arXiv subjects

Behnam Pourhassan

Publications and source records attributed to Behnam Pourhassan.

At least 37 records · Page 2Linked to original sources

arXiv2026

Holographic CFT Phase Transitions and Criticality for Einstein-Maxwell-Power-Yang-Mills AdS Black Holes

We present a comprehensive study of the thermodynamic phase structure for Anti-de Sitter black holes in Einstein-Maxwell-power-Yang-Mills gravity, reformulated through holographic duality as an ensemble problem in the dual conformal field theory (CFT). By deriving an extended first law where the central charge $C$ is a thermodynamic variable, we systematically explore both canonical and mixed ensembles. In the canonical ensemble with fixed charges, we identify a van der Waals-like phase transition between small and large black holes, marked by a characteristic swallowtail structure and coexistence curves with a negative slope. In contrast, within the mixed ensemble of fixed electric potential, the system exhibits a Hawking-Page transition between confined and deconfined phases of the boundary CFT. Our key finding is the suppressive role of the non-Abelian Yang-Mills charge $\tilde{q}$: increasing $\tilde{q}$ lowers both the minimum and the Hawking-Page transition temperatures, significantly narrowing the stability window of the confined phase. These results, supported by detailed numerical analysis, reveal a rich, ensemble-dependent phase landscape and establish the non-linear Yang-Mills sector as a critical controller of confinement physics in strongly coupled holographic systems.

hep-th↗

arXiv2026

Cosmic Hysteresis in Reconstructed $f(T)$ Bounce Models A Torsion-Based Thermodynamic Perspective

We investigate the emergence of cosmic hysteresis in cyclic and bouncing cosmologies within the framework of reconstructed $f(T)$ gravity. In contrast to curvature-based modifications of General Relativity, teleparallel gravity attributes gravitation to spacetime torsion encoded in the torsion scalar $T$. By reconstructing viable $f(T)$ functions corresponding to analytically prescribed nonsingular bouncing scale factors and coupling the geometry to a minimally interacting canonical scalar field, we demonstrate that asymmetric scalar field dynamics between expansion and contraction phases give rise to a non-vanishing thermodynamic work integral $\oint p_ϕ\, dV$ over complete cycles. This hysteresis manifests as closed loops in the $(w_ϕ,a)$ plane, signifying thermodynamic memory and irreversibility. We derive the modified Friedmann equations, establish exact bounce and turnaround conditions, and discuss the implications of torsion-induced hysteresis for the cosmological arrow of time. Our results confirm that cosmic hysteresis is a generic feature of cyclic universes in modified gravity, extending beyond curvature-based theories.

gr-qc↗

arXiv2026

Multi-probe analysis of strong-field effects in $f(Q)$ gravity

Covariant $f(Q)$ gravity is a viable extension of General Relativity, however its strong-field predictions remain largely untested. Using the static, spherically symmetric black-hole solutions of the theory, we confront it with the most stringent probes available: black-hole shadows, Event Horizon Telescope (EHT) measurements, S2-star precession, and strong gravitational lensing. We show that the two admissible solution branches behave very differently: Case~I produces negligible deviations from Schwarzschild solution, whereas Case~II yields significant, potentially observable corrections to the photon sphere and shadow size. From the EHT shadow diameters of M87* and Sgr~A*, we obtain tight bounds, which are further strengthened by strong-lensing coefficients. These results provide the sharpest strong-field constraints on covariant $f(Q)$ gravity to date, and point toward future tests using next-generation horizon-scale imaging and precision Galactic-center astrometry.

gr-qc↗

arXiv2026

A Bayesian Statistical Study of Bianchi Type-I Universe in $f(R,T^ψ)$ Modified Gravity

