arXiv subjects

Hassan Hassanabadi

Publications and source records attributed to Hassan Hassanabadi.

At least 19 recordsLinked to original sources

arXiv2026

Quasinormal-mode redshifts of black holes in galactic halos with radial pressures

We investigate the black-hole quasinormal mode redshifts induced by a galactic dark-matter halo with nonvanishing radial pressure. The halo is described by an anisotropic fluid with a generalized density profile and a constant radial equation-of-state parameter $w$. We construct the corresponding static, spherically symmetric spacetime and study the resulting corrections to the unstable light ring and quasinormal-mode spectrum. For dilute halos and $w\neq -1$, the light-ring angular frequency and Lyapunov exponent acquire the same leading-order gravitational redshift, implying a universal shift of the real and imaginary parts of the quasinormal frequencies. The magnitude of this effect depends on both the halo structure and the radial pressure. This universal redshift relation holds not only in the case of $w=0$ that has been discovered in the literature but also in a much wider scenario, supporting its importance in black hole spectroscopy. On the other hand, the case $w=-1$ is qualitatively different: the factor that corresponds to the leading common redshift vanishes, the corrections are further suppressed by the hierarchy between the black-hole and halo scales, and the universal relation is broken, with the dominant contribution controlled by the inner halo profile. Our numerical computations of scalar-field quasinormal mode spectra confirm the analytic eikonal results.

gr-qc↗

arXiv2026

Black holes in depleted Dehnen dark matter halos with physical inner edges and strong-field observables

We construct a static and spherically symmetric black hole (BH) spacetime surrounded by a depleted Dehnen dark matter (DM) halo. The model describes a final equilibrium configuration rather than dynamical accretion process: the initial halo mass inside a prescribed inner edge is incorporated into the central BH, leaving a vacuum gap between the dressed horizon and the surviving halo. Consequently, the horizon radius depends on the Dehnen cusp index $γ$ and the absorbed halo mass, while the Schwarzschild result is recovered exactly when the halo is removed. We derive general expressions for the metric functions and redshift phase and obtain explicit solutions for $γ=0,1,2,$ and $5/2$. The dominant energy condition imposes the universal local bound $κ\geq5/4$, whereas global admissibility depends additionally on the cusp index and halo compactness. A literal stable Einstein-cluster interpretation requires the stronger condition $κ\geq3$ when stable circular motion is imposed at the halo edge. We also analyze the photon sphere, shadow, and admissible parameter space, demonstrating how the halo cusp and depletion scale affect strong-field observables. Previously studied analytic BH--halo geometries and the Schwarzschild and weak-halo limits follow as special cases of the general construction.

gr-qc↗

arXiv2026

Generalized Regular Black Holes with Tunable Cores and Geodesic Properties

We introduce a family of static and spherically symmetric regular black-hole geometries characterized by a discrete core index $n$, a regularization scale $a$, and a dimensionless deformation parameter $η$. The construction starts from a positive, normalized effective density profile with a tunable central behavior. The lowest member contains the Bardeen geometry as a limiting case, whereas a nonzero deformation parameter generates a Reissner--Nordström-like asymptotic correction. The parameter $n$ controls the order of central depletion: the lowest member possesses a de Sitter-type core, whereas higher members approach a Minkowski-like center with vanishing density and curvature. We determine the horizon structure, extremality condition, Hawking temperature, entropy, and the horizon thermodynamic identity implied by the radial Einstein equation. Null geodesics are analyzed to obtain the photon sphere, critical impact parameter, and shadow radius, including analytic weak-deformation approximations, the exact Schwarzschild and Bardeen limits, and comparison with the leading Reissner--Nordström-like behavior. The photon-orbit angular frequency and Lyapunov exponent are then used to separate orbital and instability timescales; their ratio provides information not already contained in the shadow radius. Weak gravitational lensing and magnification are discussed through a controlled sector-isolated expansion of the asymptotic metric. We further study timelike geodesics, circular motion, the marginally bound orbit, and the innermost stable circular orbit, deriving exact implicit relations and perturbative expressions. The model provides a unified framework for investigating how a tunable regular core and an RN-like exterior deformation affect the thermodynamic and geodesic properties of regular black holes.

