arXiv subjects

Ali Ovgun

Publications and source records attributed to Ali Ovgun.

13 recordsLinked to original sources

arXiv2026

Josephson interferometry in an Oppenheimer--Snyder-like scale-dependent black-hole spacetime

We develop a covariant framework for Josephson transport, superconducting interference, and shunt noise in the scale-dependent exterior generated by an Oppenheimer--Snyder-like collapse. Static Josephson frequencies and transported currents referred to Killing time acquire one lapse factor, whereas power acquires two. For a junction comoving with the collapsing surface, the coordinate-time phase rate differs from the frequency received at infinity because of null propagation and Doppler effects. We derive the exact radial-null travel-time kernel and show that its local near-extremal logarithmic enhancement crosses over, at fixed emission offset, to pole-controlled extremal behavior. We also obtain the redshifted resistively and capacitively shunted-junction equation and the dc and microwave-driven two-junction interference envelopes. In the negligible-total-inductance limit, static lapse imbalance modifies lobe amplitudes without shifting their centers, whereas microwave-induced translations require dynamical fluxoid closure. In a Hartle--Hawking state, Tolman redshift produces a superconducting exclusion layer and a lapse-independent asymptotic shunt-noise spectrum; its low-frequency limit obeys a parameter-free fluctuation--delay relation. Finally, we derive a shadow--Josephson consistency relation and sensitivity bounds on the running parameter.

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arXiv2026

Optical-area minimum method for static spherical black hole shadows

We formulate a global optical-area method for shadows of static, spherically symmetric black holes. For the metric $ds^{2}=-A(r)dt^{2}+B(r)dr^{2}+C(r)d\Omega^{2}$, spherical sections of the optical geometry have area $\mathcal{A}_{\rm opt}=4\pi C/A$. A null ray with impact parameter $b$ can cross a spherical section only if $b^{2}\leq C/A$. The capture threshold is fixed by the infimum of $C/A$ on the connected interval between the observer and the black hole horizon. When attained at an interior point, the infimum gives $b_{\rm sh}^{2}=\min(C/A)$, while a static observer outside the controlling minimum, on the inward-sky branch, measures $\sin^{2}\alpha_{\rm sh}=\mathcal{A}_{*}/ \mathcal{A}_{\rm opt}(r_{\rm o})$. The usual photon-sphere equation follows when the minimum occurs at a smooth interior point. Exponential instability additionally requires the minimum to be nondegenerate. The local optical-radius and photon-sphere formulas are established results. Our contribution is to organize them into an observer-to-horizon global selection rule that compares all stationary candidates and relevant endpoint limits. The radial function $B(r)$ does not affect the shadow angle, although it enters the coordinate-time instability rate, whose numerical value also depends on the normalization of the static time coordinate. We derive compact first- and second-order formulas for deformed metrics, demonstrate candidate comparison with a synthetic two-minimum profile, and apply the construction to Reissner--Nordstr\"om, Bardeen, charged dilaton, and Kottler black holes. An explicit transformation of the charged-dilaton example from a nonareal to an areal radial coordinate verifies radial-coordinate invariance, while the Kottler example probes a nonasymptotically flat static region.

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arXiv2026

Two-scale magnetically charged regular black holes from nonlinear electrodynamics and a T-duality-inspired zero-point length

We construct a two-scale, static, spherically symmetric regular black hole in Einstein gravity sourced by magnetic nonlinear electrodynamics (NED). The zero-point length $\ell$ regularizes the mass and charge profiles, whereas $q$ is the asymptotic magnetic charge. The geometry approaches Reissner-Nordstr\"om at large radius, reduces to the neutral zero-point-length solution for $q=0$, and coincides geometrically with the Ay\'on-Beato-Garc\'ia solution for $\ell=|q|$. For $q\neq0$, inverse reconstruction gives a single-valued magnetic Lagrangian with Maxwell asymptotics and a finite strong-field limit. The center is regular and is de Sitter, locally Minkowski, or anti-de Sitter according to the sign of $2M\ell-q^2$; the weak energy condition holds globally if and only if $3M\ell\geq2q^2$. We derive the extremality curve, the exact heat capacity, and homogeneous horizon-variation and Smarr identities while retaining the Wald area entropy. We also prove that every charged black hole in this family has a nondegenerate extraordinary NED optical metric throughout the domain of outer communication. The associated capture shadow is selected by the global minimum of the optical impact-parameter function and generally differs from the background-geodesic shadow. Weak-field calculations yield the periapsis, bending, time-delay, and redshift corrections; in particular, $\ell$ first appears beyond the standard first-post-Newtonian parameters. Finally, for minimally coupled test radiation in a cold transparent plasma, we obtain exact parametric shadow relations for power-law density profiles and combine Hamiltonian ray tracing with a Novikov-Thorne disk model. A separate extraordinary NED-plasma continuation is displayed only as a phenomenological prescription because a material plasma breaks the conformal ambiguity of the vacuum characteristic metric.

