arXiv subjects

Abdullah Guvendi

Publications and source records attributed to Abdullah Guvendi.

At least 19 recordsLinked to original sources

arXiv2026

Exact scalarization thresholds and onset laws for a nonminimally coupled scalar field on the Ellis wormhole

We study a real scalar field nonminimally coupled to curvature on a fixed Ellis wormhole background. The scalar perturbation decouples at linear order, and the substitution $ x=a \tan z $ reduces its static radial equation to a trigonometric Pöschl-Teller problem. This yields the exact threshold spectrum $ ζ_{n\ell}=(n+\ell+1)^2/2 $, closed-form Gegenbauer profiles, and an exact count of negative radial eigenvalues in each angular sector. The s-wave thresholds are zero-energy resonances at which the growth rate rises linearly, whereas the $ \ell \geq1\ $ threshold states are square integrable and the growth rate rises as a square root. At the first threshold we also obtain the next-order term, $ σa=(π/4)g-(3π/16)g^2+O(g^3) $ with $g=2ζ-1 $. For a probe field with quartic self-interaction, we show that no nontrivial spherical static solution exists for $ζ\leq1/2 $ and derive local branch expansions near the s-wave thresholds. Numerical evolutions are consistent with saturation on the nodeless branch. These nonlinear results refer to the probe-field model; they do not establish the existence or stability of fully backreacted scalarized wormholes.

gr-qc↗

arXiv2026

Holonomy classification, gyroscope precession, and Landau levels in Gödel-type spacetimes and their superfluid analog

We derive the parallel-transport holonomies of circles centered on the symmetry axis in the homogeneous Gödel-type family of Rebouças and Tiomno, which includes the Gödel universe, the causal member with $m^2=4Ω^2$, and the Som--Raychaudhuri limit. The holonomy is governed by a single invariant $Δ(r)$, whose sign determines whether the Lorentz transformation is elliptic, parabolic, or hyperbolic. We determine the number of parabolic radii across the family and prove that, whenever the azimuthal circles become closed timelike curves (CTCs), the chronology radius satisfies $Δ(r_c)=-\cosh(mr_c)<0$. We also express this invariant in terms of Killing-vector scalars, derive the vector and spinor Wilson loops, give the exact condition for trivial holonomy after a finite number of turns, and analyze Fermi--Walker transport on circular CTCs. Finally, we discuss an analogy with the effective metric of a rotating superfluid, valid under stated conditions, and derive exact expressions for the radial $L^2$ localization of Klein--Gordon Landau modes relative to the Som--Raychaudhuri chronology radius. The geometric results are exact within the metric family considered; the discussion of the analog system relies on the validity of a coarse-grained effective description.

gr-qc↗

arXiv2026

Quasinormal Ringing of a Schwarzschild Black Hole Surrounded by a King Dark Matter Halo: Higher-Order WKB Analysis

We study scalar and electromagnetic perturbations of a Schwarzschild black hole (BH) surrounded by a dark matter halo described by the King density profile. The lapse function can be expressed as $1-2[M+M_{\rm DM}(r)]/r$, where $M_{\rm DM}(r)$ is the halo mass enclosed within radius $r$; thus, the halo affects the geometry through its enclosed-mass function. We compute the quasinormal frequencies using the sixth-order WKB method and its Padé-improved version. Rewriting the lapse function in terms of the inverse hyperbolic sine avoids the severe numerical cancellations that arise when evaluating its high-order derivatives. For the configurations examined, the fundamental frequencies agree with those extracted from time-domain evolution to within $0.2\%$.The halo lowers both the oscillation frequency and the damping rate. The change of the real part is almost entirely accounted for by the halo mass enclosed by the photon sphere, and the additional decrease of the damping rate follows the ratio of the Lyapunov exponent to the angular frequency of the circular null orbit. Since part of the halo lies inside the horizon in this metric, we also place the halo outside the BH by means of an Einstein cluster. The frequencies then decrease by a similar amount, but the real part follows the Schwarzschild value multiplied by the gravitational redshift factor of the halo. A jump in the density at the inner edge of the halo scatters the waves and changes the damping rate by up to about $9\%$. For a continuous density profile, deviations from the redshift relation remain below about $1.5\%$ for $r_{\rm in}=3M$ and below about $0.5\%$ for the cases with $r_{\rm in}\geq4M$. No unstable modes are found within the perturbation sectors and parameter configurations examined.