We have examined the cosmological actions of LRS (Locally Rationally Symmetric) Bianchi type-I universe model in $f(R,T^ψ)$ gravity. For this, we have estimated the Hubble parameter, the effective equation of state parameter ($ω^{eff}$), and the potential of the scalar field as a function of time using equation $H = W(ψ)$. The graphical representation of the potential function $V(ψ)$ with respect to cosmic time t is described. This study explores the dynamical properties of a Bianchi Type-I universe by utilizing Bayesian statistical techniques to constrain the model parameters and evaluate the viability of anisotropic cosmology under extended matter-geometry couplings. Also, we have applied the Markov Chain Monte Carlo (MCMC) mechanism on the derived $H(z)$ model by using observational Hubble data (OHD), the Baryon Acoustic Oscillation (BAO) dataset, and the Pantheon dataset. From the confidence-level contours and best-fit parameter values obtained, along with the corresponding reduced $χ^{2}$, it is evident that the model aligns strongly with observational data, demonstrating statistical stability and consistency in describing late-time cosmic acceleration. Likewise, the error analyses presented in this research, including a comparison between the $Λ$CDM cosmology and the reconstructed $H(z)$ model, confirm the model's compatibility with current observations by yielding a reliable and accurate account of the universe's expansion history.

gr-qc↗

arXiv2025

Statistical Constraints on Anisotropic Bianchi-III Cosmology in $f(R,T)$-Gravity Using MCMC Methods

Anisotropic Bianchi type-III cosmology is examined within the framework of f(R,T) gravity, where R denotes the Ricci scalar and T the trace of the energy-momentum tensor. In this work, we investigate the statistical constraints on anisotropic Bianchi type-III cosmology within the framework of f(R,T) gravity. The specific choice $f(R,T)=R+2f(T)$ is considered and exact solutions are derived for the background dynamics of the model. The physical parameters, such as the Hubble parameter H(z), spatial volume V(z), energy density $ρ(z)$, and pressure p(z), are derived and their evolutionary behaviors are analyzed. To examine the observational viability of the model, we employ Markov Chain Monte Carlo (MCMC) methods and perform a comprehensive statistical analysis using the latest observational datasets, including the Hubble parameter measurements, Baryon Acoustic Oscillations (BAO), and the Pantheon compilation of type Ia supernovae. The combined data analysis provides constraints on the free parameters of the model and allows a comparison with the standard $Λ$CDM cosmology. Our results show that the anisotropic Bianchi-III universe in f(R,T) gravity can successfully accommodate current observational data, offering new insights into the role of matter-geometry coupling in the late-time cosmic acceleration.

gr-qc↗

arXiv2025

Viscous Fluid Models of Cosmic Acceleration in FRW Spacetime Using MCMC Constraints

This study combines theoretical advancements with observational limitations to investigate the cosmological implications of a bulk viscous modified Chaplygin gas (MCG) in a Friedmann--Robertson--Walker (FRW) in (3+1) dimensional spacetime framework. We provide analytical solutions for both viscous and non-viscous cases, pointing out variations in the energy density evolution, the Hubble parameter dynamics, and the deceleration parameter transitions. Bulk viscosity suppresses oscillations in structure creation, a well-known drawback of Chaplygin gas models in larger dimensions, as shown by a thorough perturbation analysis. Using the bulk viscosity coefficient and Hubble expansion parameter, which are incorporated by the total pressure and the appropriate pressure and by using energy momentum conservation law determined time time-dependent density. With the help of three conditions ($ξ= 0$, $ξ\neq0$, and we neglect both bulk viscosity and presence of Chaplygin gas, i.e $A=0$ and $ξ=0$) created three different models as the Hubble parameter is a function of redshift $z$. By applying the MCMC method to these models, we have gone through observational analysis by using the Hubble and BAO datasets.

gr-qc↗

arXiv2025

Beyond Extremality: Weak Gravity Conjecture Constraints on Gravitational Lensing in Gravity's Rainbow