gr-qc↗

arXiv2026

Causal structure and optical signatures of rotating power law Kalb-Ramond geometry

We investigate a stationary and axisymmetric power law Kalb-Ramond geometry, obtained as a rotating extension of the asymptotically flat static solution. We reconstruct the exact inverse Einstein source supporting the geometry and determine its energy conditions and curvature properties. We characterize the horizon and stationary limit configurations and quantify the outer ergoregion through its angular dependent radial thickness plus its coordinate and proper spatial volumes. We derive the unstable spherical photon region and show that the Kalb-Ramond deformation shifts the prograde photon orbit into the ergoregion at lower spin, while the retrograde region remains outside throughout the entire domain. From the unstable photon region, we construct the exact vacuum critical curve and analyze its area equivalent diameter and displacement, including comparisons with M87$^\ast$ and Sgr~A$^\ast$ within the asymptotically admissible domain. We further examine the fixed-$J$ geometric behavior and rotational energy extraction, with emphasis to rotating weak deflection and two dimensional weak lensing, commenting the physical consequences of our solution in terms of observable outcomes.

gr-qc↗

arXiv2026

Geometry--gauge controlled dynamics and localization of a two-particle system on a helicoidal manifold

We study the dynamics and localization of two oppositely charged particles constrained to a helicoidal manifold in a uniform magnetic field. The embedding-induced metric and the pullback of the ambient electromagnetic gauge potential, together with restriction to the reflection-symmetric longitudinal rest frame, lead to an exact reduction of the relative dynamics to a one-dimensional Hamiltonian with a coordinate-dependent kinetic term and a gauge-shifted momentum. We analyze the corresponding effective potential, turning points, and classically allowed regions, and determine how the geometric and magnetic parameters modify the bounded relative motion. For a regularized attractive interaction, we derive the zero-energy condition for localization around the symmetric configuration and obtain the local stiffness governing its stability. When this stiffness changes sign while the quartic coefficient remains positive, the symmetric minimum undergoes a pitchfork-type bifurcation to two symmetry-related finite-separation minima of the relative coordinate. Canonical quantization of the reduced Hamiltonian then gives the low-energy spectrum in the harmonic approximation and the associated zero-energy localization thresholds. At the critical stiffness, the quadratic term vanishes and the quartic term provides the leading contribution to the local low-energy scaling. These results establish how helicoidal geometry and magnetic coupling modify the relative localization and low-energy spectral properties of the two-particle system.

quant-ph↗

arXiv2026

Astrophysical Signature and Optical Appearance of Weyl--Corrected Einstein--Maxwell Black Holes

In this work, we investigate the physics of charged black holes modified by Weyl corrections, which emerge from the non-minimal coupling between spacetime curvature and electromagnetism. We begin by revisiting the thermodynamics of these systems, deriving the Hawking temperature, Wald entropy, and heat capacity to examine how the Weyl correction parameter reshapes the landscape of thermal stability and phase transitions. Then, we apply a topological method to classify the thermodynamic states and reveal the impact of the Weyl modifications on this classification. To explore astrophysical signatures, we analyze the null trajectories and shadow cast by photons under two-photon polarization modes, deriving observational constraints on the black hole parameters. Finally, we model the accretion disk around these black holes. By calculating the energy flux, spectral luminosity, and differential luminosity, we show how these corrections leave a detectable trace on the emitted radiation.

gr-qc↗

arXiv2026

Imprints of core/cusp dark matter distributions on black hole signatures in galaxies

