arXiv subjects

Safae Dahmani

Publications and source records attributed to Safae Dahmani.

9 recordsLinked to original sources

arXiv2026

Testing the running vacuum model in light of DESI-DR2 Measurements

Motivated by the fact that quantum effects leave an imprint on the vacuum equation of state, making it depart from the standard cosmological constant relation, $P_{\rm vac}=-ρ_{\rm vac}$, this work investigates generalized running vacuum models (RVMs) by considering a dynamical vacuum equation of state in which the vacuum energy density, $ρ_{\text{vac}}$, evolves as a function of the Hubble parameter, $H$, and its time derivative, $\dot{H}$. This formulation extends previous running vacuum approaches by incorporating a dependence of the vacuum energy density on both, $H^2$ and $\dot{H}$. The corresponding Friedmann equations are derived and analyzed to study their impact on cosmic expansion. The model parameters are constrained through a joint statistical analysis combining the cosmic microwave background shift parameters, DESI-DR2 observations, PantheonPlus type Ia supernovae compilation, and Hubble rate $H(z)$ measurements. Model comparison is performed using information criteria, the Akaike Information Criterion (AIC) and Deviance Information Criterion (DIC), in order to assess the statistical performance of the RVMs relative to the standard $Λ$CDM scenario. The results show that the generalized RVMs provide a good fit to current observations and represent a statistically competitive alternative to the standard $Λ$CDM model. Notably, all three running vacuum formulations yield lower AIC and DIC values than $Λ$CDM, indicating that dynamical running vacuum energy remains a viable cosmological scenario.

physics.gen-ph↗

arXiv2026

Testing the bulk viscosity within the f(R,T) gravity in light of DESI-DR2 observations

We investigate the cosmological implications of the bulk viscosity within the framework of $f(R,T)$ gravity by considering the linear model $f(R,T)=R+2λT$, where $λ$ characterizes the matter--geometry coupling. Assuming a homogeneous and isotropic Friedmann--Lemaître--Robertson--Walker Universe, we derive analytical expressions for the Hubble parameter in two different scenarios: model A, corresponding to a pressureless viscous fluid ($ω=0$), and model B, where the equation of state (EoS) parameter is treated as a free parameter. The cosmological parameters are constrained through a Markov Chain Monte Carlo analysis. The results show that both viscous $f(R,T)$ models provide an excellent description of the current expansion history and remain compatible with the combined observational data. Model A is slightly favored over the standard cosmological model ($Λ$CDM) according to AICc, reflecting the improved fit achieved with only one additional free parameter. In contrast, the BIC continues to favor the simpler $Λ$CDM model owing to its stronger penalty on model complexity. Although model B yields the lowest minimum $χ^2$, the improvement is too small to justify the introduction of an additional free parameter, resulting in a statistical performance comparable to $Λ$CDM according to the AICc but less favorable according to the BIC.

physics.gen-ph↗

arXiv2026

Study of the cosmological tensions and DESI-DR2 in the framework of the Little Rip model

We present an analysis that investigates the $H_0$ and $S_8$ tensions by considering a dark energy model. The latter is a late-time model characterized by a future abrupt event known as the Little Rip (LR) model and characterised by one extra parameter, $β$, compared to the standard model, $Λ$CDM. To test this approach, we perform a statistical analysis by the MCMC method using the most recent observational data. We obtain a positive correlation in ($H_0$, $β$) plane. We also note that the Hubble tension is less than $3σ$ when using early measurements, i.e., Cosmic Microwave Background (CMB) data, and when combining it with Baryon Acoustic Oscillation (BAO) data, but it is no longer so when we combine early and late measurements (i.e. PantheonPlus (PP)). In addition, we test the model with DESI-DR2 combined with CMB and recent SNIa measurements. We notice that our model shifts toward the quintessence field. For a complete statistical analysis, we use the Akaike Information Criteria and Bayesian analysis of the evidence. According to Bayes factors, we find that the LR model provides an improved fit only to CMB data.

astro-ph.CO↗

arXiv2025

Emergence of running vacuum energy in $f(R,T)$ gravity : Observational constraints

In this work, we present a new analysis for $f(R,T)$ gravity by exploring the energy momentum tensor. We demonstrate that $f(R,T)$ gravity with the form $f(R,T)=R+2 κ^2 λT-2Λ$ is equivalent to Running Vacuum Energy (RVE), which interacts with the components of the cosmic fluid, namely dark matter and radiation. Interestingly, the form of such interaction is inferred from the non-conservation of the stress energy tensor in $f(R, T)$ gravity rather than being introduced in a phenomenological manner. Furthermore, the parameters that distinguish RVE from $Λ$CDM are fixed once the parameter of $f(R,T)$ gravity, $λ$, is known. To illustrate our setup, we perform a Markov Chain Monte Carlo analysis of three interaction scenarios using a combination of different data. we find that the parameters characterizing the RVE model are very small as expected. These results give an accuracy to this equivalence between $f(R,T)$ gravity under consideration and support the recent result obtained from a quantum field theory in curved space-time point of view which could open a new relationship between $f(R,T)$ gravity and quantum field theory. Finally, the interaction of the running vacuum increases the value of the current value of the Hubble rate by $3.5\%$ compared to the $Λ$CDM model, which may be a promising study for the Hubble tension.

gr-qc↗

arXiv2024

Constraints on Power Law and Exponential models in $f(Q)$ Gravity

In this paper, we observationally test the $ f(Q) $ gravity model at both background and perturbation levels using Pantheon$^+$, Hubble measurements, and Redshift Space Distortion Data. We obtain the best-fit parameters by solving numerically the modified Friedmann equations for two distinct cosmological models of $ f(Q) $ gravity namely the Power law and Exponential models. This involves performing a Markov Chain Monte Carlo analysis for these specific forms of $ f(Q) $. To evaluate the statistical significance of the $ f(Q) $ gravity models, we use the Bayesian and corrected Akaike Information Criteria. Our results indicate that the Exponential model in $ f(Q) $ gravity is statistically favored over both the Power-law model and the $ Λ$CDM model.

