arXiv subjects

M. Koussour

Publications and source records attributed to M. Koussour.

At least 37 records · Page 2Linked to original sources

arXiv2024

Constraints on bulk viscosity in $f(Q,T)$ gravity from H(z)/Pantheon+ data

In this study, we investigate the role of bulk viscosity in $f(Q,T)$ gravity in explaining late-time cosmic acceleration. This model, an extension of symmetric teleparallel gravity, introduces viscosity into cosmic matter dynamics for a more realistic representation. Specifically, we consider the linear form of $f (Q, T) =αQ + βT$, where $α$ and $β$ are free model parameters. To assess the model, we derive its exact solution and use Hubble parameter $H(z)$ data and Pantheon + SNe Ia data for parameter estimation. We employ the $χ^2$ minimization technique alongside the MCMC random sampling method to determine the best-fit parameters. Then, we analyze the behavior of key cosmological parameters, including the deceleration parameter, bulk viscous matter-dominated universe density, effective pressure, and the effective EoS parameter, accounting for the viscous type fluid. We observe a transition in the deceleration parameter from a positive (decelerating) to a negative (accelerating) phase at transition redshift $z_t$. The matter density shows the expected positive behavior, while the pressure, influenced by viscosity, exhibits negative behavior, indicative of accelerating expansion. Furthermore, we investigate the energy conditions and find that while the NEC and DEC meet positivity criteria, the SEC is violated in the present and future epochs. The $Om(z)$ diagnostic suggests that our model aligns with quintessence behavior. Finally, our $f(Q,T)$ cosmological model, incorporating bulk viscosity effects, provides a compelling explanation for late-time cosmic behavior, consistent with observational data.

astro-ph.CO↗

arXiv2024

The dynamics of matter bounce cosmology in Weyl-type $f(Q,T)$ gravity

In this work, we investigate the dynamics of bouncing cosmologies within the framework of Weyl-type $f(Q,T)$ gravity. Here, $Q$ represents the non-metricity of the space-time, is determined by the vector field $w_μ$, while $T$ represents the trace of the matter energy-momentum tensor. Our objective is to explore the feasibility of avoiding the Big Bang singularity by implementing a matter-bounce cosmology. To achieve this, we consider a specific model with the functional form $f(Q,T)=αQ+\frac{β}{6κ^2}T$, where $α$ and $β$ are model parameters. We analyze the dynamical parameters associated with this model and examine the influence of the Weyl-type $f(Q,T)$ theory on these parameters. Moreover, we assess the stability of the proposed model to ensure its viability as a cosmological scenario. Through our analysis, we aim to gain insights into the potential implications and consequences of Weyl-type $f(Q,T)$ gravity for bouncing scenarios, contributing to our understanding of alternative gravitational theories in the context of cosmology.

astro-ph.CO↗

arXiv2024

Exploring late-time cosmic acceleration: A study of a linear $f(T)$ cosmological model using observational data

Understanding the evolution of dark energy poses a significant challenge in modern cosmology, as it is responsible for the universe's accelerated expansion. In this study, we focus on a specific $f(T)$ cosmological model and analyze its behavior using observational data, including 31 data points from the CC dataset, 1048 points from the Pantheon SNe Ia samples, and 6 points from the BAO dataset. By considering a linear $f(T)$ model with an additional constant term, we derive the expression for the Hubble parameter as a function of cosmic redshift for non-relativistic pressureless matter. We obtain the best-fit values for the Hubble constant, $H_0$, and the model parameters $α$ and $β$, indicating a stable model capable of explaining late-time cosmic acceleration without invoking a dark energy component. This is achieved through modifying field equations to account for the observed accelerated expansion of the universe.

astro-ph.CO↗

arXiv2024

Cosmological constraints on time-varying cosmological terms: A study of FLRW universe models with $Λ(t)$CDM cosmology