In galactic environments, a host dark matter (DM) halo can imprint weak but coherent corrections on black hole (BH) strong-field observables. We construct an exact family of static and spherically symmetric BH spacetimes sourced by a generic core/cusp DM halo, described by an anisotropic stress-energy tensor with nonvanishing radial pressure. The resulting geometry is determined self-consistently from the Einstein equations for a broad $\{α,β,γ\}$ density profile, including the NFW, Moore, Hernquist, Jaffe, and core/cusp Dehnen models as special cases. We discuss the asymptotic structure, horizon location, curvature scale, and energy conditions of the corresponding geometries, emphasizing that the inner logarithmic slope $γ$ controls the amount of DM probed by the relativistic region. We then obtain perturbative analytic estimates for the characteristic circular geodesics, demonstrating that the leading strong-field corrections are controlled by the dimensionless compactness $q_γ$, rather than by the total halo mass alone. To leading order in $q_γ$, we derive analytic expressions for the light ring radius, angular frequency, critical impact parameter, Lyapunov exponent, innermost stable circular orbit (ISCO) radius, and ISCO frequency. For cuspy profiles, the light ring and ISCO are displaced outward, while the corresponding orbital frequencies are redshifted; for cored profiles, the light ring radius is unchanged at this order, although its frequency and capture impact parameter still carry finite environmental corrections. We also investigate the weak- and strong-deflection angles and their dependence on the halo's inner structure. We further discuss how these environmental corrections may affect ringdown physics, in particular through the perturbative imprint of the halo on quasinormal-mode redshifts and late-time wave propagation.

gr-qc↗

arXiv2026

Electromagnetic wave propagation in static black hole spacetimes: an effective refractive index description in Schwarzschild geometry

We investigate electromagnetic wave propagation in static, spherically symmetric black hole spacetimes using a covariant and gauge-invariant framework based on the established Maxwell perturbation formalism. Building upon known parity decompositions and gauge-invariant master equations, we reformulate the resulting radial dynamics entirely within Schwarzschild coordinates and introduce an effective refractive-index description of electromagnetic propagation in curved spacetime. Starting from the source-free Maxwell equations on a curved background, electromagnetic perturbations are decomposed according to parity and systematically reduced to gauge-invariant dynamical variables without introducing auxiliary coordinate transformations or horizon-regular variables. Both axial and polar sectors are shown to obey the same parityindependent master equation, and their exact isospectrality emerges naturally as a direct consequence of Maxwell theory in four dimensions. By eliminating first-derivative terms through an appropriate field redefinition, the radial dynamics is cast into a Helmholtz-type equation, which motivates the introduction of an effective, position- and frequency-dependent refractive index encoding gravitational redshift, curvature effects, and angular momentum within a unified optical framework. Specializing to the Schwarzschild geometry, we obtain the refractive index in closed analytical form and analyze its behavior in the near-horizon, intermediate, and asymptotic regimes. The resulting description provides a transparent and physically intuitive interpretation of electromagnetic evanescence, and propagation in black hole spacetimes, and establishes a robust foundation for wave-optical, semiclassical, and numerical studies in more general static gravitational backgrounds.

gr-qc↗

arXiv2026

Schwarzschild-like Black Holes Submerged in an Exponential Density Dark Matter Profile

We study a class of Schwarzschild black holes embedded in an exponential-spheroidal dark matter halo, modelled by a phenomenological density profile $ρ(r)=ρ_0 e^{-r/r_0}$. By solving the Einstein equations for a static, spherically symmetric spacetime, we obtain an analytic solution for the lapse function that reduces to the Schwarzschild spacetime in the absence of the halo and to a regular halo configuration when the central black hole mass vanishes. Indeed, the two halo parameters, $ρ_0$ and $r_0$, describe the strength and radial extent of the dark matter distribution. We analyse the curvature structure, energy conditions, shadow observables, scalar quasi-normal modes and grey-body bounds of the resulting spacetime. The Ricci scalar and the Ricci square remain finite at the origin, whilst the Kretschmann scalar retains the usual central tidal singularity in the presence of a black hole mass. The weak, null, and dominant energy conditions are satisfied, whilst the strong energy condition is violated on a finite radial interval. We also show that the halo monotonically shifts the photon sphere and the shadow radius, which allows us to derive approximate constraints from the EHT observations of M87* and Sgr A*. For scalar perturbations, the Padé-resummed WKB approximation yields stable quasinormal frequencies, while the standard WKB approximation becomes unreliable for higher overtones and strong-halo configurations. Finally, the greybody bounds indicate that the halo weakens transmission through the effective barrier, particularly at low frequencies.

