arXiv subjects

Alnadhief H. A. Alfedeel

Publications and source records attributed to Alnadhief H. A. Alfedeel.

At least 19 recordsLinked to original sources

arXiv2026

Analytic semiclassical backreaction of a Schwarzschild black hole in a finite cavity: horizon shift, temperature renormalization, and canonical stability in the Hartle-Hawking State

We construct an analytic model of static semiclassical backreaction for a Schwarzschild black hole in the Hartle--Hawking state enclosed within a finite spherical cavity. Using a minimal renormalized stress--energy tensor consistent with conservation, thermal asymptotics, and horizon regularity, we integrate the reduced semiclassical Einstein equations under Dirichlet boundary conditions at the cavity wall. This yields explicit expressions for the corrections to the mass function, redshift factor, horizon location, and surface gravity. We obtain a closed-form first-order correction to the Hawking temperature in terms of a dimensionless backreaction parameter and the cavity radius. The temperature shift decomposes into redshift renormalization, geometric horizon displacement, and a local energy-density contribution at the horizon. The perturbative expansion is controlled by a parameter of order $M_P^2/M^2$, ensuring validity for macroscopic black holes. The near-horizon geometry retains its universal Rindler$^{2}\times S^{2}$ structure, indicating that semiclassical effects renormalize rather than modify the geometric origin of Hawking radiation.

gr-qc↗

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 $f(T,L_m) = αT + βL_m$ and assume a constant bulk viscosity coefficient $ζ= ζ_0 > 0$. 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, $H_0 = 68.16 \pm 0.65$, $α= 1.53^{+0.49}_{-0.61}$, $β= 0.40 \pm 0.96$, and $ζ_0 = 2.15^{+0.69}_{-0.81}$, 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 $Λ$CDM toward a dynamical dark energy component. Although information criteria ($Δ\mathrm{AIC} = 2.2$, $Δ\mathrm{BIC} = 13.13$) slightly favor the simpler $Λ$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

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 $f(Q, L_m) = -Q + αL_m^n + β$, where $α$, $β$, 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

Quintessence phase of the late-time Universe in $f(Q,T)$ gravity

In this paper, we have studied the late-time accelerating expansion of the Universe using the matter-geometry coupled $ f(Q, T) $ gravity model, where $ Q $ is the non-metricity scalar and $ T $ represents the trace of the energy-momentum tensor. We constrain the best-fit values of cosmological parameters $Ω_{m0}, H_0, α_0~\mbox{and}~ β_0$ through the Monte Carlo Markov Chain (MCMC) simulation {using 31 Hubble parameter data points from cosmic chronometers (CC) and 26 data points from baryon acoustic oscillations (BAO), making a total of 57 datasets (labeled \texttt{CC+BAO}), as well as SNIa distance moduli measurements from the Pantheon+ sample, which consists of 1701 light curves of 1550 distinct supernovae (labeled \texttt{Pantheon +SHOES}), and their combination (labeled \texttt{CC+BAO+Pantheon +SHOES)}}. {We compare our constrained Hubble constant $H_0$ value with different late-time and early-time cosmological measurements.} Deceleration {parameter} $q(z)$, effective equation of state parameters $w_{eff}(z)$, Hubble parameter $H(z)$, and distance modulus $μ(z)$ are numerical results of dynamical quantities that show that the $f(Q, T)$ gravity model is compatible with a transition towards a quintessence-like phase in the late-time. In conformity with $Λ$CDM, we moreover take into account the geometrical interpretations by considering the state-finder parameters $r-s$ and $r-q$, which are crucial parameters for additional analysis. Additionally, the statistical analysis has been carried out for further investigation.

gr-qc↗

arXiv2024

The cosmology of $f(R, L_m)$ gravity: constraining the background and perturbed dynamics