This paper explores models of the FLRW universe that incorporate a time-varying cosmological term $Λ(t)$. Specifically, we assume a power-law form for the cosmological term as a function of the scale factor: $Λ(t)=Λ_{0} a(t)^{-α}$, where $Λ_{0}$ represents the present value of the cosmological term. Then, we derive an exact solution to Einstein's field equations within the framework of $Λ(t)$CDM cosmology and determine the best-fit values of the model parameters using the combined $H(z)$ + SNe Ia dataset and MCMC analysis. Moreover, the deceleration parameter demonstrates the accelerating behavior of the universe, highlighting the transition redshift $z_{tr}$, at which the expansion shifts from deceleration to acceleration, with confidence levels of $1-σ$ and $2-σ$. In addition, we analyze the behavior of the Hubble parameter, jerk parameter, and $Om(z)$ diagnostic. Our analysis leads us to the conclusion that the $Λ(t)$CDM model is consistent with present-day observations.

astro-ph.CO↗

arXiv2024

Exploring accelerated expansion in the universe: A study of $f(Q,T)$ gravity with parameterized EoS and cosmological constraints

The study conducted in this research paper utilizes the $f(Q,T)$ gravity, where $Q$ represents non-metricity and $T$ represents the trace of the energy-momentum tensor, to investigate the accelerated expansion of the universe. To complete the study, an effective EoS with one parameter $α$, is parameterized as $ω_{eff}=-\frac{3}{α(1+z)^{3}+3}$. The linear version of $f(Q,T)=-Q+σT$ is also considered, where $σ$ is a constant. By constraining the model with six BAO points, 57 Hubble points, and 1048 Pantheon sample datasets, the parameters $α$ and $σ$ are determined to best match the data. The cosmological parameters and energy conditions for the model are derived and examined. The results show that the model is in good agreement with observations, and can serve as a valuable starting point for analyzing FLRW models in the $f(Q,T)$ theory of gravity.

astro-ph.CO↗

arXiv2024

Exploring Universe acceleration through observational constraints via Hubble parameter reconstruction

In this article, we introduce an innovative parametric representation of the Hubble parameter, providing a model-independent means to explore the dynamics of an accelerating cosmos. The model's parameters are rigorously constrained through a Markov Chain Monte Carlo (MCMC) approach, leveraging a comprehensive dataset consisting of 31 data points from cosmic chronometers (CC), 1701 updated observations of Pantheon supernovae type Ia (SNeIa), and 6 data points from baryonic acoustic oscillations (BAO). Our analysis delves into the behavior of various cosmological parameters within the model, including the transition from a decelerating phase to an accelerating one, as well as the density parameters and the equation of state (EoS) parameter. The outcomes of our investigation reveal that the equation of state parameter aligns with characteristics reminiscent of the phantom model, supporting the prevailing understanding of our universe's current state of acceleration. This research contributes valuable insights into the ongoing cosmic expansion and underscores the utility of our novel parametric approach.

astro-ph.CO↗

arXiv2024

Bouncing behavior in $f(R,L_m)$ gravity: Phantom crossing and energy conditions

In this work, we investigate the bouncing behavior of the universe within the framework of $f(R,L_m)$ gravity, using a simple form of $f(R,L_m)=\frac{R}{2}+L_m^γ$ (where $γ$ is a free model parameter) as previously studied. The model predicts a vanishing Hubble parameter in the early and late times, with the deceleration parameter approaching a specific limit at the bouncing point. The EoS parameter is observed to cross the phantom divide line ($ω=-1$) near the bouncing point, indicating a significant transition from a contracting to an expanding phase. The model satisfies the necessary energy conditions for a successful bouncing scenario, with violations indicating exotic matter near the bouncing point. Stability conditions are satisfied for certain values of $γ$ near the bouncing point, but potential instabilities in late-time evolution require further investigation. Finally, we conclude that the $f(R,L_m)$ gravity model is promising for understanding the universe's dynamics, especially during events like the bouncing phase.