We investigate the constraints imposed by the Weak Gravity Conjecture (WGC) on gravitational lensing in gravity's rainbow, focusing in particular on scenarios beyond extremality and on the interplay between the WGC and the Weak Cosmic Censorship Conjecture (WCCC) in the context of Reissner-Nordström-Anti-de Sitter black holes modified by rainbow gravity. Using topological methods, we first analyze the configuration of photon spheres and confirm that unstable circular photon spheres with topological charge $(ω= -1)$ exist outside the event horizon throughout the parameter space, thereby verifying the simultaneous validity of both the WGC and the WCCC. The rainbow functions $f(\varepsilon)$ and $g(\varepsilon)$, which encode Planck-scale corrections through the energy ratio $(\varepsilon =E/E_P)$, modify both the spacetime metric and the extremality bound. We derive the corresponding modified extremal charge-to-mass ratio, $(q^2/m^2)>(Q^2/M^2)_{\text{ext}}$, and show that gravity's rainbow offers a natural mechanism for reconciling these two fundamental conjectures. By applying the Gauss-Bonnet theorem in conjunction with Jacobi-Maupertuis optical geometry, we compute the weak deflection angles for both photons and massive particles to second order. The rainbow function $g(\varepsilon)$ appears with powers $(g^{-2})$ and $(g^{-4})$, enhancing the deflection angle when $g(\varepsilon)<1$, while $f(\varepsilon)$ influences only the charge-dependent contributions. At extremality, the deflection angle becomes independent of $f(\varepsilon)$, yielding a universal prediction that can be tested without specifying the form of the rainbow functions. We further find that super-extremal configurations exhibit stronger lensing effects than extremal black holes, suggesting a potential observational discriminator between WGC-satisfying naked singularities and WCCC-preserving black holes.

gr-qc↗

arXiv2025

Can quantum gravity be both consistent and complete?

General relativity, despite its profound successes, fails as a complete theory due to presence of singularities. While it is widely believed that quantum gravity has the potential to be a complete theory, in which spacetime consistently emerges from quantum degrees of freedom through computational algorithms, we argue that this goal could be fundamentally unattainable. We examine how this limitation could emerge in various contexts, depending on whether or not every mathematically valid result is physically realized. In the first case, Godel's incompleteness theorems, along with related results by Tarski and Chaitin, imply that no theory formulated as a formal axiomatic system can be complete, and that within any computational framework, a fully consistent internal truth predicate is impossible. In the second case, if only a subset of mathematical truths is realized in nature, we argue that this selection cannot be determined by any purely computational process. Hence, a meta-theoretical approach based on non-algorithmic understanding is indispensable in every case. We discuss some possible consequences of this observation for describing physical systems and note that a non-algorithmic approach should be essential for any theory of everything.

gr-qc↗

arXiv2025

Spherically Symmetric Quantum-Corrected Black Holes with String Clouds: A Multi-Observable Analysis

We present an investigation of quantum-corrected black hole spacetimes coupled with clouds of strings, examining two distinct theoretical models that incorporate quantum gravitational effects through different implementations of correction terms. Our study explores the geodesic structure, focusing on photon sphere properties, black hole shadows, and innermost stable circular orbits of test particles around these exotic geometries. The analysis reveals fundamental modifications to particle trajectories that distinguish quantum-corrected solutions from their classical counterparts, with observable implications for high-energy astrophysics. We investigate quasi-periodic oscillations arising from test particle motion, deriving frequency relationships that could serve as observational probes of quantum gravity effects in accreting black hole systems. Through rigorous gravitational lensing analysis using the Gauss-Bonnet theorem, we calculate weak-field deflection angles and identify distinctive signatures that enable discrimination between the two quantum correction models. The gravitational lensing study reveals opposite dependencies on quantum parameters between the models, providing unambiguous observational discriminants. Additionally, we analyze the thermodynamic properties including temperature, entropy, and heat capacity modifications, exploring topological characteristics and phase transition behavior in these quantum-corrected systems. The investigation reveals that the two models exhibit contrasting behaviors across multiple observational channels, from gravitational lensing deflection angles to quasi-periodic oscillation frequencies, providing remarkable discriminants for testing quantum gravity theories.

astro-ph.HE↗

arXiv2025

Swampland Conjectures through ACT Observations: Observational Signatures of Radiative-Corrected Inflation

