arXiv subjects

Nasr Ahmed

Publications and source records attributed to Nasr Ahmed.

At least 19 recordsLinked to original sources

arXiv2026

Non-equilibrium thermodynamics of an expanding Swiss-Cheese braneworld cosmology with a matter bounce

We investigate the non-equilibrium thermodynamics of a Swiss-cheese (SC) braneworld universe. By employing Hayward's unified first law and the Clausius relation at the apparent horizon, we first show that the standard equilibrium treatment of the effective SC braneworld fluid, together with the usual Hawking temperature, reproduces the Bekenstein-Hawking area law without any braneworld correction to the entropy. This demonstrates that the brane quadratic energy density corrections cannot, by themselves, generate a modified entropy area relation within the equilibrium framework. We therefore formulate a non-equilibrium thermodynamic description in which deviations from the Bekenstein-Hawking entropy are accompanied by an internal entropy production term. For a general horizon entropy functional, we derive the corresponding entropy production rate and then specialize to a logarithmic and inverse area quantum-corrected entropy. The resulting irreversible contribution is expressed in terms of the Hubble parameter and its derivative. We apply the formalism to a nonsingular matter-bounce solution in the SC braneworld. During the expanding phase, the total entropy production remains positive satisfying the GSLT. Moreover, the late-time behavior of the second derivative of the total entropy becomes negative, indicating a decelerating entropy growth and an asymptotic approach toward thermodynamic equilibrium. Far from the bounce, the equation of state parameter $w \approx -1$, indicating a dark energy-dominated regime. Significant departures from $w = -1$ are confined to the bounce vicinity, where high energy braneworld corrections drive the nonsingular transition. Thus, the model naturally transitions from a high energy bounce into an asymptotic dark energy phase during late-time expansion.

gr-qc↗

arXiv2026

Quintessence-dominated cyclic universe with negative cosmological constant

We investigate two simplified non-singular cyclic models with a negative time-varying cosmological constant to represent the non-conventional mechanism of negative cosmological constant expected to address the late-time cosmic acceleration. We show that a physically acceptable evolution with positive energy density can be realized, while negative energy density dominates in case of a positive or zero cosmological constant. In the first model, we demonstrate a sign flipping of the cosmic pressure in a quintessence-dominated universe with no violation of the null energy condition. In the second model, we propose a matter-bounce scenario with showing the crossing of the phantom divide line in the vicinity of the bounce. We find that while we get positive kinetic term and scalar potential, the sum of scalar and quantum potentials is negative.

gr-qc↗

arXiv2025

A novel cosmic framework of interdependent dark matter and Holographic Dark Energy within the Bianchi type-V universe

We study the anisotropic and homogeneous Bianchi type-V Universe with holographic dark energy (HDE) and interacting dark matter (DM). The solution for the field equations have been obtained for a certain form of the deceleration parameter. As for $Λ$CDM, we show that the coincidence problem disappears for a specific choice of the dark matter-holographic dark energy interaction. In this study, the observational data combination of OHD and JLA (Yu et al., Astrophys. J. 856 (2018) 3), where $q_0$ = $-0.52$ and $H_0$ = $69.2$, have been taken into consideration. We also found that the anisotropy of the expansion achieves isotropy after some finite time. We have also explained the physical and geometrical aspects of the model. The physical acceptability and stability of the model have been examined.

gr-qc↗

arXiv2025

An oscillating Rastall universe crossing the phantom divide line

A cyclic flat universe with quintom behaviour and future big rip has been presented in the framework of Rastall gravity, which is an extension of the standard $Λ$CDM model. The Hubble parameter oscillates periodically between positive and negative values from one cycle to the next. Cosmic transit has been simulated through an oscillating time-dependent deceleration parameter, and is expected to occur at approximately $ 8.7~~ \text{Gyr}$. The causality is satisfied all the time except near the initial singularity and the future Big Rip singularity.The apparent horizon, entropy and other thermodynamical quantities associated to the current model have been analyzed. Energy conditions have been investigated.

gr-qc↗

arXiv2024

Logamediate inflation on the Swiss-cheese brane with varying cosmological constant

