arXiv subjects

M. Koussour

Publications and source records attributed to M. Koussour.

At least 19 recordsLinked to original sources

arXiv2026

Observational constraints on viscous cosmology in $f(T,L_m)$ gravity

We investigate the late-time cosmic acceleration within the framework of viscous $f(T,L_m)$ gravity, where the gravitational action depends on both the torsion scalar $T$ and the matter Lagrangian $L_m$. In this context, the Universe is modeled as a bulk viscous fluid, allowing for dissipative effects that generate an effective negative pressure capable of driving acceleration without invoking a cosmological constant. We adopt a simple linear model $f(T,L_m) = \alpha T + \beta L_m$ and assume a constant bulk viscosity coefficient $\zeta = \zeta_0 > 0$. The model parameters are constrained using a joint analysis of recent observational datasets, including 31 Hubble parameter measurements, the Pantheon+ sample of 1701 Type Ia Supernovae, and the latest baryon acoustic oscillation data from DESI, employing a Markov Chain Monte Carlo (MCMC) approach. The best-fit results, $H_0 = 68.16 \pm 0.65$, $\alpha = 1.53^{+0.49}_{-0.61}$, $\beta = 0.40 \pm 0.96$, and $\zeta_0 = 2.15^{+0.69}_{-0.81}$, are consistent with current cosmological observations and indicate that bulk viscosity plays a significant role in the late-time dynamics. The deceleration parameter $q_0 = -0.33 \pm 0.41$ confirms the current accelerated expansion, while the effective equation of state (EoS) evolves from a matter-like regime at high redshift toward a quintessence phase at late times. The $Om(z)$ diagnostic further supports this behavior, suggesting a mild deviation from $\Lambda$CDM toward a dynamical dark energy component. Although information criteria ($\Delta \mathrm{AIC} = 2.2$, $\Delta \mathrm{BIC} = 13.13$) slightly favor the simpler $\Lambda$CDM model, the viscous $f(T,L_m)$ framework remains a viable and physically motivated alternative capable of explaining cosmic acceleration through the combined effects of torsion-matter coupling and viscosity.

physics.gen-ph↗

arXiv2025

Non-singular bouncing cosmology in $f(T, \mathcal{T})$ gravity with energy condition violations

The singularity and inflationary problems have posed significant challenges for understanding the universe's origin and evolution. Bouncing cosmology has emerged as a promising alternative to standard cosmological models, offering a non-singular approach to early universe dynamics by facilitating a "bounce" rather than a singular beginning. In this study, we explore the feasibility of modeling specific bouncing scenarios within the framework of $ f(T, \mathcal{T}) $ gravity, allowing for a comprehensive coupling between the torsion scalar $T$ and the trace of the energy-momentum tensor $\mathcal{T}$. We analyze two $f(T, \mathcal{T})$ models: a linear model $f(T, \mathcal{T}) = \alpha T + \beta \mathcal{T}$ and a non-linear model $f(T, \mathcal{T}) = \alpha \sqrt{-T} + \beta \mathcal{T}$, with a parameterized scale factor $a(t) = \sqrt{a_0^2 + \gamma^2 t^2}$ to capture the bounce behavior. The analysis confirms a cosmic bounce at $t = 0 $, where the Hubble parameter $H = 0$ signals a transition from contraction to expansion. A crucial condition for achieving the bounce is the violation of the null energy condition (NEC) near the bounce, enabling the equation of state (EoS) parameter to enter the phantom region ($\omega < -1$). Both models exhibit an increase in energy density as the universe approaches the bounce, peaking at the bounce epoch and then decreasing post-bounce. Pressure remains negative throughout, with the EoS parameter crossing into the phantom region near the bounce in both positive and negative time zones. Our findings show that NEC and strong energy condition (SEC) violations are essential for the non-singular bounce, while the dominant energy condition (DEC) is satisfied, ensuring a consistent matter distribution...