gr-qc↗

arXiv2026

Optical and thermodynamic properties of Kerr-Bertotti-Robinson black holes

We investigate the thermodynamic and optical properties of Kerr--Bertotti--Robinson black holes, namely rotating black holes immersed in an external Bertotti--Robinson electromagnetic background. In the fixed-$a$ ensemble, we derive the horizon mass relation, the Hawking temperature, the entropy, the Helmholtz-type free energy, the heat capacity, and the extremal remnant configuration. These quantities reduce smoothly to their Kerr counterparts as $B\to0$. In the weak-field regime, the leading thermodynamic corrections arise at order $B^2$; the extremal radius is shifted at this order, whereas the remnant mass receives its first correction only at order $B^4$. We also introduce a formal AdS-like thermodynamic interpretation of the Bertotti--Robinson scale, treating the associated pressure as an effective response variable rather than a genuine cosmological pressure. Because the spacetime is not asymptotically flat, we further compute the finite-radius Komar mass and the Komar charge associated with the horizon generator. Using the Hamilton--Jacobi formalism, we derive the separated null-geodesic potentials, the impact parameters of spherical photon orbits, and the celestial coordinates of the shadow boundary for a finite-distance observer. We then characterize the photon-region boundaries, ergosphere thickness, photon--ergosphere gap, shadow area, and magnetic shadow susceptibility. Within the perturbative regime considered, the Bertotti--Robinson background decreases the averaged ergosphere thickness and shadow area, increases the photon--ergosphere gap, and produces a negative shadow susceptibility whose magnitude is enhanced by rotation.

gr-qc↗

arXiv2026

Phase-space structure and nonlinear dynamics of a charged particle on a helicoidal manifold under a magnetic field

We analyze the classical dynamics of a charged particle constrained to a helicoidally embedded Riemannian manifold in $\mathbb{R}^3$ under a uniform magnetic field in the ambient space. The induced metric $ds^2=du^2+(1+w^2u^2)dv^2$ and the pulled-back symmetric gauge yield an exact reduction to a one-dimensional nonlinear Hamiltonian system. The resulting effective potential couples geometry and magnetic field, producing transitions between bounded and unbounded motion and a reorganization of phase-space topology. In the asymptotic regime, the dynamics reduces to a harmonic oscillator with $ω_{\mathrm{eff}}=ω_c/2$ and $\ell=\sqrt{2}\,\ell_\mathcal{B}$. The system admits a Landau-type semiclassical spectrum and exhibits a geometry--magnetic control parameter $Λ=q\mathcal{B}+\hbar k_v w$ governing a chirality transition.

physics.class-ph↗

arXiv2026

Lorentz-violating modifications to particle dynamics, thermodynamics and vacuum energy in bumblebee gravity

We investigate how spontaneous Lorentz symmetry breaking in bumblebee gravity modifies particle dynamics, thermodynamics, and vacuum energy around a static black hole background. Starting from the optical-mechanical correspondence, we derive a modified dispersion relation that encodes the influence of the Lorentz-violating parameter $λ$ on the propagation of massive and massless modes. We analyze the resulting optical properties, including the effective refractive index, group velocity, and energy-dependent time delay, and show how the non-asymptotically flat geometry reshapes signal propagation. From the same dispersion relation, we construct the interparticle potential for massive and massless excitations and evaluate the electron scattering cross section within the Born approximation, identifying characteristic Lorentz-violating corrections. We then develop a statistical-ensemble description based on the deformed energy-momentum relation and obtain analytic expressions for the thermodynamic observables of a massless bosonic gas. The pressure, mean energy, entropy, and heat capacity are examined in three representative regimes -- extremely close to the horizon, near the photon sphere, and in the asymptotic region -- where Lorentz violation systematically increase the magnitude of these quantities and leads to finite asymptotic plateaus. Finally, we analyze the vacuum state in the curved background and compute the regularized Casimir energy at zero and finite temperature.

