arXiv subjects

Hamid Shabani

Publications and source records attributed to Hamid Shabani.

At least 19 recordsLinked to original sources

arXiv2025

The cosmic consequences and the constraints on HN-gravity

In this paper, we investigate the latetime cosmic acceleration of the Quintessence model within the framework of Hoyle Narlikar Gravity (HNG), which incorporates a creation field. Using the Hubble tension as a function of the density parameter for matter, the density parameter for radiation, and the density parameter for dark energy in the covariant formulation, we find the gravitational field equations in the spatially flat, homogeneous, and isotropic spacetime to examine the dynamical mechanism that leads to cosmic acceleration in the late-time universe. We analyze the observational constraints on the latetime density parameters using various recent observational datasets, including the Hubble datasets, Pantheon plus, and the joint compilation, Pantheon BAO. Consequently, it is explicitly demonstrated that latetime cosmic acceleration can be consistent with recent observational data in Hoyle Narlikar Gravity with nonminimal matter interaction. In contrast with other modified theories of gravity, it is observed that the creation field theory with non minimal matter interaction renders more compact constraints on the Hubble tension together with density parameters, and extensively explains the accelerating expansion of the universe, which makes it a more plausible option compared to the LCDM model. Furthermore, the ww1 phase analysis confirms alternating thawing and freezing behaviour of the model, with all trajectories ultimately converging toward the LCDM point, thereby confirming the model stability and the observational consistency.

gr-qc↗

arXiv2025

Cosmological Scenarios in $f(Q,C)$ gravity with a dynamical degree of freedom

The $f(Q,C)$ theory, which extends symmetric teleparallel gravity by including the boundary term $C$ in addition to the non-metricity scalar $Q$, provides a unifying framework that encompasses both $f(Q)$ and $f(\mathring{R})$ gravities. In this work, we develop a comprehensive dynamical system analysis of $f(Q,C)$ cosmology formulated within the non-coincident affine connection branches. Unlike the coincident case, these branches introduce a dynamical degree of freedom that significantly enriches the cosmological phase space. We show that even for simple power-law forms of the Lagrangian, the theory accommodates a broad spectrum of cosmic scenarios, including successive pressureless matter eras, stiff-matter phases between early inflation and dark matter domination, and late-time acceleration. Our analysis demonstrates that the Universe can naturally evolve from an initial de Sitter phase to a matter-dominated epoch and subsequently to a final de Sitter attractor, consistent with the observed thermal history. These results highlight the role of the dynamical connection in shaping cosmic evolution and underline the potential of $f(Q,C)$ gravity as a viable alternative framework for addressing outstanding issues in modern cosmology.

gr-qc↗

arXiv2024

Emergent Universe in f(Q) gravity theories

One resolution of the ancient cosmic singularity, i.e., the Big Bang Singularity (BBS), is to assume an inflationary stage preceded by a long enough static state in which the universe and its physical properties would oscillate around certain equilibrium points. The early period is referred to as the Einstein Static (ES) Universe phase, which characterizes a static phase with positive spatial curvature. A stable Einstein static state can serve as a substitute for BBS, followed by an inflationary period known as the Emergent Scenario. The initial need has not been fulfilled within the context of General Relativity, prompting the investigation of modified theories of gravity. The current research aims to find such a solution within the framework of symmetric teleparallel gravity, specifically in the trendy $f(Q)$ theories. An analysis has been conducted to investigate stable solutions for both positively and negatively curved spatial FRW universes, in the presence of a perfect fluid, by utilizing various torsion-free and curvature-free affine connections. Additionally, we propose a method to facilitate an exit from a stable ES to a subsequent inflationary phase. We demonstrate that $f(Q)$ gravity theories have the ability to accurately depict the emergence of the universe.