gr-qc↗

arXiv2024

Probing Weyl-type $f(Q, T)$ gravity: Cosmological implications and constraints

In this paper, we investigate the cosmological implications and constraints of Weyl-type $f(Q, T)$ gravity. This theory introduces a coupling between the non-metricity $Q$ and the trace $T$ of the energy-momentum tensor, using the principles of proper Weyl geometry. In this geometry, the scalar non-metricity $Q$, which characterizes the deviations from Riemannian geometry, is expressed in its standard Weyl form $\nabla _{μ}g_{αβ}=-w_{μ}g_{αβ}$ and is determined by a vector field $w_{μ}$. To study the implications of this theory, we propose a deceleration parameter with a single unknown parameter $χ$, which we constrain by using the latest cosmological data. By solving the field equations derived from Weyl-type $f(Q, T)$ gravity, we aim to understand the behavior of the energy conditions within this framework. In the present work, we consider two well-motivated forms of the function $f(Q, T)$: (i) the linear model represented by $f(Q, T) = αQ + \fracβ{6κ^2} T$, and (ii) the coupling model represented by $f(Q, T) = \fracγ{6H_0^2 κ^2} QT$, where $α$, $β$, and $γ$ are free parameters. Here, $κ^2 = \frac{1}{16πG}$ represents the gravitational coupling constant. In both of the models considered, the strong energy condition is violated, indicating consistency with the present accelerated expansion. However, the null, weak, and dominant energy conditions are satisfied in these models.

gr-qc↗

arXiv2024

Anisotropic f(Q) gravity model with bulk viscosity

This study investigates the dynamics of a spatially homogeneous and anisotropic LRS Bianchi type-I universe with viscous fluid in the framework of $f(Q)$ symmetric teleparallel gravity. We assume a linear form for $f(Q)$ and introduce hypotheses regarding the relationship between the expansion and shear scalars, as well as the Hubble parameter and bulk viscous coefficient. The model is constrained using three observational datasets: the Hubble dataset (31 data points), the Pantheon SN dataset (1048 data points), and the BAO dataset (6 data points). The calculated cosmological parameters indicate expected behavior for matter-energy density and bulk viscous pressure, supporting the universe's accelerating expansion. Diagnostic tests suggest that the model aligns with a $Λ$CDM model in the far future and resides in the quintessence region. These findings are consistent with recent observational data and contribute to our understanding of cosmic evolution within the context of modified gravity and bulk viscosity.

gr-qc↗

arXiv2024

Cosmic expansion beyond $Λ$CDM: Investigating power-law and logarithmic corrections

The cosmic acceleration observed in the expansion of the Universe has sparked extensive research into the nature of dark energy, which is known to constitute approximately 70\% of the Universe's energy content. In this study, we explore two parametrizations of the Hubble parameter, namely power-law and logarithmic corrections, as alternatives to the standard $Λ$CDM model. Using observational data from Cosmic Chronometers (CC), Pantheon+, and the Baryonic Acoustic Oscillations (BAO) datasets, we investigate the dynamics of essential cosmological parameters, including the deceleration parameter, energy density, pressure, and equation of state (EoS) parameter. The $Om(z)$ diagnostic test is employed to classify different dark energy models. Our cosmological models, with the power-law and logarithmic corrections, are found to provide a good fit to the recent observational data and efficiently describe the cosmic expansion scenario.

astro-ph.CO↗

arXiv2024

Cosmic expansion history analysis with Hubble parametrization in $Q^{n}$ gravity

We propose a specialized parameterization for the Hubble parameter, inspired by $Λ$CDM cosmology, to investigate the cosmic expansion history of the Universe. This parameterization is employed to analyze the Universe's late-time behavior within the context of $Q^n$ gravity, where $Q$ represents non-metricity. By using data from 57 Hubble data points, 1048 supernova (SNe) data points, and 6 baryon acoustic oscillation (BAO) data points, we determine the optimal values for the model parameters. Additionally, we explore three distinct cosmological models based on the parameter $n$, specifically when it takes on the values of $0.55$, $1.5$, and $2.0$. The results of our analysis indicate that our proposed parameterization, along with the associated models for different values of $n$, predicts an accelerated cosmic expansion phase.

astro-ph.CO↗

arXiv2024

Investigating the compatibility of exact solutions in Weyl-type $f(Q,T)$ gravity with observational data