gr-qc↗

arXiv2026

Thermodynamic Geometry, Heat Engines, and Topology of Sharma--Mittal ModMax-dRGT Black Holes

We investigate the thermodynamic structure of charged AdS black holes in ModMax nonlinear electrodynamics coupled to dRGT-like massive gravity, incorporating Sharma--Mittal entropy corrections. The thermodynamic geometry is analyzed using the Weinhold metric in the parameter space spanned by the horizon radius and electric charge. The resulting thermodynamic Ricci scalar characterizes effective microscopic interactions, with curvature singularities signaling extremal boundaries and degeneracies of the thermodynamic metric. We further construct a rectangular black hole heat engine in the extended phase space and derive an exact expression for its efficiency, demonstrating how the ModMax parameter and massive-gravity couplings influence the enthalpy-based conversion of heat into work, while the Sharma--Mittal parameters modify the Carnot bound through corrections to the black-hole temperature. Finally, a topological analysis of the corrected temperature and generalized free energy reveals both conventional and novel critical points, and the associated conserved topological charge is investigated.

gr-qc↗

arXiv2026

Extended Thermodynamics and Throttling Process of Charged AdS Black Holes in ModMax-dRGT Massive Gravity with Sharma-Mittal Entropy

We investigate the extended thermodynamics, including the Joule-Thomson expansion and $P-V$ criticality, of a four-dimensional charged anti-de Sitter (AdS) black hole within the combined framework of ModMax nonlinear electrodynamics and dRGT-like massive gravity. Operating in the extended phase space and employing the generalised Sharma-Mittal entropy to account for non-extensive statistical correlations, we derive exact analytical expressions for the modified Hawking temperature, specific heat, Joule-Thomson coefficient, and the equation of state. Our analysis of the throttling process reveals that the conformal nonlinearities of the ModMax field ($γ$) expand the physically accessible cooling domain by shifting the inversion transition to smaller horizon radii. While the Sharma-Mittal parameters ($δ$, $R$) critically govern local thermodynamic stability and the inversion radius, the global inversion phase boundary remains fundamentally dictated by the massive graviton background. Furthermore, an analysis of the Gibbs free energy uncovers a van der Waals-like first-order phase transition characterized by a distinct swallow-tail structure. We observe a clear physical decoupling in the critical regime: ModMax nonlinearities modify the critical phase boundary by suppressing electromagnetic interactions, Sharma-Mittal parameters dictate the relative thermal stability of competing phases, and massive gravity governs the overarching macroscopic phase landscape. These results highlight the sensitivity of thermodynamic phase phenomena as robust diagnostic tools for distinguishing nonlinear and non-extensive modifications to black hole physics.

gr-qc↗

arXiv2026

Thermodynamic topology of dyonic AdS black holes with quasitopological electromagnetism in Einstein-Gauss-Bonnet gravity

In this study, we investigate the thermodynamic topology of the high-dimensional dyonic AdS black holes with quasitopological electromagnetism in the Einstein-Gauss-Bonnet background. We first examine the topological charge connected to the critical point and find that the two conventional critical points $CP_{1},CP_{2}$ of the black hole are physical critical point, and the novel critical point $CP_{3}$ that lacks the capability to minimize the Gibbs free energy ($α=0.5$). The critical points $CP_{1}$ and $CP_{2}$ are observed to occur at the maximum extreme points of temperature in the isobaric curve, while the critical point $CP_{3}$, emerges at the minimum extreme points of temperature. Furthermore, the number of phases at the novel critical point exhibits an upward trend, followed by a subsequent decline at the conventional critical points. With the increase of the coupling constant ($α= 1$), although the system has three critical points, only the conventional $CP_{1}$ is a (physical) critical point, and the conventional $CP_{2}$ serves as the phase annihilation point. This means that the coupling constant $α$ has significant impact on the phase structure. Additionally, we regard dyonic AdS black holes as a topological defect within the thermodynamic space, our findings indicate that alterations in pressure can result in the system exhibiting distinct points of generation and annihilation. However, the total topological number of black holes in different dimensions is $1$, the system shares a similar topological classification as the charged RN-AdS black holes. The discovery we have made provides a crucial component in understanding the thermodynamic topology of dyonic AdS black holes.

