citations
TL;DR: In this article, the origin of dark energy is identified as a modification of gravity and the Noether symmetry with gauge term is investigated for the f(R) cosmological model.
Abstract: We discuss the f(R) gravity model in which the origin of dark energy is identified as a modification of gravity. The Noether symmetry with gauge term is investigated for the f(R) cosmological model. By utilization of the Noether Gauge Symmetry (NGS) approach, we obtain two exact forms f(R) for which such symmetries exist. Further it is shown that these forms of f(R) are stable.
81 citations
TL;DR: In this paper, a new model of spinor cosmology with Ricci scalar and non-minimally coupled spinor fields was introduced, where the equations of motion of FRW and Bianchi type-I spacetimes have been derived and solved numerically.
Abstract: F-essence is a generalization of the usual Dirac model with the nonstandard kinetic term. In this paper, we introduce a new model of spinor cosmology containing both Ricci scalar and the non minimally coupled spinor fields in its action. We have investigated the cosmology with both isotropy and anisotropy, where the equations of motion of FRW and Bianchi type-I spacetimes have been derived and solved numerically. Finally the quantization of these models through Wheeler-De Witt (WD) wave function has been discussed.
TL;DR: In this paper, the authors investigated the Noether symmetries of cosmology involving matter and dark energy and obtained the scalar potential of a canonical scalar field with a potential.
Abstract: In this paper, we investigate the Noether symmetries of $F(T)$ cosmology involving matter and dark energy. In this model, the dark energy is represented by a canonical scalar field with a potential. Two special cases for dark energy are considered including phantom energy and quintessence. We obtain $F(T)\sim T^{3/4},$ and the scalar potential $V(\phi)\sim\phi^2$ for both models of dark energy and discuss quantum picture of this model. Some astrophysical implications are also discussed.
TL;DR: In this paper, a non-minimally conformally coupled scalar field and dark matter in F(T) cosmology is introduced and the authors investigate the stability and phase space behavior of the parameters of the scalar fields by choosing an exponential potential and cosmologically viable form of F (T).
Abstract: In this paper, we introduce a non-minimally conformally coupled scalar field and dark matter in F(T) cosmology and study their dynamics. We investigate the stability and phase space behavior of the parameters of the scalar field by choosing an exponential potential and cosmologically viable form of F(T). We found that the dynamical system of equations admits two unstable critical points; thus no attractor solutions exist in this cosmology. Furthermore, taking into account the scalar field mimicking quintessence and phantom energy, we discuss the corresponding cosmic evolution for both small and large times. We investigate the cosmological implications of the model via the equation of state and deceleration parameters of our model and show that the late-time Universe will be dominated by phantom energy and, moreover, phantom crossing is possible. Our results do not lead to explicit predictions for inflation and the early Universe era.
TL;DR: In this article, the authors considered the entropy-corrected version of the holographic dark energy model in the non-flat FRW universe and proposed to replace the future event horizon area with the inverse of Ricci scalar curvature.
Abstract: Motivated by the holographic principle, it has been suggested that the dark energy density may be inversely proportional to the area A of the event horizon of the universe. However, such a model would have a causality problem. In this work, we consider the entropy-corrected version of the holographic dark energy model in the non-flat FRW universe and we propose to replace the future event horizon area with the inverse of the Ricci scalar curvature. We obtain the equation of state (EoS) parameter ω
Λ, the deceleration parameter q and $\Omega_{D}'$
in the presence of interaction between Dark Energy (DE) and Dark Matter (DM). Moreover, we reconstruct the potential and the dynamics of the tachyon, K-essence, dilaton and quintessence scalar field models according to the evolutionary behavior of the interacting entropy-corrected holographic dark energy model.
TL;DR: In this article, the cosmological implication of the interacting entropy-corrected holographic dark energy model in the framework of Brans-Dicke cosmology is studied.