gr-qc↗

arXiv2025

Modified cosmology in $f(Q, L_m)$ gravity

In this letter, we investigate cosmology within the framework of modified $f(Q, L_m)$ gravity using the non-linear model $f(Q, L_m) = -Q + \alpha L_m^n + \beta$, where $\alpha$, $\beta$, and $n$ are free parameters. The modified Friedmann equations are derived for a matter-dominated universe, and an analytical solution is obtained. Using Hubble, Pantheon+, and joint datasets, we constrain the model parameters and examine their implications. The results indicate that the model accommodates varying $H_0$ values, contributing to the Hubble tension, while $n \neq 1$ suggests deviations from general relativity (GR). The deceleration parameter confirms a transition to acceleration at $z_t \approx 0.6 - 0.8$, with present values supporting cosmic acceleration. This model offers a viable alternative to GR-based cosmology.

astro-ph.CO↗

arXiv2025

Energy conditions in $f(Q, L_m)$ gravity

We are experiencing a golden age of experimental cosmology, with exact and accurate observations being used to constrain various gravitational theories like never before. Alongside these advancements, energy conditions play a crucial theoretical role in evaluating and refining new proposals in gravitational physics. We investigate the energy conditions (WEC, NEC, DEC, and SEC) for two $f(Q, L_m)$ gravity models using the FLRW metric in a flat geometry. Model 1, $f(Q, L_m) = -\alpha Q + 2L_m + \beta$, features linear parameter dependence, satisfying most energy conditions while selectively violating the SEC to explain cosmic acceleration. The EoS parameter transitions between quintessence, a cosmological constant, and phantom energy, depending on $\alpha$ and $\beta$. Model 2, $f(Q, L_m) = -\alpha Q + \lambda (2L_m)^2 + \beta$, introduces nonlinearities, ensuring stronger SEC violations and capturing complex dynamics like dark energy transitions. While Model 1 excels in simplicity, Model 2's robustness makes it ideal for accelerated expansion scenarios, highlighting the potential of $f(Q, L_m)$ gravity in explaining cosmic phenomena.

astro-ph.CO↗

arXiv2025

Divergence-free deceleration and energy conditions in non-minimal $f(R,T)$ gravity

We investigate the divergence-free parametric form of the deceleration parameter within the simplest non-minimal matter-geometry coupling in $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ is the trace of the energy-momentum tensor. Specifically, we consider the linear model $f(R,T) = R + 2\lambda T$, where $\lambda$ governs the interaction between matter and geometry. Using this parametric form, we derive the Hubble parameter as a function of redshift $z$ and incorporate it into the modified Friedmann equations. Constraining the model with OHD and Pantheon data, we obtain precise estimates for $H_0$, the present deceleration parameter $q_0$, and its evolutionary component $q_1$, confirming a smooth transition between cosmic deceleration and acceleration. Further, we analyze the evolution of the energy density $\rho$ and total EoS parameter $\omega$ for different $\lambda$ values, highlighting deviations from $\Lambda$CDM and the role of $\lambda$ in shaping cosmic dynamics. In addition, we examine energy conditions, finding that the NEC and DEC are satisfied throughout evolution, while the SEC is violated at late times, supporting the observed acceleration. Our findings demonstrate that this divergence-free parameterization within $f(R,T)$ gravity offers a viable framework for explaining late-time cosmic acceleration while maintaining key observational and theoretical constraints.

gr-qc↗

arXiv2025

Testing the viability of $f(T, \mathcal{T})$ gravity models via effective equation of state constraints