In this study, we investigate the dynamics of the Universe during the observed late-time acceleration phase within the framework of the Weyl-type $f(Q,T)$ theory. Specifically, we consider a well-motivated model with the functional form $f(Q,T)=αQ+\frac{β}{6κ^2}T$, where $Q$ represents the scalar of non-metricity and $T$ denotes the trace of the energy-momentum tensor. In this context, the non-metricity $Q_{μαβ}$ of the space-time is established by the vector field $w_μ$. The parameters $α$ and $β$ govern the gravitational field and its interaction with the matter content of the Universe. By considering the case of dust matter, we obtain exact solutions for the field equations and observe that the Hubble parameter $H(z)$ follows a power-law behavior with respect to redshift $z$. To constrain the model parameters, we analyze various datasets including the $Hubble$, $Pantheon$ datasets, and their combination. Our results indicate that the Weyl-type $f(Q,T)$ theory offers a viable alternative to explain the observed late-time acceleration of the Universe avoiding the use of dark energy.

astro-ph.CO↗

arXiv2023

Investigating the dark energy phenomenon in $f(R,L_m)$ cosmological models with observational constraints

This paper explores the dark energy phenomenon within the context of $f(R,L_m)$ gravity theory. Two specific non-linear $f (R, L_m)$ models are considered: $f(R,L_m)=\frac{R}{2}+L_m^α$ and $f(R,L_m)=\frac{R}{2}+(1+αR)L_m$, where the parameter $α$ is free. Here, we adopt a parametrization form for the Hubble parameter in terms of redshift $z$ as $H(z)=H_0 \left[A(1+z)^3+B+ε\log(1+z)\right]^\frac{1}{2}$, which allows for deviations from the standard $Λ$CDM model at both low and high redshifts. We then incorporate the Hubble parameter solution into the Friedmann equations for both models. We employ Bayesian analysis to estimate the constraints on the free parameters $H_0$, $A$, $B$, and $ε$ using the Hubble measurements and the Pantheon dataset. Further, we investigate the evolution of key cosmological quantities, such as the deceleration parameter, energy density, pressure, EoS parameter, and energy conditions. The evolution of the deceleration parameter reveals a significant transition from a decelerating phase to an accelerating phase in the Universe. The EoS parameter exhibits quintessence-like behavior for both non-linear $f (R, L_m)$ models.

astro-ph.CO↗

arXiv2023

Scalar field dark energy: Insights into cosmological evolution and black hole accretion

We propose a novel approach to parameterize the equation of state for Scalar Field Dark Energy (SFDE) and use it to derive analytical solutions for various cosmological parameters. Using statistical MCMC with Bayesian techniques, we obtain constraint values for the model parameters and analyze three observational datasets. We find a quintessence-like behavior for Dark Energy (DE) with positive values for both model parameters $α$ and $β$. Our analysis of the $CC$+$BAO$+$SNe$ datasets reveals that the transition redshift and the current value of the deceleration parameter are $z_{tr}=0.73_{-0.01}^{+0.03}$ and $q_{0}=-0.44_{-0.02}^{+0.03}$, respectively. We also investigate the fluid flow of accretion SFDE around a Black Hole (BH) and analyze the nature of the BH's dynamical mass during accretion, taking into account Hawking radiation and BH evaporation. Our proposed model offers insight into the nature of DE in the Universe and the behavior of BHs during accretion.

astro-ph.CO↗

arXiv2023

A comprehensive parametrization approach for the Hubble parameter in scalar field dark energy models

This study proposes a novel parametrization approach for the dimensionless Hubble parameter i.e. $E^2(z)=A(z)+β(1+γB(z))$ in the context of scalar field dark energy models. The parameterization is characterized by two functions, $A(z)$ and $B(z)$, carefully chosen to capture the behavior of the Hubble parameter at different redshifts. We explore the evolution of cosmological parameters, including the deceleration parameter, density parameter, and equation of state parameter. Observational data from Cosmic Chronometers (CC), Baryonic Acoustic Oscillations (BAO), and the Pantheon+ datasets are analyzed using MCMC methodology to determine model parameters. The results are compared with the standard $Λ$CDM model using the Planck observations. Our approach provides a model-independent exploration of dark energy, contributing to a comprehensive understanding of late-time cosmic acceleration.

