arXiv subjects

Hamid Shabani

Publications and source records attributed to Hamid Shabani.

24 records · Page 2Linked to original sources

arXiv2017

Consequences of energy conservation violation: Late time solutions of $Λ(T) CDM$ subclass of $f(R,T)$ gravity using dynamical system approach

Very recently, the authors of [PRL {\bf 118} (2017) 021102] have shown that violation of energy-momentum tensor (EMT) could result in an accelerated expansion state via appearing an effective cosmological constant, in the context of unimodular gravity. Inspired by this outcome, in this paper we investigate cosmological consequences of violation of the EMT conservation in a particular class of $f(R,T)$ gravity when only the pressure-less fluid is present. In this respect, we focus on the late time solutions of models of the type $f(R,T)=R+βΛ(-T)$. As the first task, we study the solutions when the conservation of EMT is respected and then we proceed with those in which violation occurs. We have found, provided that the EMT conservation is violated, there generally exist two accelerated expansion solutions which their stability properties depend on the underlying model. More exactly, we obtain a dark energy solution for which the effective equation of state (EoS) depend on model parameters and a de Sitter solution. We present a method to parametrize $Λ(-T)$ function which is useful in dynamical system approach and has been employed in the herein model. Also, we discuss the cosmological solutions for models with $Λ(-T)=8πG(-T)^α$ in the presence of the ultra relativistic matter.

gr-qc↗

arXiv2016

Cosmological consequences and statefinder diagnosis of a non-interacting generalized Chaplygin gas in $f(R,T)$ gravity

In this paper, we investigate cosmological consequences of a scenario for recently reported accelerated expansion of the Universe, in which the generalized Chaplygin gas (GCG) along with the baryonic matter are responsible for this observed phenomenon. In the present model, we employ an isotropic and homogeneous FLRW space time in the framework of $f(R,T)$ theory of gravity. In $f(R,T)$ gravity, the conservation of energy-momentum tensor (EMT) leads to a constraint equation which enforces us to use some specific forms of type $f(R,T)=g(R)+h(T)$. In this work we choose $g(R)=R$. We consider three classes of Chaplygin gas models which include three different forms of $f(R,T)$ function; those models which employ the standard Chaplygin gas (SCG), models which use GCG in the high pressure regimes and finally, the third case is devoted to investigating high density regimes in the presence of GCG.

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arXiv2016

Stability of the Einstein static Universe in $f(R,T)$ gravity

The Einstein static (ES) universe has played a major role in various emergent scenarios recently proposed in order to cure the problem of initial singularity of the standard model of cosmology. In the herein model, we study the existence and stability of ES universe in the context of $f(R,T)$ modified theories of gravity. Considering specific forms of $f(R,T)$ function, we seek for the existence of solutions representing ES state. Using dynamical system techniques along with numerical analysis, we find two classes of solutions: the first one is always unstable of the saddle type while the second is always stable so that its dynamical behavior corresponds to a center equilibrium point. The importance of the second class of solutions is due to the significant duty they have in constructing non-singular emergent models in which the universe could have experienced past-eternally, a series of infinite oscillations about such an initial static state after which, it enters through a suitable physical mechanism, to an inflationary era. Considering specific forms for the functionality of $f(R,T)$, we show that this theory is capable of providing cosmological solutions which admit emergent universe (EU) scenarios. We also investigate homogeneous scalar perturbations for the mentioned models. The stability regions of the solutions are parametrized by a linear equation of state (EoS) parameter and other free parameters that will be introduced for the models. Our results suggest that modifications in $f(R,T)$ gravity would lead to stable solutions which are unstable in $f(R)$ gravity model.