This paper rigorously examines the potential of the $f(T, \mathcal{T})$ theory as a promising framework for understanding the dark sector of the universe, particularly in relation to cosmic acceleration. The $f(T, \mathcal{T})$ theory extends gravitational dynamics by incorporating both the torsion scalar $T$ and the trace of the energy-momentum tensor $\mathcal{T}$. Further, we explore the functional form $f(T, \mathcal{T}) = T + \beta \mathcal{T}$, where $\beta$ is a free parameter that modulates the matter's influence on spacetime evolution. To evaluate this model, we employ an effective EoS parameter dependent on redshift $z$, to solve the field equations and analyze the evolution of the Hubble parameter $H(z)$. Using a joint dataset ($H(z)+Pantheon^+$) and the Markov Chain Monte Carlo (MCMC) method with Bayesian analysis, we obtain the best-fit parameter values: $H_0 = 68.04 \pm 0.64$, $\beta = 0.14 \pm 0.17$, and $\gamma = 0.96^{+0.38}_{-0.69}$, which align well with current observational data. Our findings indicate a deceleration parameter of $q_0 = -0.51$, supporting a present-day accelerated expansion phase, with a transition redshift $z_t = 0.57$ marking the universe's shift from deceleration to acceleration. Moreover, we confirm a positive cosmic fluid energy density, reinforcing stability, and find an EoS parameter value of $\omega_0 = -0.76$, consistent with quintessence-driven acceleration. These results underscore the viability of $f(T, \mathcal{T})$ as a robust framework for addressing the accelerating universe and dark energy dynamics, paving the way for future investigations into its cosmological implications.

astro-ph.CO↗

arXiv2025

Constraints on $\Lambda(t)$CDM cosmology using Cosmic Chronometers and Supernova data

In this manuscript, we investigate the constraints on dynamical vacuum models within the framework of $\Lambda(t)$CDM cosmology by assuming a parameterization of the vacuum energy density as $\rho_{\Lambda}(t)=\rho_{\Lambda 0} \left[1 + \alpha (1 - a)\right]$, where $\rho_{\Lambda 0}$ is the present vacuum density and $\alpha$ is a free parameter. We use 31 cosmic chronometer data points and 1048 Pantheon type Ia supernova samples to constrain the model parameters. Our statistical analysis employs Markov Chain Monte Carlo (MCMC) simulations. We have found that the universe is currently undergoing accelerated expansion, transitioning from a decelerating phase. The transition redshift $z_t=0.65^{+0.03}_{-0.19}$ obtained from the combined CC+SNe dataset is consistent with recent constraints. The total EoS indicates an accelerating phase, with density parameters for matter and vacuum energy exhibiting expected behaviors. The $Om(z)$ diagnostic shows distinct behaviors for different datasets, and the present value of the jerk parameter deviates slightly from the $\Lambda$CDM model but remains consistent within uncertainties. These findings support the dynamic nature of dark energy and provide valuable constraints on the evolution of the universe.

astro-ph.CO↗

arXiv2025

Bulk viscous matter in $f(T)$ gravity: A path to cosmic acceleration

In this paper, we investigate the effects of varying bulk viscosity coefficients $\zeta(t)=\zeta_{0}+\zeta_{1}H$ on cosmic evolution within the framework of $f(T)$ teleparallel gravity. We focus on two cases: (i) $\zeta_{1} \neq0$ and (ii) $\zeta_{1} =0$, deriving the Hubble parameter $H$ as a function of redshift $z$ using a linear $f(T)$ model ($f(T) = \alpha T$ where $\alpha \neq 0$). Using the combined $H(z)+Pantheon^{+}+BAO$ dataset, we obtain observational constraints on model parameters. For Case I ($\zeta_1 \neq 0$), best-fit values are $H_0=60.0^{+2.0}_{-1.9}$ km/s/Mpc, $\alpha=1.01^{+0.10}_{-0.098}$, $\zeta_0=40.1^{+1.9}_{-2.0}$, and $\zeta_1=0.123^{+0.093}_{-0.088}$, while for Case II ($\zeta_1 = 0$), they are $H_0=67.5^{+1.3}_{-1.3}$ km/s/Mpc, $\alpha=0.94^{+0.14}_{-0.13}$, and $\zeta_0=34.7^{+2.0}_{-2.0}$. The analysis reveals a transition in the deceleration parameter, indicating a shift from deceleration to acceleration of the universe's expansion, with present-day values of $q_{0} \approx -0.49$ and $q_{0} \approx -0.32$ for the respective cases. The jerk parameter $j(z)$ and effective EoS for the cosmic viscous fluid also support the cosmic acceleration, with trajectories aligning with the quintessence scenario. These findings underscore the potential of our $f(T)$ model dominated by bulk viscous matter in explaining cosmic acceleration.