Abstract: In the derivation of holographic dark energy density, the area law of the black hole entropy plays a crucial role. However, the entropy-area relation can be modified from the inclusion of quantum effects, motivated from the loop quantum gravity, string theory and black hole physics. In this paper, we study cosmological implication of the interacting entropy-corrected holographic dark energy model in the framework of Brans–Dicke cosmology. We obtain the equation of state and the deceleration parameters of the entropy-corrected holographic dark energy in a non-flat Universe. As system’s IR cutoff we choose the radius of the event horizon measured on the sphere of the horizon, defined as L = ar(t). We find out that when the entropy-corrected holographic dark energy is combined with the Brans–Dicke field, the transition from normal state where wD > −1 to the phantom regime where wD < −1 for the equation of state of interacting dark energy can be more easily achieved for than when resort to the Einstein field equations is made.
TL;DR: In this paper, the authors investigate the holographic, new agegraphic and ghost dark energy models in the framework of fractal cosmology and obtain the equation of state parameters of the selected dark energy model in the ultraviolet regime.
Abstract: We investigate the holographic, new agegraphic and ghost dark energy models in the framework of fractal cosmology. We consider a fractal FRW universe filled with the dark energy and dark matter. We obtain the equation of state parameters of the selected dark energy models in the ultraviolet regime and discuss on their implications.
TL;DR: In this article, the Friedmann equation in the f (T ) cosmology model is taken, and the statefinder parameters { r, s } are calculated, and a model of f ( T ) which contains a constant, linear and non-linear form of torsion is considered.
Abstract: In this paper, we intend to evaluate and analyze the statefinder parameters in f ( T ) cosmology. Friedmann equation in f ( T ) model is taken, and the statefinder parameters { r , s } are calculated. We consider a model of f ( T ) which contains a constant, linear and non-linear form of torsion. We plot r and s in order to characterize this model in the { r , s } plane. We found that our model \(f(T)=2C_{1} \sqrt{-T} +\alpha T+C_{2},\) predicts the decay of dark energy in the far future while its special case namely teleparallel gravity predicts that dark energy will overcome over all the energy content of the Universe.
TL;DR: In this article, the Friedmann equation in the cosmology model was taken, and the statefinder parameters of the model were analyzed in the ${r, s}$ plane, and it was shown that this model predicts the decay of dark energy in the far future while its special case namely teleparallel gravity predicts that dark energy will overcome over all the energy content of the universe.
Abstract: In this paper, we intend to evaluate and analyze the statefinder parameters in $f(T)$ cosmology. Friedmann equation in $f(T)$ model is taken, and the statefinder parameters ${r,s}$ are calculated. We consider a model of $f(T)$ which contains a constant, linear and non-linear form of torsion. We plot $r$ and $s$ in order to characterize this model in the ${r,s}$ plane. We found that our model $f(T)=2C_1 \sqrt{-T} +\alpha T+C_2,$ predicts the decay of dark energy in the far future while its special case namely teleparallel gravity predicts that dark energy will overcome over all the energy content of the Universe.
TL;DR: In this paper, the power-law correction of entropy on the horizon has been investigated for both equilibrium and non-equilibrium cases in a flat FRW universe with the n components fluid with interactions.
Abstract: In this work, we have considered the power-law correction of entropy on the horizon. If the flat FRW Universe is filled with the n components fluid with interactions, the GSL of thermodynamics for apparent and event horizons have been investigated for equilibrium and non-equilibrium cases. If we consider a small perturbation around the de Sitter spacetime, the general conditions of the validity of GSL have been found. Also if a phantom dominated Universe has a pole-like type scale factor, the validity of GSL has also been analyzed. Further we have obtained constraints on the power-law parameter α in the phantom and quintessence dominated regimes. Finally we obtain conditions under which GSL breaks down in a cosmological background.
TL;DR: In this article, the authors study the evolution of primordial black holes by considering present universe is no more matter dominated rather vacuum energy dominated, and they also consider the accretion of radiation, matter and vacuum energy during respective dominance period.
TL;DR: In this paper, the generalized second law of thermodynamics for the Friedmann universe enclosed by a boundary is studied using the four well-known cosmic horizons as boundaries, namely, apparent horizon, future event horizon, Hubble horizon and particle horizon.