gr-qc↗

arXiv2026

Magnetic localization and spectral asymmetry of a charged particle at a screw dislocation

We study a charged particle moving outside a spiral (screw) dislocation in a uniform magnetic field. Although the geometry is locally Euclidean, its global screw identification couples the conserved axial and azimuthal quantities through the combination $\mathcal{J}=\ell-βk$ After eliminating the cyclic coordinates, the radial motion is governed by an inverse-square term determined by $\mathcal{J}$ and a harmonic confining term generated by the magnetic field. The corresponding reduced quantum problem is exactly solvable for $q\mathcal{B}_0\neq0$, yielding an equally spaced spectrum with an orientation-dependent energy shift. The radial probability density depends on $\mathcal{J}$, and opposite values of the screw-modified quantum number have identical spatial profiles but different energies for a fixed magnetic-field orientation. The radial size scales as $\sqrt{\langle r^2\rangle_n}\propto |q\mathcal{B}_0|^{-1/2}$, with the magnetic length setting the localization scale. At $q\mathcal{B}_0=0$, the confining term vanishes, and the problem reduces to a scale-free inverse-square system. Thus, the screw parameter enters the dynamics through the global coupling of the conserved axial and azimuthal quantities, whereas the magnetic field provides radial confinement and its orientation determines the spectral asymmetry.

cond-mat.mes-hall↗

arXiv2026

Charged Klein-Gordon modes on an Ellis wormhole: magnetic confinement and conditional Heun solvability

We study charged Klein-Gordon modes on a $(2+1)$-dimensional Ellis traversable wormhole in an external magnetic field defined as uniform in an auxiliary Euclidean embedding space. The spatial geometry is intrinsically catenoidal, while the embedding is used only to construct the external gauge connection. Pullback of the ambient potential yields a regular azimuthal gauge field whose associated intrinsic magnetic scalar reverses sign across the throat. Separation of the Klein-Gordon equation leads to a radial problem that is unitarily equivalent to a one-dimensional Schrödinger operator with a regular effective potential. For nonzero magnetic coupling, the quadratic asymptotic term produces radial confinement on the complete wormhole. The radial equation can be reduced to the confluent-Heun class. Polynomial truncation is possible only on constrained parameter submanifolds determined by simultaneous termination conditions and therefore does not represent the generic confined spectrum. We derive the corresponding relativistic energies and analyze the lowest polynomial sector, while distinguishing the formal continuous-$\ell$ solvability correlations from the integer values of the azimuthal separation constant required by the standard angular periodicity condition.

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

Singularity-Rank Signatures in Generalized Dirac Oscillators near a BTZ Horizon

We study singular generalized Dirac-oscillator couplings in the near-horizon region of a nonextremal BTZ black hole. An effective covariant construction adapted to the stationary horizon congruence relates the two singular profiles to the proper acceleration and its radial gradient, whose leading near-horizon behaviors are $1/ρ$ and $1/ρ^2$. This motivates the family $\mathcal{L}_p(ρ)=λ_0+β_p/ρ^p$, with $p=1,2$. The $p=1$ system has a regular singular point and is Fuchsian, whereas $p=2$ is the first irregular case; more generally, a leading $ρ^{-p}$ interaction with $p>1$ has Poincaré rank $p-1$. For $p=1$, a constant spinor transformation reduces the matrix-Coulomb system exactly to the Whittaker equation, while elimination in the original basis gives an equivalent confluent-Heun representation with an apparent finite singularity. For $p=2$, the local branches contain the essential factors $e^{\pmβ_2/ρ}ρ^{-1/2}$, and their coefficient expansions exhibit generic factorial late-order growth. The finite-radius response $\mathcal{Z}_p(Ω;ρ_0)=ψ_{2,p}(ρ_0)/ψ_{1,p}(ρ_0)$ obeys an exact Riccati equation whose large-damping expansion first becomes sensitive to the radial gradient of the interaction at order $γ^{-3}$. Even when the two couplings are matched at $ρ_0$, this response retains information about their different pole orders. The response functions provide local spinorial data for subsequent matching to the complete BTZ exterior.

hep-th↗

arXiv2026

Non-Hermitian $\mathcal{PT}$ symmetric Klein-Gordon fields in Lorentz-violating wormholes spacetime background

