arXiv subjects

M. Koussour

Publications and source records attributed to M. Koussour.

70 records · Page 4Linked to original sources

arXiv2023

Cosmic jerk parameter in symmetric teleparallel cosmology

In this paper, we have examined the recently proposed modified symmetric teleparallel gravity, in which gravitational Lagrangian is given by an arbitrary function of non-metricity scalar $Q$. We have considered a constant jerk parameter to express the Hubble rate. Moreover, we have used 31 points of OHD datasets and 1701 points of Pantheon+ datasets to constraint our model parameters by means of the Markov Chain Monte Carlo analysis. The mean values and the best fit obtained give a consistent Hubble rate and deceleration parameter compared to the observation values. In order to study the current accelerated expansion scenario of the Universe with the presence of the cosmological fluid as a perfect fluid, we have considered two forms of teleparallel gravity. We have studied the obtained field equations with the proposed forms of $f(Q)$ models, specifically, linear $f\left( Q\right) =αQ+β$ and non-linear $f\left( Q\right) =Q+mQ^{n}$ models. Next, we have discussed the physical behavior of cosmological parameters such as energy density, pressure, EoS parameter, and deceleration parameter for both model. To ensure the validity of our proposed cosmological models, we have checked all energy conditions. The properties of these parameters confirm that our models describe the current acceleration of the expansion of the Universe. This result is also corroborated by the energy conditions criteria. the Finally, the EoS parameter for both models indicates that the cosmological fluid behaves like a quintessence dark energy model.

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arXiv2023

A new parametrization of Hubble parameter in $f(Q)$ gravity

In this paper, we examine the accelerated expansion of the Universe at late-time in the framework of $f\left( Q\right) $ gravity theory in which the non-metricity scalar $Q$ describes the gravitational interaction. To this, we propose a new parametrization of the Hubble parameter using a model-independent way and apply it to the Friedmann equations in the FLRW Universe. Then we estimate the best fit values of the model parameters by using the combined datasets of updated $H(z)$ consisting of $57$ points, the Pantheon consisting of $1048$ points, and BAO datasets consisting of six points with the Markov Chain Monte Carlo (MCMC) method. The evolution of deceleration parameter indicates a transition from the deceleration to the acceleration phase of the Universe. In addition, we investigate the behavior of statefinder analysis and Om diagnostic parameter, Further, to discuss other cosmological parameters, we consider a $f\left( Q\right) $\ model, specifically, $f\left( Q\right) =Q+mQ^{n}$, where $m$ and $n$ are free parameters. Finally, we find that the model supports the present accelerating Universe, and the EoS parameter behaves like the quintessence model.

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arXiv2023

Constant-roll and primordial black holes in f(Q,T) gravity

In this study, we investigate the consequence of the constant-roll condition and examine the role of $f(Q,T)$ gravity in the cosmological inflation process. We analyze the inflationary scenario by calculating modified Friedmann equations, and giving an alternative technique that enables relating modified slow-roll parameters to the constant roll parameter $β$. Considering both chaotic and hilltop models, we calculate the spectral index and the tensor-to-scalar ratio and compare their compatibility with Planck's data for different choices of the constant roll parameter $β$. We examine the evolution of primordial black holes in our chosen modified gravity model taking into account the accretion process and the evaporation due to Hawking radiation. We compute the evaporation and accretion masses rate and provide an analytic estimation of the primordial black holes masse and of the radiation in the $f(Q,T)$ gravity model.

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arXiv2022

Thermodynamical aspects of Bianchi type-I Universe in quadratic form of $f\left( Q\right) $ gravity and observational constraints

In this paper, we discuss the Bianchi type-I cosmological model in the framework of symmetric teleparallel gravity say $f(Q)$ gravity in which the non-metricity term $Q$ is responsible for the gravitational interaction. We consider a special form of the $f\left( Q\right) $ function which can be cast as $f\left( Q\right) =λQ^{n}$, where $λ$ and $n$ both are the dynamical model parameters. Such a choice can be viewed as a hybrid scale factor that leads to a relation between cosmic time and redshift as $% t=\left( \frac{αt_{0}}{β}\right) W\left[ \frac{β}{α}e^{% \frac{β-\ln \left( 1+z\right) }{α}}\right] $ which describes a $% Λ$CDM model of the Universe with the expansion evolving from decelerating to an acceleration phase. The best values for the model parameters i.e. $α$ and $β$ that would accord with the most current observational datasets are then estimated. We make use of 57 points from the Hubble dataset, 1048 points from the supernovae of type Ia dataset and 6 points from the BAO dataset. We use the Markov Chain Monte Carlo (MCMC) technique in conjunction with Bayesian analysis and the likelihood function. Further, we study the validity of our model with the investigation of the thermodynamical quantities, energy conditions along with some physical variables such as the EoS, and jerk parameters. Next, our results are discussed in light of current observational data and trends.