This paper delves into the late-time accelerated expansion of the universe and the evolution of cosmic structures within the context of a specific $ f(R, L_m) $ gravity model, formulated as $ f(R, L_m) = λR + βL_m^α+ η$. To study the cosmological viability of the model, we employed the latest cosmic measurement datasets: i) 57 observational Hubble parameter data points (\texttt{OHD}); ii) 1048 distance moduli data points (\texttt{SNIa}); iii) a combined dataset (\texttt{OHD+SNIa}); and large scale structure datasets, including iv) 14 growth rate data points (\texttt{f}); and v) 30 redshift space distortion data points (\texttt{f}$σ_8$). These datasets facilitated the constraint of the $ f(R, L_m) $-gravity model via MCMC simulations, followed by a comparative analysis with the $Λ$CDM model. A comprehensive statistical analysis has been conducted to evaluate the $ f(R, L_m) $-gravity model's efficacy in explaining both the accelerated expansion of the universe and the growth of cosmic structures.

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↗

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

Perturbations in Bianchi Type-I spacetimes with varying Lambda, G

In this paper, we investigate the evolution of cosmological perturbations within the context of Bianchi Type$-I$ spacetimes. We consider models containing viscous fluids with evolving cosmological (\LL) and Newtonian gravitational (G) parameters. The investigation of how over-densities in the viscous matter content in the Bianchi Type$-I$ model is our primary emphasis. In particular, we investigate the generation and propagation of signals associated with large-scale structures in this setting. We contrast our findings with the predictions of the classic $Λ$CDM ($Λ$-Cold-Dark-Matter) cosmological model to draw relevant contrasts and insights. Our findings emphasise the need to incorporate viscous fluids into the Bianchi Type$-I$ geometry, as well as the dynamic fluctuations of \LL and $G$. These factors influence the rate of structure growth in the cosmos as a whole. Thus, our findings offer light on the complex dynamic interaction between viscosity, changing cosmological parameters, and the growth of large-scale structures in an anisotropic universe.

gr-qc↗

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↗

arXiv2023

Modeling cosmic acceleration with a generalized varying deceleration parameter

Understanding the accelerating expansion of the Universe remains a fundamental challenge in modern cosmology. In this paper, we investigate a cosmological model parametrized by a generalized variable deceleration parameter to elucidate the dynamics driving cosmic acceleration. By employing constraints from the latest observational datasets, including Cosmic Chronometers (CC), Type Ia Supernovae (SNe), and Baryon Acoustic Oscillations (BAO), we assess the compatibility of the model with observational data. The chosen parametrization aligns with thermodynamic constraints on the deceleration parameter, further validating its reliability. Further, we estimate the present value of the Hubble parameter, transition redshift, deceleration parameter, and EoS parameter, which align with observational data. Lastly, our stability analysis confirms the model's stability against small perturbations.

gr-qc↗

arXiv2023

Constrained evolution of effective equation of state parameter in non-linear $f(R, L_m)$ dark energy model: Insights from Bayesian analysis of cosmic chronometers and Pantheon samples

We conduct a Bayesian analysis of recent observational datasets, specifically the Cosmic Chronometers (CC) dataset and Pantheon samples, to investigate the evolution of the EoS parameter in dark energy models. Our study focused on the effective EoS parameter, which is described by the parametric form $ω_{eff}=-\frac{1}{1+m(1+z)^n}$, where $m$ and $n$ are model parameters. This parametric form is applicable within the framework of $f(R,L_m)$ gravity, where $R$ represents the Ricci scalar and $L_m$ is the matter Lagrangian. Here, we examine a non-linear $f(R,L_m)$ model characterized by the functional form $f(R,L_m)=\frac{R}{2}+L_m^α$, where $α$ is the free parameter of the model. We examine the evolution of several cosmological parameters, including the effective EoS parameter $ω_{eff}$, the deceleration parameter $q$, the density parameter $ρ$, the pressure $p$, and the statefinder parameters. Our analysis revealed that the constrained current value of the effective EoS parameter, $ω_{eff}^{0}=-0.68\pm0.06$ for both the CC and Pantheon datasets, points towards a quintessence phase. Moreover, at redshift $z=0$, the deceleration parameter, $q_0 = -0.61^{+0.01}_{-0.01}$, indicates that the present Universe is undergoing accelerated expansion.