astro-ph.CO↗

arXiv2023

Square-Root parametrization of dark energy in $f(Q)$ cosmology

This paper is a parametrization of the equation of state (EoS) parameter of dark energy (DE), which is parameterized using Square-Root (SR) form i.e. $ω_{SR}=\text{$ω_{0}$}+\text{$ω_{1}$}\frac{z}{\sqrt{z^{2}+1}}$, where $ω_{0}$ and $ω_{1}$ are free constants. This parametrization will be examined in the context of the recently suggested $f(Q)$ gravity theory as an alternative to General Relativity (GR), in which gravitational effects are attributed to the non-metricity scalar $Q$ with the functional form $f(Q)=Q+αQ^{n}$, where $α$ and $n$ are arbitrary constants. We derived observational constraints on model parameters using the Hubble dataset with 31 data points and the Supernovae (SNe) dataset from the Pantheon samples compilation dataset with 1048 data points. For the current model, the evolution of the deceleration parameter, density parameter, EoS for DE, and $Om(z)$ diagnostic have all been investigated. It has been shown that the deceleration parameter favors the current accelerated expansion phase. It has also been shown that the EoS parameter for DE has a quintessence nature at this time.

astro-ph.CO↗

arXiv2023

Constraining the cosmological model of modified $f(Q)$ gravity: Phantom dark energy and observational insights

Despite the significant accomplishments of general relativity, numerous unresolved issues persist in our understanding of the cosmos. One of the most perplexing challenges is the ongoing accelerated expansion of the Universe, which continues to elude a complete explanation. Consequently, scientists have proposed various alternative theories to GR in pursuit of a deeper understanding. In our analysis, we delve into the recently proposed modified $f(Q)$ gravity, where $Q$ represents the non-metricity scalar responsible for gravitational effects. Specifically, we investigate a cosmological model characterized by the functional form $f(Q) = Q+αQ^n$, where $α$ (with $α\neq 0$) and $n$ serve as free parameters. Utilizing this functional form, we construct our Hubble rate, incorporating a specific equation of state to describe the cosmic fluid. Furthermore, we leverage a dataset consisting of 31 data points from Hubble measurements and an additional 1048 data points from the Pantheon dataset. These data serve as crucial constraints for our model parameters, and we employ the Markov Chain Monte Carlo (MCMC) method to explore the parameter space and derive meaningful results. With our parameter values constrained, our analysis yields several noteworthy findings. The deceleration parameter suggests a recent accelerated phase in the cosmic expansion. In addition, the EoS parameter paints a portrait of dark energy exhibiting phantom-like characteristics. Furthermore, we delve into the application of cosmological diagnostic tools, specifically the statefinder and the $Om(z)$ diagnostics. Both of these tools align with our previous conclusions, confirming the phantom-like behavior exhibited by our cosmological model. These results collectively contribute to our understanding of the dynamic interplay between gravity, dark energy, and the expanding cosmos.

astro-ph.CO↗

arXiv2023

Cosmological implications of the constant jerk parameter in $f(Q,T)$ gravity theory

This study delves into modified gravity theories that are equivalent to General Relativity but involve the torsion or non-metricity scalar instead of the curvature scalar. Specifically, we focus on $f(Q,T)$ gravity, which entails an arbitrary function of the non-metricity scalar $Q$ non-minimally coupled to the trace of the stress-energy tensor $T$. We investigate the functional form $f(Q,T)=f(Q)+f(T)$, where $f(Q) =Q+α\,Q^2$ represents the Starobinsky model in $f(Q)$ gravity and $f(T)=2\,γ\,T$, with $α$ and $γ$ are constants. To obtain solutions for the Friedmann equations, we introduce the concept of a constant jerk and employ its definition to trace the evolution of other kinematic variables, including the deceleration parameter, energy density, EoS parameter, and various energy conditions. These analyses serve to validate the proposed model. We constrain our constant jerk model using the most available Pantheon set of data, the Hubble set of data, and the BAO set of data. Further, we employ $Om(z)$ diagnostic as a means to differentiate between different theories of dark energy. Throughout our examination, all cosmological parameters under scrutiny consistently indicate an accelerating Universe.

gr-qc↗