gr-qc↗

arXiv2024

The cosmological significance of boundary term in non-metricity gravity

Within the context of metric-affine gravity, we examine the significance of the boundary term in symmetric teleparallel gravity by employing the cosmological dynamical system analysis method. We focus on the novel gravity models characterized by the functions $f(Q,C)$, where $f$ is a smooth function of the non-metricity scalar $Q$ and the associated boundary term $C$. In a cosmological setting adopting three different classes of symmetric teleparallel affine connections, we investigate a model $f(Q,C)=Q^{s}+eC^{r}$, and some special cases of this model. We show that the boundary term which is added to the Einsteinian field equations (or equivalently to $f(Q)=Q$ ones) are capable of bringing forward solutions corresponding to the early accelerated expansion. This alludes the physics behind the boundary terms which usually are discarded in the most gravitational theories.

gr-qc↗

arXiv2024

Late time cosmological solutions in $f(R,T)$ gravity in a viscous Universe

Considering the condition on conservation of energy momentum tensor (EMT), we study late time cosmological solutions in the context of $f(R,T)=R+\alpha T^{n}$ gravity (where $\alpha$ and $n$ are constants) in a flat FLRW spacetime. The present model discusses the case of a barotropic perfect fluid as the matter content of the Universe, along with the case when dissipative effects are taken into account. Briefly, assuming a single perfect fluid we find that the mentioned model is not capable of presenting an observationally consistent picture of the late time accelerated expansion of the Universe; nevertheless, the model leads to admissible solutions when the bulk viscosity is included. In this regard, a consistent setting is found considering the Eckart, Truncated and Full Israel-Stewart theories which determine the behavior of bulk viscosity. In the presence of bulk viscosity, the behavior of the deceleration parameter (DP) shows that the underlying model can describe an acceptable evolution even if the isotropic pressure of matter content of the Universe is negligible.

gr-qc↗

arXiv2023

Cosmology of $f(Q)$ gravity in non-flat Universe

We investigate the cosmological implications of $f(Q)$ gravity, which is a modified theory of gravity based on non-metricity, in non-flat geometry. We perform a detailed dynamical-system analysis keeping the $f(Q)$ function completely arbitrary. As we show, the cosmological scenario admits a dark-matter dominated point, as well as a dark-energy dominated de Sitter solution which can attract the Universe at late times. However, the main result of the present work is that there are additional critical points which exist solely due to curvature. In particular, we find that there are curvature-dominated accelerating points which are unstable and thus can describe the inflationary epoch. Additionally, there is a point in which the dark-matter and dark-energy density parameters are both between zero and one, and thus it can alleviate the coincidence problem. Finally, there is a saddle point which is completely dominated by curvature. In order to provide a specific example, we apply our general analysis to the power-law case, showing that we can obtain the thermal history of the Universe, in which the curvature density parameter may exhibit a peak at intermediate times. These features, alongside possible indications that non-zero curvature could alleviate the cosmological tensions, may serve as advantages for $f(Q)$ gravity in non-flat geometry.

gr-qc↗

arXiv2023

Phase-space analysis of a novel cosmological model in $f(Q)$ theory

The vanishing affine connections have been used solely while adopting the modified $f(Q)$ gravity theory to the cosmology. Consequently, researchers could not get beyond what is already known in $f(T)$ theory earlier. To alleviate this problem, in the present manuscript we investigate a recently proposed construction of $f(Q)$ theory using non-vanishing affine connection in the spatially flat FLRW spacetime. We then investigate the cosmological solutions of $f(Q)$ theory for a perfect fluid through the phase space analysis. We introduce few variables and dimensionless parameters to construct the corresponding equations suitable for the dynamical system approach. The conservation of the energy-momentum tensor leads to a constraint equation that relates the dynamical variables. Briefly, both unstable and stable de Sitter solutions appear which correspond to early and late times accelerated expansions. Also, unstable points corresponding to the matter dominated and radiation dominated eras have been found which do exist for every $f(Q)$ function. As a result, the present discussion shows that $f(Q)$ gravity endowed by non-vanishing affine connections is capable of explaining a true sequence of cosmic eras.

