arXiv subjects

M. Monshizadeh

Publications and source records attributed to M. Monshizadeh.

5 recordsLinked to original sources

arXiv2025

Cosmological Tensions with Non-Extensive Entropic Cosmology: A Modified Stress-Energy Approach

We perform a comprehensive cosmographic analysis of Friedmann cosmologies modified by non-extensive entropy frameworks, focusing on Tsallis, Rényi, and Kaniadakis entropies within a novel modified energy-momentum tensor approach. In our approach the microscopic matter density remains $ρ=m~n$ while the horizon thermodynamics of non-extensive entropy modifies the effective source that drives expansion, $ρ_{eff}=f(ρ)ρ$ which reduces to the standard case for $f(ρ)=1$. By deriving the generalized Friedmann equations for each entropy type, we calculate analytical expressions for key cosmographic parameters, including the deceleration ($q_0$), jerk ($j_0$), snap ($s_0$), and lerk ($l_0$) parameters, and examine their behavior compared to the standard $Λ$CDM model. Our results reveal significant differences between the traditional formal approach and the modified energy-momentum approach, particularly in the Tsallis model where cosmographic parameters and the dimensionless Hubble function $E(z)$ show notable deviations for deformation parameters away from unity. Moreover, both Rényi and Kaniadakis models exhibit increasing tension with $Λ$CDM at higher redshifts, while remaining similar at low redshifts. Importantly, given the persistent $\sim5σ$ Hubble tension between local and global measurements, our analysis indicates that the flexibility of the modified entropic cosmology framework to alter the expansion rate could potentially alleviate this discrepancy by modifying the effective expansion history in a way compatible with observations. Future work will involve full Bayesian analyses with Pantheon+, DESI, and Planck data to further assess the viability of these models in resolving current cosmological tensions.

gr-qc↗

arXiv2023

Exploring Modifications to FLRW Cosmology with General Entropy and Thermodynamics: A new Approach

The investigation of modifications to the FLRW cosmology resulting from the consideration of a general entropy for the cosmological apparent horizon is the subject of this study. Building upon the work of Nojiri and collaborators in 2022, who introduced a class of generalized entropies with four parameters capable of converging to familiar entropies and addressing specific cosmological issues, our research explores the impact of correcting the entropy on the energy-momentum tensor of the cosmic fluid from the outset. Our calculations demonstrate that, by employing a correction function $f(ρ)$ to modify the energy-momentum density tensor, the entropic area law (Bekenstein-Hawking entropy) can still be regarded as a general entropy. The construction of the function $f(ρ)$ is facilitated through considerations of the thermodynamics associated with the apparent horizon. Additionally, we investigate the first and second laws of thermodynamics within this framework and illustrate how the limitations imposed on the equation of state of the cosmic fluid can be resolved through the incorporation of this correction function. Finally, we compute cosmography parameters to analyze the kinematics of the universe, with particular attention given to the notable influence of the correction function $f(ρ)$ on these parameters. This paper provides valuable insights into the application of general entropies to the apparent horizon of the universe.

gr-qc↗

arXiv2013

Statefinder diagnostic of logarithmic entropy corrected holographic dark energy with Granda-Oliveros IR cut-off

In this work, we have studied the logarithmic entropy corrected holographic dark energy (LECHDE) model with Granda-Oliveros (G-O) IR cutoff. The evolution of dark energy (DE) density $Ω'_D$, the deceleration parameter, $q$, and equation of state parameter (EoS), $ω_Λ$, are calculated. We show that the phantom divide may be crossed by choosing proper model parameters, even in absence of any interaction between dark energy and dark matter. By studying the statefinder diagnostic and $ω_Λ-ω_Λ^{\prime}$ analysis, the pair parameters $\{r,s\}$ and $(ω_Λ-ω_Λ^{\prime})$ is calculated for flat GO-LECHDE universe. At present time, the pair $\{r,s\}$ can mimic the $Λ$CDM scenario for a value of $α/β\simeq 0.87$, which is lower than the corresponding one for observational data ($α/β=1.76$) and for Ricci scale ($α/β=2$). We find that at present, by taking the various values of ($α/β$), the different points in $r-s$ and $(ω_Λ-ω_Λ^{\prime})$ plans are given. Moreover, in the limiting case for a flat dark dominated universe at infinity ($t\rightarrow \infty$), we calculate $\{r,s\}$ at G-O scale. For Ricci scale ($α= 2$, $β= 1$) we obtain $\{r=0,s=2/3\}$.

gr-qc↗

arXiv2012

Reconstruction of modified gravity with ghost dark energy models

In this work, we reconstruct the $f(R)$ modified gravity for different ghost and generalized ghost dark energy models in FRW flat universe, which describe the accelerated expansion of the universe. The equation of state of reconstructed $f(R)$ - gravity has been calculated. We show that the corresponding $f(R)$ gravity of ghost dark energy model can behave like phantom or quintessence. We also show that the equation of state of reconstructed $f(R)$ gravity for generalized ghost model can transit from quintessence regime to the phantom regime as indicated by recent observations.

hep-th↗

arXiv2009

Thermodynamics of Taub-NUT/Bolt-AdS Black Holes in Einstein-Gauss-Bonnet Gravity

We give a review of the existence of Taub-NUT/bolt solutions in Einstein Gauss-Bonnet gravity with the parameter $α$ in six dimensions. Although the spacetime with base space $S^{2}\times S^{2}$ has curvature singularity at $r=N$, which does not admit NUT solutions, we may proceed with the same computations as in the $\mathbb{CP}^{2}$ case. The investigation of thermodynamics of NUT/Bolt solutions in six dimensions is carried out. We compute the finite action, mass, entropy, and temperature of the black hole. Then the validity of the first law of thermodynamics is demonstrated. It is shown that in NUT solutions all thermodynamic quantities for both base spaces are related to each other by substituting $α^{\mathbb{CP}^{k}}=[(k+1)/k]α^{S^{2} \times S^{2}\times >...S_{k}^{2}}$. So no further information is given by investigating NUT solution in the $S^{2}\times S^{2}$ case. This relation is not true for bolt solutions. A generalization of the thermodynamics of black holes to arbitrary even dimensions is made using a new method based on the Gibbs-Duhem relation and Gibbs free energy for NUT solutions. According to this method, the finite action in Einstein Gauss-Bonnet is obtained by considering the generalized finite action in Einstein gravity with an additional term as a function of $α$. Stability analysis is done by investigating the heat capacity and entropy in the allowed range of $α$, $Λ$ and $N$. For NUT solutions in $d$ dimensions, there exist a stable phase at a narrow range of $α$. In six-dimensional Bolt solutions, metric is completely stable for $\mathcal{B}=S^{2}\times S^{2}$, and is completely unstable for $\mathcal{B}=\mathbb{CP}^{2}$ case.

hep-th↗