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arXiv2022

Bulk viscous fluid in extended symmetric teleparallel gravity

In this paper, we investigate the existence of bulk viscous FLRW cosmological models in a recently proposed extended symmetric teleparallel gravity or $f\left( Q,T\right) $ gravity in which $Q$ is the non-metricity and $T$ is the trace of the energy-momentum tensor. We consider a simple coupling between matter and non-metricity, specifically, $f\left( Q,T\right) =αQ^{m+1}+λT$ and $f\left( Q,T\right) =αQ+λT$ where $α$, $λ$ and $m$ are free model parameters. The exact cosmological solutions are found by assuming the scale factor in the form of the hybrid expansion law. This type of relation generates a time-varying deceleration parameter with the transition of the Universe from the early decelerating phase to the present accelerating phase. In the presence of viscous fluid, we analyze some cosmological parameters of our cosmological model such as the energy density, bulk viscous pressure, bulk viscous coefficient, equation of state parameter, and energy conditions. Finally, we conclude our $f\left( Q,T\right) $\ cosmological models agree with the recent astronomical observations.

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arXiv2022

Bianchi type-I Barrow holographic dark energy model in symmetric teleparallel gravity

In this work, we have discussed a spatially homogeneous and anisotropic Bianchi type-I space-time in the presence of Barrow holographic dark energy (infrared cut-off is the Hubble's horizon) proposed by Barrow recently (Physics Letters B 808 (2020): 135643) and matter in the framework of $f(Q)$ gravity where the non-metricity $Q$ is responsible for the gravitational interaction for the specific choice of $f(Q)=λQ^{2}$ (where $λ<0$ is a constant). To find the exact solutions to the field equations we consider the deceleration parameter $q$ is a function of the Hubble's parameter $H$ i.e. $q=b-\frac{n}{H}$ (where $b$ and $n$ are constants). We have studied the physical behavior of important cosmological parameters such as the EoS parameter, BHDE and matter density, skewness parameter, squared sound speed, and $ω_{B}-ω_{B}^{^{\prime }}$ plane. Also, we constrain the values of the model parameters $b$ and $n$ using $57$ Hubble's parameter measurements.

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arXiv2022

Anisotropic background for two fluids: matter and holographic dark energy

We discuss a spatially homogeneous and anisotropic Bianchi type-I space-time with two fluids as the content of the Universe: matter and holographic dark energy in the framework of general relativity. To get the exact solutions of Einstein's field equations, we choose the scale factor as a hyperbolic function, specifically, $a\left( t\right) =\sinh ^{\frac{1}{n}}\left( γt\right) $, where $γ$ and $n>0$ are arbitrary constants, which gives us a time-dependent deceleration parameter. Then we study our cosmological model under the conditions of the parameters as: $γ$ fixed and $n>1$. Our cosmological solutions led to an early deceleration phase followed by the current observed acceleration phase. Further, the anisotropic parameter and some other physical parameters are discussed. We conclude that our cosmological model is consistent with the results of recent astronomical observations.

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arXiv2022

Quintessence Universe and cosmic acceleration in $f(Q,T)$ gravity

The problem of cosmic acceleration and dark energy is one of the mysteries presently posed in the scientific society that general relativity has not been able to solve. In this work, we have considered alternative models to explain this late-time acceleration in a flat Friedmann-Lemaî% tre-Robertson-Walker (FLRW) Universe within the framework of the $% f\left(Q,T\right) $\ modified gravity theory (where $Q$ is the non-metricity and $T$ is the trace of the energy-momentum tensor) recently proposed by Xu et al. (Eur. Phys. J. C 79 (2019) 708), which is an extension of $f\left( Q\right) $ gravity with the addition of the $T$ term. Here, we presume a specific form of $f\left( Q,T\right) =αQ+βQ^{2}+γT$ where $% α$, $β$ and $γ$ are free model parameters, and obtained the exact solutions by assuming the cosmic time-redshift relation as $t(z)=\frac{% nt_{0}}{m}g(z)$ which produces the Hubble parameter of the form $H(z)=\frac{% mH_{0}}{m+n}\left[ \frac{1}{g(z)}+1\right] $ where $m$ and $n$ are the non-negative constants, we find the best values for them using 57 data points of the Hubble parameter $H\left( z\right) $. Also, we find the behavior of different cosmological parameters as the deceleration parameter $% \left( q\right) $, energy density $\left( ρ\right) $, pressure $% \left(p\right) $ and EoS parameter $\left( ω\right) $ and compare them with the observational results. To ensure the validity of the results, we studied the energy conditions along with jerk parameter. Finally, we find that our model behaves similarly to the quintessence Universe.