We investigate a $\mathcal{PT}$-symmetric Klein--Gordon (KG) oscillator in a $(3+1)$-dimensional Lorentz-violating (LV) traversable wormhole spacetime. The geometry is characterized by a smooth throat of radius $a$ and a constant lapse function, $\mathcal{A}(x)=1$, so that no Killing horizons occur and the two asymptotic regions at $x\to \pm\infty$ remain connected through the throat. The Lorentz violation modifies the radial geometry and the oscillator scale according to $\widetildeΩ=Ω/\sqrt{1-ζ}$. A nonminimal non-Hermitian coupling, $\mathcal{F}_t(x)=i\widetildeΩx$, leads to a $\mathcal{PT}$-symmetric KG oscillator with a regular effective potential at the throat and a quadratic confinement at large $|x|$. The radial equation can be mapped to the confluent Heun equation, and polynomial truncation gives a conditionally exactly solvable sector in which the oscillator frequency, throat radius, LV parameter, angular momentum, and radial quantum number satisfy algebraic constraints. For the degree-1 sector, these conditions restrict the allowed angular momenta and correlate them with the LV parameter. The resulting bound-state energies are real and discrete, with exact particle--antiparticle symmetry, $E^{-}=-E^{+}$, while the corresponding wave functions are regular and localized on the two-sided wormhole geometry. These results show how Lorentz violation and the $\mathcal{PT}$-symmetric coupling modify the relativistic bound-state spectrum through the geometry of the wormhole.

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

Optical and Thermodynamic Signatures of Lorentz Symmetry Breaking in Bumblebee AdS Black Holes

We investigate the impact of spontaneous Lorentz symmetry breaking on scalar wave propagation, null geodesics, and thermodynamic behavior of four-dimensional asymptotically AdS black holes in bumblebee gravity. The static, spherically symmetric solutions are characterized by a dimensionless parameter $\ell > -1$ arising from the vacuum expectation value of the bumblebee vector field, which globally rescales the radial geometry. Massless scalar fields are analyzed via the radial Klein--Gordon equation cast into a generalized Helmholtz form, yielding an effective frequency-dependent refractive index that identifies oscillatory and evanescent regions, classical turning points, and confinement induced by curvature and Lorentz violation. In the high-frequency limit, wave propagation coincides with null geodesics, with $\ell$ controlling radial scaling and governing the geometric-optics limit. The AdS boundary reflects waves, while the horizon acts as a one-way absorber. Thermodynamic analysis in non-extended and extended phase spaces confirms the first law and Smarr relation, with $\ell$ influencing heat capacity, free energy, and stability. \textcolor{black}{Modeling these black holes as heat engines, we construct explicit cycles and show that efficiency increases with $\ell$, leading to an upper bound imposed by $η\leq 1$. Our results provide a framework connecting Lorentz violation, wave propagation, geometric optics, and AdS black hole thermodynamics in bumblebee gravity.

gr-qc↗

arXiv2026

Spectroscopic structure of Non-Hermitian $\mathcal{PT}$-Symmetric Klein--Gordon Fields in a Magnetized Cosmic String Spacetime

We investigate non-Hermitian $\mathcal{PT}$-symmetric Klein--Gordon (KG) fields in a magnetized cosmic string spacetime. A complex non-minimal scalar interaction, $\mathcal{F}_μ=(0,\mathcal{F}_{r},0,0)$ with $\mathcal{F}_{r}=ωr+ib\in\mathbb{C}$, indulged with a magnetic charge $q=ie$, is shown to introduce an effective $\mathcal{PT}$-symmetric Klein--Gordon oscillator supplemented by complex Coulombic and linear interactions. The resulting radial equation is shown to be conditionally exactly solvable using a biconfluent Heun series-to-polynomial approach. To observe non-Hermitian $\mathcal{PT}$ symmetrization, we compare with the Hermitian counterpart by mapping $e=-i\tilde e \Rightarrow q\in \mathbb R$ and $b=-i\tilde b \Rightarrow \mathcal F_r\in \mathbb R$. In the limiting case $ω=0$, the system is shown to reduce to an exactly solvable non-Hermitian $\mathcal{PT}$ symmetric Coulomb-type KG equation using confluent hypergeometric series/polynomials. Hereby, for both $\mathcal{PT}$ symmetric models considered, we show that non-Hermitian $\mathcal{PT}$ symmetrization introduces an upper limit for the allowed energies, unlike the corresponding Hermitian cases. These results provide an analytically tractable framework for exploring non-Hermitian relativistic quantum fields in curved spacetimes and demonstrate the role of $\mathcal{PT}$-symmetry in regulating the allowed spectrum.