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arXiv2014

Cosmological and Solar System Consequences of f(R,T) Gravity Models

To find more deliberate f(R,T) cosmological solutions, we proceed our previous paper further by studying some new aspects of the considered models via investigation of some new cosmological parameters/quantities to attain the most acceptable cosmological results. Our investigations are performed by applying the dynamical system approach. We obtain the cosmological parameters/quantities in terms of some defined dimensionless parameters that are used in constructing the dynamical equations of motion. The investigated parameters/quantities are the evolution of the Hubble parameter and its inverse, the "weight function", the ratio of the matter density to the dark energy density and its time variation, the deceleration, the jerk and the snap parameters, and the equation-of-state parameter of the dark energy. We numerically examine these quantities for two general models $R+αR^{-n}+\sqrt{-T}$ and $R\log{[αR]}^{q}+\sqrt{-T}$. All considered models have some inconsistent quantities (with respect to the available observational data), except the model with n=-0.9 which has more consistent quantities than the other ones. By considering the ratio of the matter density to the dark energy density, we find that the coincidence problem does~not refer to a unique cosmological event, rather, this coincidence also occurred in the early universe. We also present the cosmological solutions for an interesting model $R+c_{1}\sqrt{-T}$ in the non--flat FLRW metric. We show that this model has an attractor solution for the late times, though with $w^{(\textrm{DE})}=-1/2$. This model indicates that the spatial curvature density parameter gets negligible values until the present era, in which it acquires the values of the order $10^{-4}$ or $10^{-3}$. As the second part of this work, we consider the weak-field [It continues ...]

gr-qc↗

arXiv2013

On Exact Anisotropic Solutions of Kasner type in Higher Order Gravities

This thesis has considered the existence of anisotropic exact vacuum solutions in the context of higher order gravities. The investigated models generally are a function of three scalars R, $R_{αβ}R^{αβ}$ and $R_{αβμν}R^{αβμν}$. Near singularity, dominant terms in the expansion of analytic type of these functions are in terms of $R^{n}$ (practically $R^{1+δ}$, for indicating deviation from Einstein-Hilbert action), ($R_{αβ}R^{αβ})^{n}$ or ($R_{αβμν}R^{αβμν})^{n}$. Investigation shows that there always exists Kasner type solutions in $R^{n}$ and $R_{αβ}R^{αβ})^{n}$ models. But, in the third type model, anisotropic Kasner type solution has not been found. Furthermore, the behavior of these models in the presence of matter has been investigated, and it has been revealed that these solutions are always valid for relativistic matter, however this is not true for some non relativistic matter. Moreover, the energy condition for $R^{1+δ}$ model in vacuum has been investigated and has been shown that the application of four energy conditions (weak, null, strong and dominant) leads to one constraint on $δ$.

gr-qc↗

arXiv2013

f(R,T) Cosmological Models in Phase Space

We investigate cosmological solutions of f(R,T) modified theories of gravity for perfect fluid in spatially FLRW metric through phase space analysis, where R is Ricci scalar and T denotes the trace of energy-momentum tensor of matter content. We explore and analyze three general theories with Lagrangians of minimal $g(R)+h(T)$, pure non--minimal g(R)h(T) and non-minimal $g(R)(1+h(T))$ couplings through dynamical systems approach. We introduce a few variables and dimensionless parameters to simplify the equations in more concise forms. The conservation of energy-momentum tensor leads to a constraint equation that, in the minimal gravity, confines functionality of h(T) to a particular form, hence, relates the dynamical variables. In this case, acceptable cosmological solutions that contain a long enough matter dominated era followed by a late-time accelerated expansion are found. To support theoretical results, we also obtain numerical solutions for a few functions of g(R), and results of the corresponding models confirm the predictions. We classify solutions into six classes which demonstrate more acceptable solutions and there is more freedom to have the matter dominated era than in the f(R) gravity. In particular, there is a new fixed point which can represent late-time acceleration. We draw different diagrams of the matter densities (consistent with the present values), the related scale factors and effective equation of state. The corresponding diagrams of parameters illustrate that there is a saddle acceleration era which is a middle era before final stable acceleration de Sitter era for some models. All presented diagrams determine radiation, matter and late-time acceleration eras very well.

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