astro-ph.CO↗

arXiv2024

Observational constraints on a generalized equation of state model

We investigate the cosmological implications of a generalized total equation of state (EoS) model by constraining its parameters using observational datasets to effectively characterize the universe's expansion history and its dynamic properties. We introduce three parameters: $\alpha$, $\beta$, and $n$ to capture the EoS behavior across different evolutionary phases. Our analysis indicates that at high redshifts ($z \gg 1$), the EoS approaches a matter- or radiation-dominated regime, transitioning to a dark energy-dominated phase as $z \to -1$, where it tends towards a constant value $\alpha$. Using a Markov Chain Monte Carlo (MCMC) method, we analyze a combined dataset that includes 31 data points from $H(z)$ and 1701 data points from the Pantheon+ dataset. The results reveal a smooth transition from deceleration to acceleration in the universe's expansion, with current EoS values suggesting quintessence-like behavior. The model aligns with observations and indicates that dark energy is dynamically evolving rather than acting as a cosmological constant. Furthermore, energy conditions and stability analyses highlight the nature and future of dark energy. This parametrized EoS model thus offers a robust framework for understanding the complexities of dark energy and the evolution of the cosmos.

astro-ph.CO↗

arXiv2024

Observational constraints on freezing quintessence in a non-linear $f(R, L_m)$ gravity

In this paper, we investigate the freezing quintessence scenario in late-time cosmic expansion using a non-linear $f(R, L_m)$ gravity model, $f(R,L_m)=\frac{R}{2}+L_m^\alpha$, where $\alpha$ is a free parameter. We consider a solution for this model using an appropriate parametrization of the scale factor, and then the model is constrained by observational datasets, including CC, Pantheon+ (SN), and CC+SN+BAO. Our analysis yields results aligning closely with observational data. The Hubble parameter, deceleration parameter, matter-energy density, and EoS parameter of our model exhibit expected trends over cosmic time, supporting its physical validity. Furthermore, the model demonstrates consistency with the $\Lambda$CDM model in late times, displaying freezing behavior in the $\omega - \omega'$ plane and stability against density perturbations. Our findings suggest that the modified $f(R, L_m)$ gravity model is a credible approach to describing the universe's accelerating phase.

astro-ph.CO↗

arXiv2024

Observational analysis of late-time acceleration in $f(Q, L_m)$ gravity

In this study, we explored late-time cosmology within an extended class of theories based on $f(Q, L_m)$ gravity. This theory generalizes $f(Q)$ gravity by incorporating a non-minimal coupling between the non-metricity $Q$ and the matter Lagrangian $L_m$, analogous to the $f(Q,T)$ theory. The coupling between $Q$ and $L_m$ leads to the non-conservation of the matter energy-momentum tensor. We first investigated a cosmological model defined by the functional form $f(Q, L_m) = \alpha Q + \beta L_m^n$, where $\alpha$, $\beta$, and $n$ are constants. The derived Hubble parameter $H(z) = H_0 (1+z)^{\frac{3n}{2(2n-1)}}$ indicates that $n$ significantly influences the scaling of $H(z)$ over cosmic history, with $n > 2$ suggesting accelerated expansion. We also examined the simplified case of $n = 1$, leading to the linear form $f(Q, L_m) = \alpha Q + \beta L_m$, consistent with a universe dominated by non-relativistic matter. Using various observational datasets, including $H(z)$ and Pantheon, we constrained the model parameters. Our analysis showed that the $f(Q, L_m)$ model aligns well with observational results and exhibits similar behavior to the $\Lambda$CDM model. The results, with $q_0 = -0.22 \pm 0.01$ across all datasets, indicate an accelerating universe, highlighting the model's potential as an alternative to $\Lambda$CDM.

astro-ph.CO↗

arXiv2024

Constraints on anisotropic properties of the universe in $f(Q, T)$ gravity theory