We investigate the consistency of radiatively corrected inflationary models with both the latest observational data from the Atacama Cosmology Telescope (ACT) combined with Planck 2018 and Baryon Acoustic Oscillation (BAO) measurements, and the theoretical constraints imposed by the swampland program. We systematically test two distinct models against three key swampland conjectures: the further refined de Sitter swampland conjecture (FRDSSC), the scalar weak gravity conjecture (SWGC), and the strong scalar weak gravity conjecture (SSWGC). Model I, based on radiatively corrected Higgs inflation, satisfies the FRDSSC and remains consistent with current observational constraints ($n_s = 0.9743 \pm 0.0034$, $r < 0.038$), but fails to meet the SWGC and SSWGC requirements, indicating limited theoretical compatibility with quantum gravity principles. In contrast, Model II, incorporating radiative corrections with scalar sectors, demonstrates full consistency by satisfying all three swampland conjectures simultaneously while maintaining observational viability. The compatibility of Model II is highly sensitive to the non-minimal coupling $ξ$ and renormalization scales $μ_b$, with larger values extending the range of swampland-consistent solutions. Our results highlight the critical role of radiative corrections in achieving simultaneous theoretical and observational consistency, and identify Model II as a promising candidate for a fully viable inflationary scenario within the swampland framework. This work provides a methodology for classifying inflationary models based on their swampland compatibility, demonstrating that satisfaction of the FRDSSC alone is insufficient for full theoretical consistency.

astro-ph.CO↗

arXiv2025

Analytic and Numerical Constraints on QPOs in EHT and XRB Sources Using Quantum-Corrected Black Holes

This investigation examines QPOs in two quantum-corrected BH spacetimes that preserve general covariance while incorporating quantum gravitational effects through a dimensionless parameter ζ. We combine analytical derivations of epicyclic frequencies with comprehensive numerical simulations of BHL accretion to explore how quantum corrections manifest in observable astrophysical phenomena. Using a fiducial BH mass of M=10M_\odot representative of stellar-mass X-ray binaries, we demonstrate that the two models exhibit fundamentally different behaviors: Model-I modifies both temporal and radial metric components, leading to innermost stable circular orbit migration proportional to ζ^4 and dramatic stagnation point evolution from 27M to 5M as quantum corrections strengthen. Model-II preserves the classical temporal component while altering only spatial geometry, maintaining constant stagnation points and stable cavity structures throughout the parameter range. Our numerical simulations reveal distinct QPO generation mechanisms, with Model-I showing systematic frequency evolution and cavity shrinkage that suppresses oscillations for ζ\geq 3M, while Model-II maintains stable low-frequency modes up to ζ\geq 5M. Power spectral density analyzes demonstrate characteristic frequency ratios (3:2, 2:1, 5:3) consistent with observations from X-ray binaries, providing specific targets for discriminating between quantum correction scenarios. The hydrodynamically derived constraints (ζ\lesssim 4M) show remarkable agreement with independent EHT limits for M87* and Sgr A*, validating our theoretical framework through multiple observational channels. These results establish QPO frequency analysis as a probe for detecting quantum gravitational effects in astrophysical BHs and demonstrate the complementary nature of timing and imaging observations in constraining fundamental physics.

astro-ph.HE↗

arXiv2025

Testing the Generalized Second Law in $(2+1)$-Dimensional Cosmology: Holographic Entropy Bounds and Observational Constraints

We investigate the validity of the Generalized Second Law (GSL) of thermodynamics in a $(2+1)$-dimensional holographic cosmological model with a negative cosmological constant. Adopting a horizon thermodynamics framework, we examine two prominent entropy bounds, the Fischler--Susskind (FS) bound and the Hubble Entropy (HE) bound, in both expanding and contracting universes, including the effects of quantum entropy corrections. Our theoretical analysis shows that the FS bound is intrinsically incompatible with the GSL in contracting $(2+1)$-dimensional universes, regardless of spatial curvature or exotic matter content, and that this incompatibility persists even when quantum corrections are considered. In contrast, the HE bound is consistent with the GSL in expanding universes under classical conditions and can also be reconciled in certain contracting scenarios when quantum effects are included. To complement the theoretical study, we perform a Markov Chain Monte Carlo (MCMC) analysis using recent Baryon Acoustic Oscillations (BAO), Cosmic Chronometer (CC), and Hubble parameter datasets to constrain the model parameters. The best-fit results reveal good cross-dataset consistency, with the cosmological constant parameter $ψ$ remaining stable across all probes. These findings identify the HE bound as a more robust candidate for holographic constraints in lower-dimensional cosmology, while demonstrating the limitations of the FS bound. Our results not only clarify the status of the GSL in $(2+1)$-dimensional settings but also provide a framework for testing entropy bounds with future high-precision cosmological data.