The existence of Schwarzchild black holes in the structure of Swiss-cheese brane-world led to the conclusion that this specific brane-world scenario is more realistic than the FLRW branes. In this paper, we show that a Logamediate inflation on the Swiss-cheese brane with time-dependent cosmological constant $Λ(H)$ leads to a positive kinetic term and a negative potential with AdS minimum. The cosmic pressure $p$ is always positive but the energy density $ρ$ starts to get negative after a finite time. However, there is a time interval where they both are positive. Although this behavior of $ρ$ can be considered as a drawback of Swiss-cheese brane where positive energy dominates the present universe, it has been suggested that the presence of some source of negative energy could have played a significant role in early cosmic expansion. The model suffers from the eternal inflation problem which appears from the evolution of the first slow-roll parameter $ε$. Due to the existence of $ρ^2$ term, we have tested the new nonlinear energy conditions. The slow-roll parameters have been investigated and compared to Planck 15 results.

gr-qc↗

arXiv2024

Investigating the hyperbolic and hybrid scalar field cosmologies with varying cosmological constant in $f(R,T)$ gravity

This paper investigated two scalar field cosmological models in $f(R,T)$ gravity with cosmic transit and varying cosmological constant $Λ(t)$.The cosmological constant tends to have a tiny positive value in the current epoch.The scalar field pressure $p_ϕ$ shows a sign reversal for a normal scalar field. For the phantom field, the scalar potential $V(ϕ)$ is negative and the energy density $ρ_ϕ=E_k+V$ takes negative values when the equation of state parameter $ω_ϕ$ is less than $-1$. While the weak energy condition WEC implies that the total energy density $ρ=\sum_iρ_i\geq 0$, we still can have a negative $ρ$ term as long as the total $ρ$ is positive. In the current work we argue that the WEC, $ρ=\sum_i ρ_i \geq 0$ and $p_i+ρ_i \geq 0$, is not violated but with an instability for the second model at late-times. For a scalar field $ϕ$, The condition $ρ_ϕ+p_ϕ=ρ_ϕ (1+ω_ϕ)=2E_k\geq 0$ allows for $ρ_ϕ<0$ if $ω_ϕ<-1$. The causality and energy conditions have been discussed for both models. The cosmology in both models was studied using a given function $a(t)$ derived from the desired cosmic behavior, which is the opposite of the traditional view.

gr-qc↗

arXiv2023

Big rip in Swiss-cheese Brane-worlds with cosmic transit

We study the big rip scienario in Swiss-cheese Brane-worlds. The results obtained have been found to be independent of the value of the cosmological constant $Λ$ whether its positive, negative or zero. Negative tension branes are not allowed in the current model. There is a sign flipping in cosmic pressure corresponding to the sign flipping in the deceleration parameter from positive to negative. The evolution of the EOS parameter shows the presence of three phases: the matter dominant decelerating era,The accelerated-Quitessence phase, and the phantom phase. The evolution of the potential and Kinetic term also shows a change of sign. The energy conditions and cosmographic parameters have also been investigated.

gr-qc↗

arXiv2022

Some cosmological features of 4D Gauss-Bonnet gravity with varying cosmological constant

We explore some cosmological features of the newly suggested 4D Gauss-Bonnet gravity through two different models assuming a varying cosmological constant. Observational constraints, such as the cosmic transit and the flat curvature, have been considered in constructing the models. The cosmology in the current work has been probed using a given scale factor derived from the desired cosmic behavior which is the inverse of the usual viewpoint. The stability and cosmography have been studied for the two models.

gr-qc↗

arXiv2022

Cosmographic analysis of a closed bouncing universe with the varying cosmological constant in $f(R,T)$ gravity

Modeling of matter bounce in $f(R,T)$ gravity has been presented with no violation of the null energy condition. Only a closed universe with negative pressure is allowed in good agreement with some recent observations which favor a universe with positive curvature. Our results agree with some recent works in which a combination of positive curvature and vacuum energy leads to non-singular bounces with no violation of the null energy condition. The stability of the model has been discussed. The cosmographic parameters are developed for the derived model to explain the accelerated expansion of the universe.

gr-qc↗

arXiv2022

A non-singular closed bouncing universe without violation of null energy condition