Abstract: In the framework of Fractional Action Cosmology (FAC), we study the generalized second law of thermodynamics for the Friedmann Universe enclosed by a boundary. We use the four well-known cosmic horizons as boundaries namely, apparent horizon, future event horizon, Hubble horizon and particle horizon. We construct the generalized second law (GSL) using and without using the first law of thermodynamics. To check the validity of GSL, we express the law in the form of four different scale factors namely emergent, logamediate, intermediate and power law. For Hubble, apparent and particle horizons, the GSL holds for emergent and logamediate expansions of the universe when we apply with and without using first law. For intermediate scenario, the GSL is valid for Hubble, apparent, particle horizons when we apply with and without first law. Also for intermediate scenario, the GSL is valid for event horizon when we apply first law but it breaks down without using first law. But for power law expansion, the GSL may be valid for some cases and breaks down otherwise.
TL;DR: In this article, the authors studied the holographic dark energy in Brans-Dicke gravity with the Granda-Oliveros cut-off proposed recently in literature and found that the transition from normal state where wD > −1 to the phantom regime where WD < −1 for the equation of state of dark energy can be more easily achieved for than when resort to the Einstein field equations is made.
Abstract: Motivated by the recent works of one of us [1, 2], we study the holographic dark energy in Brans-Dicke gravity with the Granda-Oliveros cut-off proposed recently in literature. We find out that when the present model is combined with Brans-Dicke field the transition from normal state where wD > −1 to the phantom regime where wD < −1 for the equation of state of dark energy can be more easily achieved for than when resort to the Einstein field equations is made. Furthermore, the phantom crossing is more easily achieved when the matter and the holographic dark energy undergo an exotic interaction. We also calculate some relevant cosmological parameters and their evolution.
TL;DR: In this paper, the Ricci-PLECHDE model was studied in the non-flat Friedmann?Robertson?Walker universe, where the future event horizon was replaced by the average radius of Ricci scalar curvature, and the potential and dynamics of scalar field models were reconstructed according to the evolutionary behaviour of the interacting entropy-corrected holographic DE model.
Abstract: Motivated by the holographic principle, it has previously been suggested that the dark energy (DE) density can be inversely proportional to the area A of the event horizon of the Universe. However, this kind of model would have a casuality problem. In this work, we study the power-law entropy-corrected holographic DE (PLECHDE) model in the non-flat Friedmann?Robertson?Walker universe, with the future event horizon replaced by the average radius of the Ricci scalar curvature. We derive the equation of state parameter ??, the deceleration parameter q and the evolution of energy density parameter ?D? in the presence of interaction between DE and dark matter. We consider the correspondence between our Ricci-PLECHDE model and the modified Chaplygin gas and the tachyon, K-essence, dilaton and quintessence scalar fields. The potential and dynamics of the scalar field models have been reconstructed according to the evolutionary behaviour of the interacting entropy-corrected holographic DE model.
TL;DR: In this paper, the Ricci-PLECHDE model was studied in the non-flat Friedmann-Robertson-Walker universe, with the future event horizon replaced by the average radius of Ricci scalar curvature.
Abstract: Motivated by the holographic principle, it has been suggested that the Dark Energy (DE) density can be inversely proportional to the area $A$ of the event horizon of the universe. However, this kind of model would have a casuality problem. In this work, we study the power-law entropy corrected holographic DE (PLECHDE) model in the non-flat Friedmann-Robertson-Walker universe, with the future event horizon replaced by the average radius of the Ricci scalar curvature. We derive the equation of state parameter $\omega_{\Lambda}$, the deceleration parameter $q$ and the evolution of energy density parameter $\Omega_D'$ in presence of interaction between DE and Dark Matter (DM). We consider the correspondence between our Ricci-PLECHDE model and the Modified Chaplygin Gas (MCG) and the tachyon, K-essence, dilaton and quintessence scalar fields. The potential and the dynamics of the scalar field models have been reconstructed according to the evolutionary behaviour of the interacting entropy-corrected holographic DE model.