Motivated by anomalies in cosmic microwave background observations, we investigate the implications of $f(Q, T)$ gravity in Bianchi type-I spacetime, aiming to characterize the universe's spatially homogeneous and anisotropic properties. By using a linear combination of non-metricity $Q$ and the energy-momentum tensor trace $T$, we parametrize the deceleration parameter and derive the Hubble solution, which we then impose in the Friedmann equations of $f(Q, T)$ gravity. Bayesian analysis is employed to find the best-fit values of model parameters, with $1-\sigma$ and $2-\sigma$ contour plots illustrating the constraints from observational data, including $H(z)$ data and the Pantheon+ sample. Our analysis reveals a transition from a decelerated to an accelerated expansion phase, with the present deceleration parameter indicating an accelerating universe. The energy density gradually decreases over time, approaching zero for the present and future, indicating continuous expansion. The anisotropic pressure, initially notably negative, transitions to slightly negative values, suggesting the presence of dark energy. The evolving equation of state parameter $\omega$ exhibits behavior akin to phantom energy, influenced by spacetime anisotropy. Violations of the null energy condition and the strong energy condition imply phantom-like behavior and accelerated expansion.

astro-ph.CO↗

arXiv2024

Exploring cosmological evolution and constraints in $f(T)$ teleparallel gravity

This study explores the extension of teleparallel gravity within the framework of general relativity, introducing an algebraic function $f(T)$ dependent on the torsion scalar $T$. Motivated by the teleparallel formulation, we investigate cosmological implications, employing the simplest parametrization of the dark energy equation of state. Our chosen $f(T)$ function, $f(T)=\alpha(-T)^n$, undergoes stringent constraints using recent observational data ($H(z)$, SNeIa, BAO, and CMB). The model aligns well with cosmic dynamics, exhibiting quintessence behavior. The evolution of the deceleration parameter, the behavior of dark energy components, and the $Om(z)$ diagnostic further reveal intriguing cosmological phenomena, emphasizing the model's compatibility with quintessence scenarios.

astro-ph.CO↗

arXiv2024

Bouncing cosmologies and stability analysis in symmetric teleparallel $f(Q)$ gravity

This paper is devoted to examining cosmological bouncing scenarios in the framework of the recently proposed symmetric teleparallel gravity (or $f(Q)$ gravity), where the non-metricity scalar $Q$ represents the gravitational interaction. We assume an $f(Q)$ model in the form of $f(Q)=αQ^n$, where $α$ and $n$ are free model parameters. To obtain a bouncing universe, we consider a special form of the scale factor $a(t)$ in terms of cosmic time, specifically $a(t) = (1+λt^2)^{1/3}$, where $λ$ is an arbitrary constant. We derive the field equations for the flat FLRW universe and obtain the corresponding exact solution. We investigate the physical behavior of various cosmological parameters such as the deceleration parameter, pressure, and equation of state (EoS) parameter with the energy conditions for our bounce cosmological model. Furthermore, we investigate the behavior of the perturbation terms $δ_m(t)$ and $δ(t)$ with respect to cosmic time $t$ using the scalar perturbation approach. We found that although the model exhibits unstable behavior at the beginning for a brief period, it shows mostly stable behavior for most of the time. Finally, we conclude that the EoS parameter crosses the quintom line $ω=-1$ in the vicinity of the bouncing point $t=0$, which confirms the success of our bounce cosmological model.

astro-ph.CO↗

arXiv2024

Late-time cosmology in $f(Q, L_m)$ gravity: Analytical solutions and observational fits

In this study, we examined the late-time cosmic expansion of the universe within the framework of $f(Q, L_m)$ gravity, where $Q$ denotes the non-metricity and $L_{m}$ represents the matter Lagrangian. We analyzed a linear $f(Q, L_m)$ model of the form $f(Q, L_m) = -αQ + 2 L_{m} + β$. Using MCMC methods, we constrained the model parameters $H_0$, $α$, and $β$ with various datasets, including $H(z)$, Pantheon+SH0ES, and BAO data. For the $H(z)$ dataset, we found $H_0 = 67.90 \pm 0.66$, $α= 0.1072_{-0.0069}^{+0.0054}$, and $β= -1988.2 \pm 1.0$. For the Pantheon+SH0ES dataset, $H_0 = 70.05 \pm 0.68$, $α= 0.0916_{-0.0033}^{+0.0028}$, and $β= -1988.3 \pm 1.0$. For the BAO dataset, $H_0 = 68.1 \pm 1.0$, $α= 0.1029_{-0.0052}^{+0.0041}$, and $β= -1988.24 \pm 0.99$. Moreover, the energy density remains positive and approaches zero in the distant future, and the deceleration parameter indicates a transition from deceleration to acceleration, with transition redshifts of $z_t = 0.60$, $z_t = 0.78$, and $z_t = 0.66$ for the respective datasets. These findings align with previous observational studies and contribute to our understanding of the universe's expansion dynamics.