A matter bouncing entropy-corrected cosmological model has been suggested. The model allows only positive curvature with negative pressure and no violation of the null energy condition. The result obtained in this paper is supported by some recent theoretical works where the combination of positive spatial curvature and vacuum energy leads to non-singular bounces with no violation of the null energy condition. An important feature of the current model is that evolutions of the cosmic pressure, energy density and equation of state parameter are independent of the values of the prefactors $α$ and $β$ in the corrected entropy-area relation. The validity of the classical and the new nonlinear energy conditions has been discussed. The cosmographic parameters have been analyzed

gr-qc↗

arXiv2021

A new topological perspective of expanding space-times with applications to cosmology

We discuss the possible role of the Tietze extension theorem in providing a rigorous topological base to the expanding space-time in cosmology. A simple toy model has been introduced to show the analogy between the topological extension from a circle $S$ to the whole space $M$ and the cosmic expansion from a non-zero volume to the whole space-time in non-singular cosmological models. A topological analogy to the cosmic scale factor function has been suggested, the paper refers to the possible applications of the topological extension in mathematical physics.

physics.gen-ph↗

arXiv2021

Note on Dark Energy and Cosmic Transit in a scale-invariance cosmology

In general, the laws of physics are not invariant under a change of scale. To find out whether the 'scale-invariance hypothesis' corresponds to nature or not, a careful examination to its implications is required. As a consequence, the scale-invariance cosmological models need to be carefully checked with many tests in order to confirm or disconfirm them. In this paper, three different toy models have been introduced in the framework of a scale-invariance cosmology to examine dark energy and cosmic transit. Although cosmic transit exists in the three models, the pressure stays always negative during cosmic evolution. In addition, there is always a singularity in the evolution of the equation of state parameter which is not suitable for a complete investigation of dark energy evolution. The undesirable features of the parameters have been discussed, and a comparison with other cosmological contexts has been done.

gr-qc↗

arXiv2020

The possibility of a stable flat dark energy-dominated Swiss-cheese Brane-world universe with a deceleration-acceleration transition

In this paper, we study the possibility of obtaining a stable flat dark energy-dominated universe in a good agreement with observations in the framework of Swiss-cheese Brane-world cosmology. Two different Brane-world cosmologies with black strings have been introduced for any cosmological constant $Λ$ using two empirical forms of the scale factor. In both models, we have performed a fine-tuning between the brane tension and the cosmological constant so that the EoS parameter $ω(t)\rightarrow -1$ for the current epoch where the redshift $z\simeq 0$. We then used these fine-tuned values to calculate and plot all parameters and energy conditions. The deceleration-acceleration cosmic transition is allowed in both models, and the jerk parameter $j\rightarrow 1$ at late-times. Both solutions predict a future dark energy-dominated universe in which $ω=-1$ with no crossing to the phantom divide line. While the pressure in the first solution is always negative, the second solution predicts a better behavior of cosmic pressure where the pressure is negative only in the late-time accelerating era but positive in the early-time decelerating era. Since black strings have been proved to be unstable by some authors, this instability can actually reflect doubts on the stability of cosmological models with black strings (Swiss-cheese type brane-worlds cosmological models). For this reason, we have carefully investigated the stability through energy conditions and sound speed. Because of the presence of quadratic energy terms in Swiss-cheese type brane-world cosmology, we have tested the new nonlinear energy conditions in addition to the classical energy conditions. We have also found that constructing non-singular and cyclic solutions through certain ansatze in Swiss-cheese Brane-worlds are not possible.

gr-qc↗

arXiv2020

Probing $κ(R,T)$ cosmology via empirical approach

In this paper, a stable flat cosmological model has been constructed and the evolution of dark energy has been investigated in the framework of the recently suggested $κ(R,T)$ gravity. The empirical approach we adopt in the current work reveals some interesting cosmological features consistent with observations and the standard $Λ$CDM model. The evolution of cosmic pressure shows a positive-to-negative transition corresponding to the cosmic deceleration-acceleration transition, both the deceleration parameter and the cosmic pressure have the positive-to-negative sign flipping. While this behavior can provide explanation for the deceleration-acceleration transition, the reason behind this positive-negative transition itself is still missing.