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arXiv2026

Scattering, Hawking Radiation and Neutrino Energy Deposition in Euler-Heisenberg Black Holes Surrounded by Perfect Fluid Dark Matter

We study the dynamical and scattering properties for the Euler-Heisenberg BH surrounded by perfect fluid dark matter. The geometry contains a compact non-linear electrodynamic correction governed by the EH coupling and a logarithmic dark-matter contribution governed by the surrounding PFDM halo. We study the scalar, electromagnetic and a particular effective axial spin-2 channel constructed on the fixed EH plus PFDM background, acting as a proxy for the gravitational-like perturbation problem and not for the fully coupled gravitational perturbation problem. We compute the quasinormal-mode spectrum employing a thirteenth-order WKB method supplemented with Pad\'e resummation and compare it with the eikonal prediction calculated in terms of the angular frequency of the photon sphere and the Lyapunov exponent. Moreover, we study the near-extremal configurations and derive a purely imaginary branch of quasinormal frequencies in the near-horizon region, whose damping rate increases with the PFDM parameter and is nearly spin-independent. We then compute exact greybody factors by direct numerical integration of the radial wave equation and compare them to analytical lower bounds. We also analyze the absorption cross sections and the Hawking emission spectra. We also calculate the relativistic enhancement of the neutrino-antineutrino annihilation channel outside the EHPFDM black hole. We find that the PFDM parameter contracts the optical structure, increases the oscillation frequency, enhances the damping rate and suppresses transmission. On the other hand, the Euler-Heisenberg correction leads to a weaker near-horizon deformation whose effect becomes relevant for sufficiently large charge. These results provide a common scattering framework for comparing the impact of dark-matter environments and nonlinear electrodynamics

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arXiv2026

Breaking Parameter Degeneracies in a Magnetically Charged Black Hole Embedded in a Hernquist Dark-Matter Halo: A Multi-Observable Analysis

We study the degeneracy of intrinsic and environmental parameters for BH observables in a static spacetime sourced by a nonlinear magnetic monopole immersed in a Hernquist dark-matter halo. We explore four complementary probes; the shadow radius $R_{sh}$, eikonal quasinormal-mode frequencies $M\omega_R$, weak gravitational lensing $\hat{\theta}_\infty$, and neutrino-antineutrino annihilation $\dot{Q}/\dot{Q}_{Newt}$, and map their degeneracy contours in the $(g/\mathcal{M},\alpha/\mathcal{M})$ plane at fixed $\beta/\mathcal{M}$. Different parameter combinations yield signatures nearly indistinguishable from a Schwarzschild black hole single-observable diagnostics cannot uniquely constrain the magnetic charge and halo amplitude. The degeneracy contours are, however, mutually non-parallel: the slopes $d\alpha/dg$ along constant-$R_{sh}$ and constant-$M\omega_R$ contours differ by a factor $\sim 5$, so their combination breaks the remaining degeneracy and constrains both parameters simultaneously. We compute the QNMs spectra using a high-order WKB method with Pad\'e resummation. The magnetic charge raises the real oscillation frequency while the halo lowers it; the cancellation is observable-dependent and does not persist across all four channels. An expansion around an asymptotically renormalized Schwarzschild background of mass $\mathcal{M}=M+\alpha$ shows that at fixed $\mathcal{M}$ both sectors reduce $R_{sh}$ at first perturbative order. For weak lensing, $\mathcal{M}$ alone determines the leading deflection, first subleading correction depends on $\mathcal{Q}=g^2+4\alpha\beta$, separating total halo mass from halo concentration. For neutrino-pair annihilation, the magnetic charge suppresses the deposition rate by raising the lapse, while the halo enhances it through the reverse mechanism.

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arXiv2026

Echoes of a hairy black hole from gravitational decoupling

We study axial gravitational perturbations of a hairy black hole constructed within the framework of gravitational decoupling and investigate the geometric origin of echo-like late-time signals in this spacetime. We derive the odd-parity master equation and the corresponding effective potential, and compute the quasinormal mode spectrum using frequency-domain and time-domain methods. We show that the axial potential develops a double-peak structure that supports a trapping cavity and gives rise to echo-like waveforms. The representative double-peak configurations also exhibit an unstable--stable--unstable light-ring structure, which provides the null-geodesic counterpart of the trapping cavity in the eikonal limit. Therefore, the echoes are not produced by an artificially imposed near-horizon condition, but instead emerge dynamically from the geometric structure of the effective potential. Our results provide a useful framework for probing black hole hair through gravitational-wave ringdown and for exploring possible observational departures from the standard no-hair paradigm.