astro-ph.CO↗

arXiv2024

Observational constraints on the equation of state of viscous fluid in $f(R, T)$ gravity

In this paper, we investigate a cosmological model based on viscous $f(R,T)$ gravity as a potential alternative to dark energy. This model incorporates bulk viscosity and is analyzed using an effective equation of state. We consider the simplest specific model, $f(R,T)=R+λT$, where $λ$ is a constant. The exact solution of our viscous $f(R,T)$ cosmological model is derived, and then we use the combined datasets consisting of 31 $H(z)$ data points and 1701 Pantheon+ SNe data points to determine the best-fit values of the model parameters. We find good agreement with observations, particularly at higher redshifts. Our model's behavior, including energy density, pressure with viscosity, effective equation of state, and deceleration parameter, is analyzed. It indicates a shift from decelerated to accelerated phases of the universe's expansion, suggesting that bulk viscosity in the cosmic fluid could effectively generate the negative pressure necessary for cosmic expansion. Finally, we explore statefinder diagnostics to differentiate between various dark energy models, revealing that our model resides in the quintessence region.

astro-ph.CO↗

arXiv2024

Constraining $f(Q, L_m)$ gravity with bulk viscosity

We investigate the influence of bulk viscosity on late-time cosmic acceleration within an extended $f(Q, L_m)$ gravity framework, where the non-metricity $Q$ is non-minimally coupled with the matter Lagrangian $L_m$. Analyzing the function $f(Q, L_m) = \alpha Q + \beta L_m$, we derive exact solutions under non-relativistic matter domination. Using observational datasets ($H(z)$, Pantheon supernovae, and their combination), we constrain the model parameters $H_0$, $\alpha$, $\beta$, and $\zeta$. The deceleration parameter transitions from positive to negative values around redshifts $z_t \approx 0.80$ to $0.99 $, indicating current accelerated expansion. Moreover, the effective equation of state parameter, $\omega_{eff}$, resembles quintessence dark energy ($-1 < \omega_{eff} < -\frac{1}{3}$), with corresponding values from respective datasets. Finally, we use the $Om(z)$ diagnostic, which confirms that our model demonstrates quintessence-like behavior. Our findings underscore the significant role of bulk viscosity in understanding accelerated expansion in the universe within alternative gravity theories.

astro-ph.CO↗

arXiv2024

Signature flips in time-varying $Λ(t)$ cosmological models with observational data

In this study, we investigate signature flips within the framework of cosmological models featuring a time-varying vacuum energy term $Λ(t)$. Specifically, we consider the power-law form of $Λ=αH^n$, where $α$ and $n$ are constants. To constrain the model parameters, we use the MCMC technique, allowing for effective exploration of the model's parameters. We apply this approach to analyze 31 points of observational Hubble Data (OHD), 1048 points from the Pantheon data, and additional CMB data. We consider three scenarios: when $n$ is a free parameter (Case I), when $n=0$ (Case II), and when $n=1$ (Case III). In our analysis across all three cases, we observe that our model portrays the universe's evolution from a matter-dominated decelerated epoch to an accelerated epoch, as indicated by the corresponding deceleration parameter. In addition, we investigate the physical behavior of total energy density, total EoS parameter, and jerk parameter. Our findings consistently indicate that all cosmological parameters predict an accelerated expansion phase of the universe for all three cases ($q_0<0$, $ω_0<-\frac{1}{3}$, $j_0>0$). Furthermore, our analysis reveals that the $Om(z)$ diagnostics for Cases I and III align with the quintessence region, while Case II corresponds to the $Λ$CDM model.

astro-ph.CO↗