gr-qc↗

arXiv2019

Crossing the phantom divide line in universal extra dimensions

We investigate the cosmic acceleration and the evolution of dark energy across the cosmological constant boundary in universal extra dimensions UED. We adopt an empirical approach to solve the higher-dimensional cosmological equations so that the deceleration parameter $q$ is consistent with observations. The expressions for the jerk and deceleration parameters are independent of the number of dimensions $n$. The behavior of pressure in $4$D shows a positive-to-negative transition corresponding to the deceleration-to-acceleration cosmic transition. This pressure behavior helps in providing an explanation to the cosmic deceleration-acceleration transition although the reason behind the transition itself remains unknown. In the conventional $4$D cosmology, there is a no-go theorem prevents the EoS parameter of a single perfect fluid in FRW geometry to cross the $ω=-1$ boundary. The current model includes a single homogenous but anisotropic perfect fluid in a homogenous FRW metric with two different scale factors in the ordinary $4$D and the UED. In contrast to the conventional $4$D cosmology, we have found that the dark energy evolution in UED shows $ω=-1$ crossing. however, the no-go theorem is still respected in $4$D where the EoS parameter doesn't cross the $ω=-1$ boundary.

gr-qc↗

arXiv2019

Ricci-Gauss-Bonnet holographic dark energy in Chern-Simons modified gravity: A flat FLRW quintessence-dominated universe

We discuss the recently suggested Ricci-Gauss-Bonnet holographic dark energy in Chern-Simons modified gravity. We have tested some general forms of the scale factor $a(t)$, and used two physically reasonable forms which have been proved to be consistent with observations. Both solutions predict a sign flipping in the evolution of cosmic pressure which is positive during the early-time deceleration and negative during the late-time acceleration. This sign flipping in the evolution of cosmic pressure helps in explaining the cosmic deceleration-acceleration transition, and it has appeared in other cosmological models in different contexts. However, the current work shows a pressure singularity which needs to be explained. The evolution of the equation of state parameter $ω(t)$ shows the same asymptotic behavior for both solutions indicating a quintessence-dominated universe in the far future. We also note that $ω(t)$ goes to negative values (leaving the decelerating dust-dominated era at $ω=0$) at exactly the same time the pressure becomes negative. Again, there is another singularity in the behavior of $ω(t)$ which happens at the same cosmic time of the pressure singularity.

gr-qc↗

arXiv2019

A stable flat entropy-corrected FRW universe

In this paper, a general entropy-corrected FRW cosmological model has been presented in which a deceleration-to-acceleration transition occurs according to recent observations. We found that the case for the flat universe ($k=0$), supported by observations, is the most stable one where it successfully passes all stability tests. The stability of the model has been studied through testing the sound speed, the classical and the new nonlinear energy conditions. The model predicts a positive pressure during the early-time decelerating epoch, and a negative pressure during the late-time accelerating epoch in a good agreement with cosmic history and dark energy assumption. We have investigated all possible values of the prefactors $α$ and $β$ in the corrected entropy-area relation to find the best values required for a stable flat universe. We have also made use of the evolution of the equation of state parameters $ω(t)$ in predicting the correct values of $α$ and $β$. The jerk and density parameters have been calculated where a good agreement with observations and $Λ$CDM model has been obtained. Two dark energy proposals have been investigated in this model, the entropy-corrected holographic dark energy and the modified holographic Ricci dark energy.

gr-qc↗

arXiv2019

Cosmological determination to the values of the prefactors in the logarithmic corrected entropy-area relation

In this paper, we continue investigating the possible values of the pre-factors $α$ and $β$ in the logarithmic corrected entropy-area relation based on cosmological stability arguments. In a previous study, we have investigated the stability of the entropy-corrected cosmology using an empirical hyperbolic form of the scale factor. We found that the zero values of the two pre-factors are necessary to obtain a stable flat universe with a deceleration-acceleration transition and no causality violation. The necessity of the zero values of the two pre-factors has also been reached in the current work using a hybrid scale factor Ansatz in the entropy-corrected cosmological equations. Investigating the corrected entropy-area relation in different gravitational and cosmological contexts can provide an accurate estimation to the correct values of the pre-factors. The current work opens a discussion on the validity of the correction terms in the logarithmic corrected entropy-area relation on the cosmological scale. The evolution of the cosmic pressure, energy density, equation of state parameter, jerk parameter and the nonlinear energy conditions has been analyzed.

gr-qc↗