gr-qc↗

arXiv2023

A non-singular bouncing cosmology in $ f(R,T) $ gravity

We investigate a bounce realization in the framework of higher order curvature in $ f(R,T) $ modified theory of gravity. We perform a detailed analysis of the cosmological parameters to explain the contraction phase, the bounce phase, and the expansion phase. Furthermore, we observe a violation of the null energy condition, instability of the model, and a singularity upon deceleration at the bouncing point, which are the supporting results for a bouncing cosmology. The outcome of the slow roll parameters is satisfactory to understand the inflation era and the equation of state parameter exhibits a ghost condensate behavior of the model near the bounce. Additionally, we discuss the stability of the model using linear perturbations in the Hubble parameter as well as the energy density.

gr-qc↗

arXiv2022

Einstein Static Universe and its Stability in Generalized Rastall Gravity

The Einstein static (ES) state is a good candidate for describing the very early universe in terms of a regular cosmological model in which the Big Bang singularity is avoided. In the present study we propose an ES solution in the framework of generalized Rastall gravity (GRG), a modified version of original Rastall theory in which the coupling parameter is allowed to vary with respect to the spacetime coordinates. Introducing an ansatz for the Rastall parameter, existence and the corresponding stability of the solutions are investigated. We show that the GRG is capable of describing a stable singularity-free state for the universe. The problem of transition from an ES to an inflationary state is also addressed. We find that a time variation of the equation of state parameter from values lying in the range |w|<1/3 to the value w=-1/3 can give rise to such a phase transition. The vector and tensor perturbations around the ES solution are studied, as well. In the case of GRG, the vector perturbations remain frozen, nevertheless, the tenor perturbations can grow in such a way that the ES solution remains stable provided the ratio of Rastall gravitational constant to the Einsteinian one always exceeds a minimum value for each tensor mode.

gr-qc↗

arXiv2022

A dynamical system representation of generalized Rastall gravity

In this work we study the phase-space analysis of generalized Rastall gravity (GRG) which has recently been introduced as a modification to the original version of Rastall gravity (RG). In GRG, the coupling parameter assumes a dynamical feature and may play the role of dark energy (DE) which is responsible for the present accelerating expansion of the Universe. Our investigation shows that such a modification of General Relativity (GR) admits a stable critical point corresponding to the late time accelerated expansion of the Universe. Assuming both dark matter (DM) and an ultra-relativistic perfect fluid (radiation) as the cosmic ingredients, we find that the underlying model presents a viable sequence of cosmic evolution, beginning from radiation dominated era passing then through DM domination and finally reaches the late time DE dominated era. Moreover, taking into account the contribution due to the spatial curvature within the total energy density leads to a growing mode for it in such a way that its present values are consistent with those reported in recent observations. We also present a numerical simulation of the model which particularizes our dynamical system representation. These studies demonstrate the ability of GRG to describe the present accelerated expansion of the Universe.

gr-qc↗

arXiv2022

Cosmological aspects of a hyperbolic solution in $f(R,T)$ gravity

This article deals with a cosmological scenario in $ f(R,T) $ gravity for a flat FLRW model of the universe. We consider the $ f(R,T) $ function as $ f(R)+f(T) $ which starts with a quadratic correction of the geometric term $ f(R) $ having structure $ f(R)=R+αR^2 $, and a linear matter term $ f(T)=2λT $. To achieve the solution of the gravitational field equations in the $ f(R,T) $ formalism, we take the form of a geometrical parameter, i.e. scale factor $ a(t)= sinh^{\frac{1}{n}}(βt) $ \cite{cha}, where $ β$ and $ n $ are model parameters. An eternal acceleration can be predicted by the model for $ 0<n<1 $, while the cosmic transition from the early decelerated phase to the present accelerated epoch can be anticipated for $ n\geq 1 $. The obtained model facilitate the formation of structure in the Universe according to the Jeans instability condition as our model transits from radiation dominated era to matter dominated era. We study the varying role of the equation of state parameter $ ω$. We analyze our model by studying the behavior of the scalar field and discuss the energy conditions on our achieved solution. We examine the validity of our model via Jerk parameter, Om diagnostic, Velocity of sound and Statefinder diagnostic tools. We investigate the constraints on the model parameter $ n $ and $ H_0 $ (Hubble constant) using some observational datasets: $SNeIa$ dataset, $ H(z)$ (Hubble parameter) dataset, $ BAO $ (Baryon Acoustic Oscillation data) and their combinations as joint observational datasets $ H(z)$ + $ SNeIa $ and $ H(z)$ + $ SNeIa $ + $ BAO $. It is testified that the present study is well consistent with these observations. We also perform some cosmological tests and a detailed discussion of the model.