astro-ph.CO↗

arXiv2023

Impact of dark energy on the equation of state in light of the latest cosmological data

We reconstruct the effective Equation of State (EoS) within the framework of General Relativity (GR) theory in a homogeneous and isotropic FLRW Universe, which is assumed to be composed of matter and Dark Energy (DE). Our analysis employs a dataset consisting of 31 Cosmic Chronometer (CC) data points, six data points of Baryon Acoustic Oscillations (BAO), and 1048 Type Ia Supernovae (SN) from the Pantheon sample, and we determine the best-fit values of the model parameters through Markov Chain Monte Carlo (MCMC) simulation. We then use these parameter values to calculate various cosmological parameters, such as the DE EoS parameter, the energy density, the deceleration parameter, the state finder parameters, and the $Om(z)$ diagnostic. All the analyzed cosmological parameters show behavior consistent with the accelerated Universe scenario.

gr-qc↗

arXiv2023

Constraining the $f(R,T) = R + 2λT$ cosmological model using recent observational data

In this paper, we consider a comprehensive investigation of the cosmological model described by $f(R,T) = R + 2λT$ (where $λ$ represents a free parameter) in light of the most recent observational data. By constraining the model using $Hubble$ and $Pantheon$ datasets, we determine its compatibility with the observed behavior of the Universe. For this purpose, we adopt a parametric form for the effective equation of state (EoS) parameter. This parametric form allows us to describe the evolution of the EoS parameter with respect to redshift and investigate its behavior during different cosmic epochs. The analysis of the deceleration parameter reveals an accelerating Universe with a present value of $q_0=-0.64^{+0.03}_{-0.03}$, indicating the current phase of accelerated expansion. The transition redshift is found to be $z_{tr}=0.53^{+0.04}_{-0.03}$, marking the epoch of transition from deceleration to acceleration. We also analyze the evolution of important cosmological parameters including density parameter, pressure, effective EoS, and stability. These findings collectively demonstrate the viability of the $f(R,T)$ cosmological model as a robust candidate capable of engendering the requisite negative pressure, thereby efficiently propelling cosmic expansion. Moreover, the undertaken stability analysis underscores the model's stability within the broader cosmic landscape. By providing the best-fit values for the coupling parameter $λ$, this approach motivates and encourages further explorations into the extensive landscape of this model and its potential applications across diverse realms of cosmology and astronomy.

astro-ph.CO↗

arXiv2022

A cosmological model with time dependent $Λ$, $G$ and viscous fluid in General Relativity

In this paper, we investigate Bianchi type$-V$ cosmological models with bulk viscous fluid and time varying cosmological $Λ$ and Newtonian $G$ parameters. The Einstein's field equations have been transformed into a coupling non-linear, first-order differential equations, and the fourth-order Runge-Kutta method of numerical integration has been used to integrate the differential equations with appropriate initial conditions consistent with current cosmological observations. We show that the model describes a universe that starts off with a negative cosmological term, as well as a matter-dominated and decelerated early epoch that, eventually becomes $Λ$-dominated and expanding with acceleration, in concordance with current observations.

physics.gen-ph↗

arXiv2018

A Generalized Solution of Bianchi Type-V Models with Time-dependent $G$ and $Λ$

We study the homogeneous but anisotropic Bianchi type-V cosmological model with time-dependent gravitational and cosmological "constants". Exact solutions of the Einstein field equations (EFEs) are presented in terms of adjustable parameters of quantum field theory in a spatially curved and expanding background. It has been found that the general solution of the average scale factor R as a function of time involved the hypergeometric function. Two cosmological models are obtained from the general solution of the hypergeometric function and the Emden-Fowler equation. The analysis of the models shows that for a particular choice of parameters in our first model, the cosmological "constant" decreases whereas the Newtonian gravitational "constant" increases with time, and for another choice of parameters, the opposite behaviour is observed. The models become isotropic at late times for all parameter choices of the first model. In the second model of the general solution, both the cosmological and gravitational "constants" decrease while the model becomes more anisotropic over time. The exact dynamical and kinematical quantities have been calculated analytically for each model.

gr-qc↗