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arXiv2022

Barrow holographic dark energy models in $f\left( Q\right)$ symmetric teleparallel gravity with Lambert function distribution

The paper presents Barrow holographic dark energy (infrared cut-off is the Hubble horizon) suggested by Barrow recently (Physics Letters B 808 (2020): 135643) in an anisotropic Bianchi type-I Universe within the framework of $% f\left( Q\right) $ symmetric teleparallel gravity, where the non-metricity scalar $Q$ is responsible for the gravitational interaction. We consider two cases: Interacting and non-interacting models of pressureless dark matter and Barrow holographic dark energy by solving $f\left( Q\right) $ symmetric teleparallel field equations. To find the exact solutions of the field equations, we assume that the time-redshift relation follows a Lambert function distribution as $t\left( z\right) =\frac{mt_{0}}{l}g\left( z\right) $, where $g\left( z\right) =LambertW\left[ \frac{l}{m}e^{\frac{l-\ln \left( 1+z\right) }{m}}\right] $, $m$ and $l$ are non-negative constants and $t_{0}$ represents the age of the Universe. Moreover, we discuss several cosmological parameters such as energy density, equation of state (EoS) and skewness parameters, squared sound speed, and $(ω_{B}-ω_{B}^{^{\prime }})$ plane. Finally, we found the values of the deceleration parameter (DP) for the Lambert function distribution as $q_{(z=0)}=-0.45$ and $q_{(z=-1)}=-1$ which are consistent with recent observational data, i.e. DP evolves with cosmic time from initial deceleration to late-time acceleration.

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arXiv2022

Accelerating Universe scenario in anisotropic $f\left( Q\right) $ cosmology

In this paper, we investigate the exact solution of an anisotropic space-time in the context of $f(Q)$ gravity, where $f\left( Q\right) $ is the arbitrary function of the non-metricity scalar $Q$. Here, we consider a specific power-law form as $f\left( Q\right) =αQ^{n+1}+β$, where $α$, $β$, and $n$ are free model parameters. Using power-law cosmology ($a\left( t\right) \propto t^{m}$), we analyze the physical behavior of cosmological parameters such as the energy density, pressure, EoS parameter, skewness parameter. Further, we validated the model with energy conditions and found that our $f\left( Q\right) $\ cosmological model behaves like the quintessence model in the present and $% Λ$CDM in the future.

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arXiv2022

Flat FLRW Universe in logarithmic symmetric teleparallel gravity with observational constraints

In this paper, we investigate the homogeneous and isotropic flat FLRW Universe in the logarithmic form of $f\left( Q\right) $ gravity, where $% Q $ is the non-metricity scalar, specifically, $f\left( Q\right) =α+β\log \left( Q\right) $, where $α$ and $β$ are free model parameters. In this study, we consider a parametrization of the Hubble parameter as $H\left( z\right) =η\left[ (z+1)^{-γ}+1\right] $, where $γ$ and $η$ are model/free parameters which are constrained by an $R^{2}$-test from 57 points of the Hubble datasets in the redshift range $0.07<z<2.36$. Further, we investigate the physical properties of the model. We analyze the energy conditions to check the compatibility of the model. We found the SEC is violated for the logarithmic form of $f\left( Q\right) $ gravity due to the reality that the Universe in an accelerating phase. Finally, we discuss some important cosmological parameters in this context to compare our model with dark energy models such as jerk parameter and statefinder parameters.