14 Mar 2012
TL;DR: In this article, a power-law weight function in the action was chosen to obtain a varying gravitational coupling constant, which was then used to model dark energy in this paradigm and obtain relevant cosmological parameters.
Abstract: Motivated by an earlier work on fractional-action cosmology with a periodic weight function [1], we extend it by choosing a power-law weight function in the action. In this approach, we obtain a varying gravitational coupling constant. We then model dark energy in this paradigm and obtain relevant cosmological parameters.
TL;DR: In this article, Sadjadi et al. extended the generalized second law (GSL) of thermodynamics for a system comprising of a Schwarzschild black hole accreting a test non-self-gravitating fluid namely phantom energy in FRW universe.
Abstract: Motivated by some earlier works [G. Izquierdo and D. Pavon, Phys. Lett. B 639 (2006) 1; H. M. Sadjadi, Phys. Lett. B 645 (2007) 108.] dealing with the study of generalized second law (GSL) of thermodynamics for a system comprising of a Schwarzschild black hole accreting a test nonself-gravitating fluid namely phantom energy in FRW universe, we extend them when the entropy of horizons of black hole and the cosmological undergo quantum corrections. Two types of such corrections are relevant here including logarithmic and power-law, while both are motivated from different theoretical backgrounds. We obtain general mathematical conditions for the validity of GSL in each case. Further we find that GSL restricts the mass of black hole for accretion of phantom energy. As such we obtain upper bounds on the mass of black hole above which the black hole cannot accrete the phantom fluid, otherwise the GSL is violated.
TL;DR: In this article, the deceleration parameter, statefinder parameters and EoS parameters for different dark energy models with variable G correction in homogeneous, isotropic and non-flat universe for Kaluza-Klein Cosmology were calculated.
Abstract: In this work, we have calculated the deceleration parameter, statefinder parameters and EoS parameters for different dark energy models with variable G correction in homogeneous, isotropic and non-flat universe for Kaluza-Klein Cosmology. The statefinder parameters have been obtained in terms of some observable parameters like dimensionless density parameter, EoS parameter and Hubble parameter for holographic dark energy, new agegraphic dark energy and generalized Chaplygin gas models.
TL;DR: In this paper, the cosmological implications of the entropy-corrected holographic dark energy model were investigated in the framework of Brans-Dicke theory, and it was shown that the model can provide the present cosmic acceleration and even the equation of state parameter of this model can cross the phantom line.
Abstract: Considering the power-law corrections to the black hole entropy, which appear in dealing with the entanglement of quantum fields inside and outside the horizon, the holographic energy density is modified accordingly. In this paper we study the power-law entropy-corrected holographic dark energy in the framework of Brans-Dicke theory. We investigate the cosmological implications of this model in detail. We also perform the study for the new agegraphic dark energy model and calculate some relevant cosmological parameters and their evolution. As a result we find that this model can provide the present cosmic acceleration and even the equation of state parameter of this model can cross the phantom line w
D
=−1 provided the model parameters are chosen suitably.
TL;DR: In this article, the authors derived exact solutions of static wormholes in f(T) gravity by considering independent cases of the pressure components including isotropic and anisotropic pressure.
Abstract: In this paper, we derive some new exact solutions of static wormholes in f(T) gravity. We discuss independent cases of the pressure components including isotropic and anisotropic pressure. Lastly we consider radial pressure satisfying a barotropic equation of state. We also check the behavior of null energy condition (NEC) for each case and observe that it is violated for the anisotropic case, while it is satisfied for isotropic and barotropic cases.
TL;DR: In this article, the deceleration parameter and statefinder parameters have been calculated in terms of dimensionless density parameters, which can be fixed by observational data, and the interaction between three fluids has been analyzed due to constant G.
Abstract: In this work, we have considered variable G in flat FRW universe filled with the mixture of dark energy, dark matter and radiation. If there is no interaction between the three fluids, the deceleration parameter and statefinder parameters have been calculated in terms of dimensionless density parameters which can be fixed by observational data. Also the interaction between three fluids has been analyzed due to constant G. The statefinder parameters also calculated in two cases: pressure is constant and pressure is variable.