gr-qc↗

arXiv2025

Thermodynamics and Joule-Thomson Expansion for Schwarzschild-AdS Black Holes with Cloud of Strings and Quintessential-like Fluid

In this study, we explore the thermodynamic properties of a Schwarzschild black hole (BH) embedded in an anti-de Sitter (AdS) background, which is further coupled with a cloud of strings and surrounded by a quintessence-like fluid. Beginning with the formulation of BH mass in terms of the event horizon radius, we incorporate the concept of pressure as related to the AdS curvature radius within the framework of extended phase space thermodynamics. Using this setup, we derive key thermodynamic quantities, including the Gibbs free energy and internal energy, to characterize the energetic behavior of the black hole system. To assess the stability of the black hole, we compute the specific heat capacity and analyze how it is influenced by external parameters, such as the string cloud and the quintessence-like fluid. These geometric and matter fields are shown to significantly modify the thermal response of the BH. Furthermore, we examine the inversion temperature associated with the black hole and highlight its distinction from the standard Hawking temperature, providing deeper insight into the phase structure. Additionally, we investigate the Joule-Thomson expansion process and demonstrate how the aforementioned parameters affect this thermodynamic phenomenon, showing important aspects of BH cooling and heating behavior in an extended thermodynamic context.

gr-qc↗

arXiv2025

Schwarzschild-AdS Black Holes with Cloud of Strings and Quintessence: Geodesics, Thermodynamic Topology, and Quasinormal Modes

In this study, we explore a Schwarzschild-anti de-Sitter black hole (BH) coupled with a cloud of strings (CoS) possessing both electric- and magnetic-like components of the string bivector, embedded in a Kiselev-type quintessence fluid (QF). We analyze the dynamics of photons and test particles, focusing on trajectories, photon spheres, BH shadows, and innermost stable circular orbits (ISCO), highlighting how CoS and QF parameters affect these features. We then examine the thermodynamic topology of the system by analyzing vector field zeros, showing that varying CoS leads to distinct topological configurations with total charges of either $0$ or $+1$, corresponding to known classes like RN and AdS-RN. Additionally, we study scalar field dynamics via the massless Klein-Gordon equation, reformulated into a Schr$\ddot{o}$dinger-like form to derive the effective potential. We compute the quasinormal modes (QNMs) of scalar perturbations, showing how CoS and QF influence oscillation frequencies and damping rates, with implications for gravitational confinement and thermalization in the AdS/CFT context.

gr-qc↗

arXiv2025

Probing the Weak Gravity Conjecture: Novel Aschenbach Signatures in Superextremal Non-Linear Charged AdS Black Holes

This study investigates the nonlinear charged Anti-de Sitter (AdS) black hole solution within the framework of massive gravity, motivated by recent advancements linking the Weak Gravity Conjecture (WGC) to phenomena such as Weak Cosmic Censorship Conjecture (WCCC) and photon sphere dynamics. Building on these foundations, we focus on the Aschenbach effect-a relativistic phenomenon intricately tied to the geometry of photon spheres and known to occur in some special sub-extremal non rotating black holes. Our primary objective is to determine whether this effect persists not only up to the extremal limit but also beyond, into the superextremal regime, thus probing the stability and validity of black hole characteristics in these extreme conditions. By analyzing the nonlinear charged AdS black hole solutions in massive gravity, we demonstrate that the Aschenbach effect remains a robust feature across both extremal and superextremal configurations. This extension suggests that key relativistic signatures and the underlying spacetime structures associated with high-spin black holes continue to hold beyond classical boundaries. Our results provide new insights into the behavior of ultra-compact objects and highlight promising directions for exploring the limits of general relativity, as well as potential generalizations of the WGC and WCC in strong gravitational fields.

