arXiv subjects

R. Jalalzadeh

Publications and source records attributed to R. Jalalzadeh.

7 recordsLinked to original sources

arXiv2026

$q$-Deformed Quantum Mechanics and the Thermodynamics of Black Hole/White Hole Spectral pair

In this work, we investigate the thermodynamics of Schwarzschild black and white holes within a $q$-deformed Wheeler--DeWitt framework. By introducing a $q$-deformed Heisenberg--Weyl algebra at a root of unity, we derive a finite-dimensional Hilbert space, a bounded mass spectrum, and an adiabatic invariant leading to a bounded entropy-mass relation. The deformation results in a universal logarithmic correction, as well as a minimum temperature and a maximum entropy that matches the de Sitter bound. Also, we examine the interpretation of a cold remnant, which is dynamically stable because its radiation rate approaches zero, even though its heat capacity remains negative. We also explore the holographic implications of this limited entropy. Our results thus provide a consistent semiclassical picture, where quantum deformation naturally introduces an entropy bound, avoids divergences at the final evaporation stage, and suggests a smooth transition from quantum gravity to cosmology.

gr-qc↗

arXiv2026

Constraining fractionality using some observational tests

Recently, a fractional version of the Schwarzschild-Tangherlini black hole with a fractal horizon has been introduced. Motivated by the key role of the Schwarzschild solution in gravitational and astrophysical studies, some consequences of this fractional-fractal generalization of the Schwarzschild black hole have been investigated. In this line, the corresponding i) Shapiro and Sagnac time delays, ii) shadow, iii) orbital precession, and iv) gravitational lensing are studied and confronted with observational data. MCMC analysis also unveils i) the potential of this metric in dealing with the solar-system tests and ii) the necessity of studying fractional spacetimes and objects.

gr-qc↗

arXiv2025

Observational constraints on New Tsallis holographic energy in Rastall theory

The cosmological implications of New Tsallis holographic dark energy (NTHDE) in Rastall theory have been studied. Using the data set that includes DESI BAO (DR2), PantheonPlus SNe Ia, H(z) measurements, and BBN and the MCMC analysis, the key cosmological and model-specific parameters are constrained. The result is compared with that of the ΛCDM model indicating that in addition to providing a viable dynamical dark energy framework, predictions for H(z) are slightly more consistent with intermediate-redshift observations. Generally, the model remains compatible with current data and offers testable deviations from ΛCDM for upcoming surveys. It is also seen that when the energy density of quantum fields in vacuum, exposed by NTHDE, is combined with the Rastall correction term to the general relativity, a plausible candidate for dynamical dark energy is obtained that mimic the current value of the dark energy density parameter reported in the ΛCDM model. The latter cannot be repeated by NTHDE alone. The study also confirms previous theoretical and observational constraints on the Rastall parameter obtained by focusing on the thermodynamics, early universe, pulsars, and the early-type galaxies.

astro-ph.CO↗

arXiv2025

Cosmological Singularities in Brane Gravity

We present a comprehensive study of cosmological singularities within the framework of Covariant Extrinsic Gravity (CEG), addressing both the initial Big Bang singularity and potential finite-time future singularities. Through detailed analysis of the emergent universe scenario, we systematically examine homogeneous and inhomogeneous perturbations (encompassing scalar, vector, and tensor modes) in a 4D FLRW brane geometry. Our work establishes rigorous existence criteria and stability conditions for a nonsingular Einstein static initial state, demonstrating that such a configuration remains stable for well-defined parameter ranges in CEG - thereby providing a compelling resolution to the long-standing initial singularity problem. Extending our analysis to late-time cosmology, we perform a complete classification of future singularity types following Barrow et al.'s formalism, deriving precise conditions that determine whether the universe in CEG evolves toward or avoids these singular states.

gr-qc↗

arXiv2024

Observational constraints on FLRW, Bianchi type I and V brane models

This study explores the compatibility of Covariant Extrinsic Gravity (CEG) with current cosmological observations. We employ the chi-square statistic and Markov Chain Monte Carlo (MCMC) methods to fit the FLRW and Bianchi type-I and V brane models to the latest datasets, including Hubble, Pantheon+ Supernova samples, Big Bang Nucleosynthesis (BBN), Baryon Acoustic Oscillations (BAO), and the structure growth rate, $fσ_8(z)$. Parameters for FLRW universe consist $\left(Ω^{\text{(b)}}_0, Ω^{\text{(cd)}}_0, Ω^{\text{(k)}}_0, H_0, γ, σ_8\right)$, while for the Bianchi model are $\left(Ω^{\text{(b)}}_0, Ω^{\text{(cd)}}_0, Ω^{(β)}_0, H_0, γ, Ω^{(θ)}_0, σ_8\right)$. We determine the best values for cosmological parameters. For the FLRW model, these values depend on the sign of $γ$: $γ> 0$ yields $γ=0.00008^{+0.00015}_{-0.00011}$, and $Ω^{\text{(k)}}_0=0.014^{+0.024}_{-0.022}$ and $γ< 0$ leads to $γ=-0.0226^{+0.0054}_{-0.0062}$, and $Ω^{\text{(k)}}_0=0.023^{+0.039}_{-0.041}$. In both cases $Ω^{\text{(k)}}_0>0$ represents a closed universe. Similarly, for the Bianchi type-V brane model, the parameter values vary with the sign of $γ$, resulting in $γ= 0.00084^{+0.00019}_{-0.00021}$, $Ω^{(β)}_0 =0.0258^{+0.0052}_{-0.0063} $, and $Ω^θ_0(\times 10^{-5} ) = 4.19^{+0.67}_{-0.75}$ (as with the density parameter of stiff matter) for $γ> 0$, and $γ= -0.00107^{+0.00019}_{-0.00020}$, $Ω^{(β)}_0 = 0.0259^{+0.0050}_{-0.0062} $, and $Ω^θ_0(\times 10^{-5} ) = 4.17^{+0.91}_{-0.98}$ for $γ< 0$. In both cases $Ω^{(β)}_0>0$, which represents the Bianchi type-V, because in the Bianchi type-I, $β=0$. Utilizing these obtained best values, we analyze the behavior of key cosmological parameters.

gr-qc↗

arXiv2024

Friedmann equations of the fractal apparent horizon

From a fractal perspective, the entropy bound of gravitational systems undergoes changes. Furthermore, in the cosmological setting, the conservation law of a perfect fluid is also altered in such systems, affecting spatial elements like volume, area, and radius. By applying the first law of thermodynamics and deriving the Friedmann equations, we can gain insight into the evolution of such a fractal cosmos. However, observations continue to necessitate the existence of a dark energy source. To address this, in this article, we have created a novel fractal $Λ$CDM cosmological model and determined the fractal cosmological observables. We show that the spatial fractal dimension is two, and the age of the Universe is 13.91 Gyr, by fitting the model's parameters to cosmological data.

gr-qc↗

arXiv2023

Probing extra dimensions through cosmological observations of dark energy

We investigate the isometrically embedded Bianchi type-V cosmology braneworld model in a $D$-dimensional bulk space. The model provides a fluid of geometric dark energy (GDE) and unification of fundamental forces similar to the Kaluza--Klein (KK) theory. The Planck energy density, the fine structure constant, the muon mass, and the number of extra dimensions are all factors that determine the density of the induced GDE. The model also predicts that graviton has mass, which is determined by the induced cosmological constant (CC). Our results are compatible with observations of the standard model of cosmology if the Universe has 22 non-compact extra dimensions. Our model provides an alternative method for probing extra dimensions.

gr-qc↗