gr-qc↗

arXiv2026

Rotating Black Holes Surrounded by Massive Vector Fields in Kaluza Klein Gravity

In this paper, we introduce a rotating Kaluza-Klein black hole characterized by a massive vector field and a scalar field. We begin by identifying the horizons and mapping the allowed parameter space to differentiate black hole solutions from naked singularities. The thermodynamic analysis shows a phase transition by examining Hawking temperature and heat capacity. We also conduct a topological study of the thermodynamic potentials. The Hawking temperature indicates a conventional critical point, while the off-shell generalized free energy classifies the system into a specific universal group. We further investigate the geometry of the ergosphere and how it relates to the black holes spin. Additionally, we look at astrophysical signs, such as the black hole shadow and the features of the thin accretion disk. Our results indicate that while the extra-dimensional changes significantly shift phase transition points and modify the shadow size, the essential topological class remains stable. This study provides a solid framework for distinguishing higher-dimensional gravity models through both thermodynamic and observational signs.

gr-qc↗

arXiv2026

Scalar-Wave Signatures of Wormholes in Dark Matter Halos

We identify scalar-wave signatures of massless fields propagating in static, spherically symmetric wormholes embedded within realistic dark matter halos. Starting from a general line element with arbitrary redshift and shape functions, we recast the radial Klein-Gordon equation in Schrödinger form, explicitly separating contributions from gravitational redshift, spatial curvature, and angular momentum. The dynamics reduce to a generalized Helmholtz equation with a space- and frequency-dependent effective refractive index that encodes the throat geometry, halo curvature, and centrifugal effects, asymptotically recovering free-space propagation. Applying this framework to Navarro-Frenk-White, Thomas-Fermi Bose-Einstein condensate, and Pseudo-Isothermal halo models, and considering zero, Teo-type, and cored redshift functions, we uncover evanescent regions and suppression of high-angular-momentum modes in the vicinity of the throat. High-frequency waves approach the geometric-optics regime, whereas low-frequency modes exhibit strong curvature-induced localization. In the geometric-optics limit, the effective refractive index reproduces null-geodesic trajectories, while finite-frequency effects capture evanescent zones and tunneling phenomena. This work establishes the first exact, non-perturbative framework linking wormhole geometry and realistic dark matter halos to observable scalar-wave propagation phenomena, including evanescence, mode suppression, and frequency-dependent localization.

gr-qc↗

arXiv2026

Charged particle dynamics in singular spacetimes: hydrogenic mapping and curvature-corrected thermodynamics

We analyze the dynamics of charged test particles in a singular, horizonless spacetime arising as the massless limit of a charged wormhole in the Einstein--Maxwell--Scalar (EMS) framework. The geometry, sustained solely by an electric charge $Q$, features an infinite sequence of curvature singularity shells, with the outermost at $ r_* = \frac{2|Q|}π $ acting as a hard boundary for nonradial motion, while radial trajectories can access it depending on the particle charge-to-mass ratio $ |q|/m $. Exploiting exact first integrals, we construct the effective potential and obtain circular orbit radii, radial epicyclic frequencies, and azimuthal precession rates. In the weak-field limit ($ r \gg |Q| $), the motion reduces to a Coulombic system with small curvature-induced retrograde precession. At large radii, the dynamics maps to a hydrogenic system, with curvature corrections inducing perturbative energy shifts. Approaching $ r_* $, the potential diverges, producing hard-wall confinement. Curvature corrections also modify the spectral thermodynamics, raising energies and slightly altering entropy and heat capacity. Our results characterize the transition from Newtonian-like orbits to strongly confined, curvature-dominated dynamics.

gr-qc↗