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

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

Schrödinger and Klein-Gordon oscillators in Eddington-inspired Born-Infeld gravity: Degree-one Confluent Heun polynomial correspondence

We investigate Schrödinger and Klein-Gordon (KG) oscillators in the spacetime of a global monopole (GM) within Eddington inspired Born-Infeld (EiBI) gravity, including, in the relativistic sector, the coupling to a Wu-Yang magnetic monopole (WYMM). By reducing the radial equations to the confluent Heun form and enforcing termination of the Heun series, we obtain conditionally exact solutions in which the radial eigenfunctions truncate to polynomials of degree $(n+1)\geq 1$. This truncation imposes algebraic constraints that quantize the oscillator frequency and restrict the values allowed for the orbital angular momenta $\ell$. In the lowest nontrivial case $n=0$, the degree-one Heun polynomial yields a closed analytic expression for the frequency and determines a finite upper bound on $\ell$, dictated jointly by the EiBI deformation and the GM deficit. The resulting parametric correlations reveal a sharp geometric control of the spectrum: EiBI nonlinearities and the angular deficit fix the admissible bound states through polynomial truncation conditions. The confluent Heun correspondence is made explicit, providing a rigorous and reproducible framework for extracting analytical solutions from otherwise non-polynomial Heun structures. Applying the same method to the KG oscillator with a WYMM, we derive conditionally exact particle and antiparticle energies in a closed form. The relativistic spectrum exhibits perfect charge symmetry and a precise dependence on the WYMM strength, the EiBI parameter and the angular momentum constraint. To the best of our knowledge, this constitutes the first unified and fully consistent treatment of conditionally exact Schrödinger and Klein-Gordon oscillators in EiBI gravity based on a degree-one confluent Heun polynomial.

physics.gen-ph↗

arXiv2026

Lorentz-Violating Wormhole Optics

We study massless spin-1 field propagation in a static, circularly symmetric $(2+1)$-dimensional wormhole with spatial Lorentz-violating anisotropy characterized by the throat radius $a$ and deformation parameter $η$. The geometry is horizon-free, geodesically complete, and asymptotically flat, with negative Gaussian curvature localized near the throat. Using the fully covariant vector boson formalism and covariant Maxwell theory, we derive an exact Schrödinger-type radial equation with a curvature-induced effective potential. Recasting the dynamics in Helmholtz form yields an effective refractive-index profile, showing that the wormhole acts as an inhomogeneous optical medium with position-dependent refractive index and frequency-dependent confinement, where low-frequency modes are strongly trapped while high-frequency modes propagate almost freely. A differential-geometric correspondence with helicoidal surfaces is established via $1/[a^2(1-η)] \leftrightarrow w^2$, demonstrating that Lorentz-violation-induced curvature is mathematically equivalent to curvature generated by geometric twist and linking the model to twisted graphene nanoribbons as analog-gravity platforms. These results provide a geometric framework for curvature-driven localization, dispersion, and anisotropic wave propagation in topologically nontrivial $(2+1)$-dimensional backgrounds.

gr-qc↗

arXiv2026

Wave Propagation and Effective Refraction in Lorentz-Violating Wormhole Geometries

We study the propagation of massless scalar waves in static, spherically symmetric Lorentz-violating wormhole spacetimes within a geometric-optical framework. Starting from a general metric characterized by an arbitrary lapse function and areal radius, we derive curvature invariants, establish regularity conditions at the wormhole throat, and reduce the Klein-Gordon equation to a Helmholtz-type radial wave equation. This formulation naturally leads to a position- and frequency-dependent effective refractive index determined by the underlying spacetime geometry and Lorentz-violating structure, resulting in effective frequency-dependent wave-optical behavior. We show that divergences of the refractive index coincide with Killing horizons, while curvature-induced turning points control reflection, transmission, and confinement of scalar waves. By analyzing constant, linear, and quadratic lapse profiles, we identify horizonless transmission regimes, asymmetric wave propagation, and multi-horizon trapping structures. Our results reveal that Lorentz violation can significantly modify wave-optical properties of curved spacetime, generating graded-index analogues and geometric confinement of modes without curvature singularities. This unified optical perspective provides a robust framework for investigating wave scattering, resonances, and potential observational signatures in Lorentz-violating gravitational backgrounds.

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↗