gr-qc↗

arXiv2025

Cosmic-Quantum Connections: Assessing the Viability of Weak Gravity and Weak Cosmic Censorship Conjectures in Kerr-Newman-Kiselev-Letelier Black Hole

This paper addresses a potential validation of the weak gravity conjecture (WGC) with the weak cosmic censorship conjecture (WCCC), as a significant challenge in quantum gravity. We explore the viability of the WGC and WCCC in the context of the Kerr-Newman-Kiselev-Letelier (KNKL) black hole. Although these conjectures appear unrelated, but surprising connection between these conjectures, It establishes a bridge between the quantum and the cosmic. By imposing specific constraints on the black hole's parameters, we demonstrate that the WGC and WCCC can be compatible in certain regions. We examine the properties of the KNKL black hole for $q/m > (Q/M )_{ext}$, where $(Q/M )_{ext}$ is the charge-to-mass ratio of a large extremal black hole. We present figures to test the validity of both conjectures simultaneously. Without the spin parameter $a$, the cloud of string parameter $b$, quintessence parameter $γ$, and equation of state parameter $ω$, the black hole either has two event horizons if $Q^2/M^2 \leq 1$ or none event horizon if $Q^2/M^2 > 1$ which leads to a naked singularity that contradicts the WCCC. However, when $a$, $b$, $γ$, and $ω$ are present, the black hole has event horizons in some regions in the $Q^2/M^2 > 1$ that ensure the singularity is covered and both the WGC and WCCC are fulfilled. Actually, we face this issue in the extremality state of the black hole viz these conjectures remain viable, with the black hole maintaining an event horizon. We conclude that certain regions of $a$, $b$, $γ$, and $ω$ parameters can make the WGC and WCCC compatible, indicating their agreement when these parameters are present.

hep-th↗

arXiv2025

Non-Linear Dynamics and Critical Phenomena in the Aretakis Instability of Extremal Black p-Branes

We present the first comprehensive investigation of the non-linear evolution of the Aretakis instability in extremal black $p$-branes. Building on recent insights into the linear behavior of perturbations in near-horizon AdS$_{p+2} \times S^{D-p-2}$ geometries, we explore the full non-linear regime using a combination of analytical scaling arguments and numerical simulations. We uncover a universal critical behavior governed by scaling exponents that depend only on the spacetime dimension $D$ and the brane worldvolume dimension $p$. Near the threshold of instability, the system exhibits power-law evolution toward dynamically generated extremal attractors, while supercritical perturbations lead to singular growth. Through the AdS/CFT correspondence, we compute entanglement entropy, correlation functions, spectral densities, and out-of-time-ordered correlators in the dual field theory, finding universal scaling across all observables. These results establish a deep connection between geometric instability in the bulk and quantum information dynamics on the boundary. The emergence of universal scaling and phase transitions in extremal geometries suggests that non-linear Aretakis dynamics may serve as a general framework for studying holographic criticality and late-time behavior in strongly coupled systems.

hep-th↗

arXiv2025

Warm Inflation with Barrow Holographic Dark Energy

In this work, we study the warm inflation mechanism in the presence of the Barrow holographic dark energy model. Warm inflation differs from other forms of inflation primarily in that it assumes that radiation and inflaton fields exist and interact throughout the inflationary process. After the warming process, energy moves from the inflaton to the radiation as a result of the interaction, keeping the cosmos warm. Here we have set up the warm inflationary mechanism using Barrow holographic dark energy as the driving agent. Warm inflation has been explored in a highly dissipative regime, and interesting results have been obtained. It is seen that the Barrow holographic dark energy can successfully drive a warm inflationary scenario in the early universe. Finally, the model was compared with the observational data, and compliance was found.

gr-qc↗