gr-qc↗

arXiv2024

Observational constraints on the growth index parameters in $f(Q)$ gravity

In this study, we analyse constraints on the growth index of matter perturbations, $γ$, within the framework of $f(Q)$ gravity, using recent cosmological observations, at the background and the perturbation levels, including Pantheon$^{+}$, Cosmic Chronometer (CC), and Redshift Space Distortion (RSD) datasets. Our analysis focuses on quantifying the distortion parameter, which measures the deviation of the $f(Q)$ gravity model from the concordance $Λ$CDM cosmology at the background level. Specifically, we investigate two cases of the growth index parameter: a constant $γ$ and a time-varying $γ(z)$. We investigate various parametrizations of the growth index $γ$, expressed as $γ= γ_{0} +γ_{1} y(z)$, where the function $y(z)$ assumes different forms, including constant ($Γ_{0}$), Taylor expansion around $z = 0$ ($Γ_{1}$), Taylor expansion around the scale factor ($Γ_{2}$), and an exponential form ($Γ_{3}$). By employing the Akaike Information Criterion and Bayesian Information Criterion, we find that the combined Pantheon$^{+}$+ CC+ RSD datasets impose stringent constraints on the value of the growth index. For the $Γ_{0}$ model, our results indicate that within the concordance $Λ$CDM model, $γ$ is constrained to $0.545 \pm 0.096$, showing strong agreement with the theoretical expectation of $γ_Λ = \frac{6}{11}$. However, within the framework of $f(Q)$ gravity, we observe $γ= 0.571^{+0.095}_{-0.110}$, slightly exceeding the $Λ$CDM value by 4.66 $\%$. Furthermore, when considering a time-varying growth index, our analysis reveals that the range of $γ_{0}$ spans from $0.596$ to $0.62$ across the $Γ_{1-3}$ models.

gr-qc↗

arXiv2024

Cosmological constraints on dynamical dark energy model in $F(Q)$ gravity

Extended teleparallel gravity, characterized by $F(Q)$ function where $Q$ is the non-metricity scalar, is one of the most promising approaches to general relativity. In this paper, we reexamine a specific dynamical dark energy model, which is indistinguishable from the $Λ$CDM model at present time and exhibits a special event in the future, within $F(Q)$ gravity. To constrain the free parameters of the model, we perform a Markov Chain Monte Carlo (MCMC) analysis, using the last data from Pantheon$^{+}$ and the latest measurements of the H(z) parameter combined. On the basis of this analysis, we have find that our dynamical dark energy model, in the context of F(Q) gravity, lies in the quintessence regime rather than in the phantom regime as in the case of general relativity. Furthermore, this behaviour affects the future expansion of the Universe as it becomes decelerating at $1σ$ confidence level for $z<-0.5$ and showing a bounce at $z_{\text{B}}\approx -0.835$. Finally, we have support our conclusion with a cosmographic analysis.

gr-qc↗

arXiv2024

Constraining dark energy equations of state in $F(R,T)$ gravity

In this paper, we examine the acceleration of the Universe's expansion in $F(R,T)$ gravity, where $R$ denotes the Ricci scalar and $T$ the trace of energy-momentum tensor. Indeed, the unknown nature of the source controlling this acceleration in general relativity leads scientists to investigate its properties by means of some alternative theories to general relativity. Our study is restricted to the particular case where $F(R,T)=R+2κ^2 λT$ , with $λ$ being a constant. We use a Bayesian analysis of current observational datasets, including the type Ia supernovae constitution compilation and $H(z)$ measurements, to constrain free parameters of the model. To parametrize dark energy, we consider two well known equations of state. We find the best fit values for each model by running a Markov chain Monte Carlo technic. The best fit parameters are used to compare both models to $Λ$CDM by means of the Akaike information criterion and the Bayesian information criterion. We show that the Universe underwent recently a transition from a deceleration to an acceleration for both models. Furthermore, the data shows a phantom nature of the equation of state for both models.

gr-qc↗

arXiv2023

Smoothing the $H_0$ tension with a dynamical dark energy model

The discrepancy between Planck data and direct measurements of the current expansion rate $H_0$ and the matter fluctuation amplitude $S_8$ has become one of the most intriguing puzzles in cosmology nowadays. The $H_0$ tension has reached $4.2σ$ in the context of standard cosmology i.e $Λ$CDM. Therefore, explanations to this issue are mandatory to unveil its secrets. Despite its success, $Λ$CDM is unable to give a satisfying explanation to the tension problem. Unless some systematic errors might be hidden in the observable measurements, physics beyond the standard model of cosmology must be advocated. In this perspective, we study a phantom dynamical dark energy model as an alternative to $Λ$CDM in order to explain the aforementioned issues. This phantom model is characterised by one extra parameter, $Ω_{pdde}$, compared to $Λ$CDM. We obtain a strong positive correlation between $H_0$ and $Ω_{pdde}$, for all data combinations. Using Planck measurements together with BAO and Pantheon, we find that the $H_0$ and the $S_8$ tensions are $3σ$ and $2.6σ$, respectively. By introducing a prior on the absolute magnitude, $M_B$, of the SN Ia, the $H_0$ tension decreases to $2.27σ$ with $H_0 = 69.76_{-0.82}^{+0.75}$ km s$^{-1}$ Mpc$^{-1}$ and the $S_8$ tension reaches the value $2.37σ$ with $S_8 =0.8269_{-0.012}^{+0.011}$.

astro-ph.CO↗