gr-qc↗

arXiv2021

Non-minimal coupling inspires the Dirac cosmological model

In the framework of the generalized Rastall theory (GRT), we study the ability of a non-minimal coupling between geometry and matter fields in order to provide a setting which allows for a variable G during the cosmic evolution. In this regard, the compatibility of this theory with Dirac hypothesis on the variations of G is investigated, and additionally, the possibility of obtaining the current accelerated universe is also addressed. In summary, our study indicates that, in GRT, having in hand the G profile, one may find the corresponding non-minimal coupling between the energy source and geometry and vise versa, in a compatible way with the current accelerated universe.

gr-qc↗

arXiv2020

A connection between Rastall-type and f(R, T) gravities

Recently a Lagrangian formulation for Rastall Gravity (RG) has been proposed in the framework of f(R, T) gravity. In the present work we obtain Lagrangian formulation for the standard Rastall theory assuming the matter content is a perfect fluid with linear Equation of State (EoS). We therefore find a relation between the coupling constant of the Lagrangian, the Rastall gravitational coupling constant and the EoS parameter. We also propose a Lagrangian for the Generalized Rastall Gravity (GRG). In this case the Rastall parameter which is a constant is replaced by a variable one. More exactly, it appears as a function of the Ricci scalar and the trace of the Energy Momentum Tensor (EMT). In both mentioned models, the Lagrangians are constructed by a linear function of R and T.

gr-qc↗

arXiv2019

Stability of the Einstein static Universe in Einstein-Cartan-Brans-Dicke gravity

In the present work we consider the existence and stability of Einstein static {\sf ES} Universe in Brans-Dicke ({\sf BD}) theory with non-vanishing spacetime torsion. In this theory, torsion field can be generated by the {\sf BD} scalar field as well as the intrinsic angular momentum (spin) of matter. Assuming the matter content of the Universe to be a Weyssenhoff fluid, which is a generalization of perfect fluid in general relativity ({\sf GR}) in order to include the spin effects, we find that there exists a stable {\sf ES} state for a suitable choice of the model parameters. We analyze the stability of the solution by considering linear homogeneous perturbations and discuss the conditions under which the solution can be stable against these type of perturbations. Moreover, using dynamical system techniques and numerical analysis, the stability regions of the {\sf ES} Universe are parametrized by the {\sf BD} coupling parameter and first and second derivatives of the {\sf BD} scalar field potential, and it is explicitly shown that a large class of stable solutions exists within the respective parameter space. This allows for non-singular emergent cosmological scenarios where the Universe oscillates indefinitely about an initial {\sf ES} solution and is thus past eternal.

physics.gen-ph↗

arXiv2018

Static Vacuum Solutions on Curved Spacetimes with Torsion

The Einstein-Cartan-Kibble-Sciama ({\sf ECKS}) theory of gravity naturally extends Einstein\rq{}s general relativity ({\sf GR}) to include intrinsic angular momentum (spin) of matter. The main feature of this theory consists of an algebraic relation between spacetime torsion and spin of matter which indeed deprives the torsion of its dynamical content. The Lagrangian of {\sf ECKS} gravity is proportional to the Ricci curvature scalar constructed out of a general affine connection so that owing to the influence of matter energy-momentum and spin, curvature and torsion are produced and interact only through the spacetime metric. In the absence of spin, the spacetime torsion vanishes and the theory reduces to {\sf GR}. It is however possible to have torsion propagation in vacuum by resorting to a model endowed with a non-minimal coupling between curvature and torsion. In the present work we try to investigate possible effects of the higher order terms that can be constructed from spacetime curvature and torsion, as the two basic constituents of Riemann-Cartan geometry. We consider Lagrangians that include fourth-order scalar invariants from curvature and torsion and then investigate the resulted field equations. The solutions that we find show that there could exist, even in vacuum, nontrivial static spacetimes that admit both black holes and naked singularities.