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arXiv2022

Late-time acceleration in $f\left( Q\right) $ gravity: Analysis and constraints in an anisotropic background

This paper is devoted to investigate the anisotropic locally rotationally symmetric (LRS) Bianchi type-I space-time in the context of the recently proposed $f(Q)$ gravity in which $Q$ is the non-metricity scalar. For this purpose, we consider a linear form of $f\left( Q\right) $ gravity model, specifically, $f\left( Q\right) =αQ+β$, where $α$ and $% β$ are free parameters and we analyzed the exact solutions of LRS Bianchi type-I space-time. The modified Friedmann equations are solved by presuming an expansion scalar $θ\left( t\right) $ is proportional to the shear scalar $σ\left( t\right) $ which leads to the relation between the metric potentials as $A=B^{n}$ where $n$ is an arbitrary constant. Then we constrain our model parameters with the observational Hubble datasets of 57 data points. Moreover, we discuss the physical behavior of cosmological parameters such as energy density, pressure, EoS parameter, and deceleration parameter. The behavior of the deceleration parameter predicts a transition from deceleration to accelerated phases in an expanding Universe. Finally, the EoS parameter indicates that the anisotropic fluid behaves like the standard $Λ$CDM model.

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arXiv2022

Anisotropic nature of space-time in $f(Q)$ gravity

In this paper, we consider the Bianchi type-I space-time with perfect fluid as matter content of the Universe in the framework of $f(Q)$ gravity (where $% Q$ is the non-metricity scalar) recently proposed by Jiménez et al. (Phys. Rev. D 98.4 (2018): 044048). We find exact solutions of the field equations using the anisotropic property of space-time for volumetric hybrid expansion. The cosmological parameters for the linear form of non-metricity scalar i.e. $f\left( Q\right) =αQ+β$ (where $α$ and $β$ are free model parameters) are discussed and compared with recent Hubble measurements. Also, we have obtained the best fitting values of the model parameters $k, n$ and $H_{0}$ by constraining our model with updated Hubble datasets consisting of 57 data points ($31$ (DA) and $26$ (BAO+other)) along with also examined the stability of the model and test its validity by energy conditions.

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arXiv2022

Stability analysis of anisotropic Bianchi type-I cosmological model in teleparallel gravity

In this work, we study a cosmological model of Bianchi type-I Universe in teleparallel gravity for a perfect fluid. To obtain the cosmological solution of the model, we assume that the deceleration parameter is a linear function of the Hubble parameter $H$ i.e. $q=-1+βH$ (where $β$ as a positive constant). Consequently, we get a model of our Universe, where it goes from the initial phase of deceleration to the current phase of acceleration. We have discussed some physical and geometric properties such as Hubble parameter, deceleration parameter, energy density, pressure, and equation of state (EoS) parameter and study their behavior graphically in terms of redshift and compare it with observational data such as Type Ia supernovae (SNIa). We also discussed the behavior of other parameters such as the Jerk parameter, Statefinder parameters and we tested the validity of the model by studying the stability analysis and energy conditions.

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arXiv2022

Holographic dark energy in Gauss-Bonnet gravity with Granda-Oliveros cut-off

In this paper, we have studied a model of holographic dark energy (HDE) with a homogeneous and anisotropic Bianchi type-I Universe in the framework of Gauss-Bonnet (GB) gravity or $f(G)$ gravity. To find an exact solution of the field equations, we assume that the deceleration parameter varies with cosmic time $\left( t\right) $. We plot some physical quantities and give interpretations of the results obtained. Finally, we compared our model with the $ΛCDM$ model by analyzing the Jerk parameter, and we examine the equivalence between the HDE of the present work and the generalized HDE.

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arXiv2022

Cosmic acceleration and energy conditions in $Λ$CDM symmetric teleparallel $f(Q)$ gravity

In this paper, we study a spatially homogeneous and isotropic FLRW cosmological model in framework of the symmetric teleparallel $f(Q)$ gravity proposed by Jimenez et al. (Physical Review D 98.4 (2018): 044048), where the non-metricity $Q$ is responsible for the gravitational interaction. For the whole analysis we have considered a special form of $f\left(Q\right)$ gravity function i.e. $f\left( Q\right) =αQ^{n}+β$ (where $α$, $β$, and $n$ are the dynamical model constant parameters) in the direction of $t\left( z\right) =\frac{kt_{0}}{b}f\left( z\right)$, where, $f\left( z\right) =W\left[ \frac{b}{k}e^{\frac{b-\ln \left(1+z\right) }{k}}\right] $ and $W$ denotes the Lambert function. We have discussed two cases of the function: linear $f(Q)$ model towards $ n=1$ and quadratic for $n=2$. In both the model we had verified the validity of the models with the help of energy conditions along with some physical parameters like equation of state parameter, jerk parameter, statefinder parameters are also discussed in details and compared it with the observational data.

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