gr-qc↗

arXiv2018

Bouncing cosmological solutions from f(R,T) gravity

In this work we study classical bouncing solutions in the context of $f({\sf R},{\sf T})={\sf R}+h({\sf T})$ gravity in a flat {\sf FLRW} background using a perfect fluid as the only matter content. Our investigation is based on introducing an effective fluid through defining effective energy density and pressure; we call this reformulation as the "effective picture". These definitions have been already introduced to study the energy conditions in $f({\sf R},{\sf T})$ gravity. We examine various models to which different effective equations of state, corresponding to different $h({\sf T})$ functions, can be attributed. It is also discussed that one can link between an assumed $f({\sf R},{\sf T})$ model in the effective picture and the theories with generalized equation of state ({\sf EoS}). We obtain cosmological scenarios exhibiting a nonsingular bounce before and after which the Universe lives within a de-Sitter phase. We then proceed to find general solutions for matter bounce and investigate their properties. We show that the properties of bouncing solution in the effective picture of $f({\sf R},{\sf T})$ gravity are as follows: for a specific form of the $f({\sf R,T})$ function, these solutions are without any future singularities. Moreover, stability analysis of the nonsingular solutions through matter density perturbations revealed that except two of the models, the parameters of scalar-type perturbations for the other ones have a slight transient fluctuation around the bounce point and damp to zero or a finite value at late times. Hence these bouncing solutions are stable against scalar-type perturbations. It is possible that all energy conditions be respected by the real perfect fluid, however, the null and the strong energy conditions can be violated by the effective fluid near the bounce event.

gr-qc↗

arXiv2017

Interpretation of $f({\sf R},{\sf T})$ gravity in terms of a conserved effective fluid

In the present work we introduce a novel approach to study $f({\sf R},{\sf T})$ gravity theory from a different perspective. Here, ${\sf T}$ denotes the trace of energy-momentum tensor ({\sf EMT}) of matter fluids. The usual method (as discussed in the literature) is to choose an $h({\sf T})$ function and then solve for the resulted Friedman equations. Nevertheless, our aim here is, without loss of generality, to reformulate a particular class of $f({\sf R},{\sf T})$ gravity models in which the Einstein-Hilbert action is promoted by an arbitrary function of the trace of {\sf EMT}. The strategy is the redefinition of the equation of motion in terms of the components of an effective fluid. We show that in this case the {\sf EMT} is automatically conserved. As we shall see, adopting such a point of view (at least) in $f({\sf R},{\sf T})$ gravity is accompanied by two significant points. On one hand, $h({\sf T})$ function is chosen based upon a physical concept and on the other, we clearly understand the overall or effective behavior of matter in terms of a conserved effective fluid. To illustrate the idea, we study some models in which different physical properties for the effective fluid is attributed to each model. Particularly, we discuss models with constant effective density, constant effective pressure and constant effective equation of state ({\sf EoS}) parameter. Moreover, two models with a relation between the effective density and the effective pressure will be considered. An elegant result is that in $f({\sf R},{\sf T})$ gravity, there is a possibility that a perfect fluid could effectively behave as a modified Chaplygin gas with four free parameters.

gr-qc↗

arXiv2017

Late-time cosmological evolution of a general class of f(R, T) gravity with minimal curvature-matter coupling

In this work, we study the late-time cosmological solutions of f(R,T)=g(R)+h(-T) models assuming that the conservation of the energy-momentum tensor (EMT) is violated. We perform our analysis through constructing an autonomous dynamical system for the equations of motion. We study the stability properties of solutions via considering linear perturbations about the related equilibrium points. These parameters which are constructed out of the functions g(R) and h(-T) play the main role in finding the late time behavior of the solutions.

gr-qc↗