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arXiv2025

Ringdown of a black hole sourced by a Burkert-density effective anisotropic source

We construct a static, spherically symmetric black hole spacetime sourced by an effective anisotropic matter distribution whose energy density follows the cored Burkert profile. The source should be understood as a Burkert-density effective fluid, rather than as a microscopic model of pressureless collisionless dark matter. Solving the Einstein equations under this closure condition, we obtain an analytic Schwarzschild-like metric that reduces smoothly to the vacuum Schwarzschild solution when the halo contribution vanishes. We then study axial gravitational perturbations of this geometry and determine the associated quasinormal spectrum using complementary frequency-domain and time-domain methods. We find that increasing either the core radius $r_0$ or the central density $\rho_0$ shifts the ringdown toward lower frequency and weaker damping, with the effect of $r_0$ being more pronounced. The close agreement among the numerical extractions supports the reliability of the results. Our analysis provides a useful benchmark for assessing how a cored Burkert-type effective environment can modify black hole ringdown.

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arXiv2025

Probing Quantum Gravity in Stellar Spacetimes: Phenomenological Insights

We explore the weak-field phenomenology of a compact star spacetime modified by quantum gravitational corrections derived from the effective field theoretical (EFT) approach by Calmet et al. [1]. These corrections, encoded in non-local curvature-squared terms, distinguish matter-supported geometries from vacuum solutions by contributing nontrivial modifications at second order in G. Using the corrected metric, we analytically derive expressions for the deflection of light and time-like particles via the Gauss-Bonnet theorem. We further compute the perihelion advance of Mercury, Shapiro time delay, and gravitational redshift within this framework. Each classical observable acquires quantum corrections that, though exceedingly small represent potential imprints of quantum gravity. The Shapiro delay and redshift likewise exhibit finite, source-dependent deviations from their general relativistic predictions due to the modified temporal metric component. While current observational capabilities remain insufficient to detect these minute effects, the analysis demonstrates that quantum gravitational signatures are embedded even in weak-field observables. Last, we study massless scalar perturbations in static, spherically symmetric spacetimes by analyzing their quasinormal modes (QNMs) and greybody factors using the WKB method and Pade resummation. Our findings demonstrate that increasing the coupling parameter enhances spacetime stability and significantly influences emission spectra through frequency-dependent transparency. Moreover, the results underscore that quantum-corrected star metrics yield phenomenological distinctions from classical black holes, particularly near the Planck scale, where vacuum solutions lose validity.

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arXiv2025

Phenomenology of Schwarzschild-like Black Holes with a Generalized Compton Wavelength

We investigate the influence of the generalized Compton wavelength (GCW), emerging from a three-dimensional dynamical quantum vacuum (3D DQV) on Schwarzschild-like black hole spacetimes, motivated by the work of Fiscaletti [10.1134/S0040577925020096] \cite{Fiscaletti:2025iuh}. The GCW modifies the classical geometry through a deformation parameter $ \varepsilon $, encoding quantum gravitational backreaction. We derive exact analytical expressions for the black hole shadow radius, photon sphere, and weak deflection angle, incorporating higher-order corrections and finite-distance effects of a black hole with generalized Compton effect (BHGCE). Using Event Horizon Telescope (EHT) data, constraints on $ \varepsilon $ are obtained: $ \varepsilon \in [-2.572, 0.336] $ for Sgr. A* and $ \varepsilon \in [-2.070, 0.620] $ for M87*, both consistent with general relativity yet allowing moderate deviations. Weak lensing analyses via the Keeton-Petters and Gauss-Bonnet formalisms further constrain $ \varepsilon \approx 0.061 $, aligning with solar system bounds. We compute the modified Hawking temperature, showing that positive $ \varepsilon $ suppresses black hole evaporation. Quasinormal mode frequencies in the eikonal limit are also derived, demonstrating that both the oscillation frequency and damping rate shift under GCW-induced corrections. Additionally, the gravitational redshift and scalar perturbation waveform exhibit deformations sensitive to $ \varepsilon $. Our results highlight the GCW framework as a phenomenologically viable semiclassical model, offering testable predictions for upcoming gravitational wave and VLBI observations.

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arXiv2023

Probing Schwarzschild-like Black Holes in Metric-Affine Bumblebee Gravity with Accretion Disk, Deflection Angle, Greybody Bounds, and Neutrino Propagation

In this paper, we investigate Schwarzschild-like black holes within the framework of metric-affine bumblebee gravity. We explore the implications of such a gravitational setup on various astrophysical phenomena, including the presence of an accretion disk, the deflection angle of light rays, the establishment of greybody bounds, and the propagation of neutrinos. The metric-affine bumblebee gravity theory offers a unique perspective on gravitational interactions by introducing a vector field that couples to spacetime curvature. We analyze the behavior of accretion disks around Schwarzschild-like black holes in this modified gravity scenario, considering the effects of the bumblebee field on the accretion process. Furthermore, we scrutinize the deflection angle of light rays as they traverse the gravitational field, highlighting potential deviations from standard predictions due to the underlying metric-affine structure. Investigating greybody bounds in this context sheds light on the thermal radiation emitted by black holes and how the modified gravity framework influences this phenomenon. Moreover, we explore neutrino propagation around Schwarzschild-like black holes within metric-affine bumblebee gravity, examining alterations in neutrino trajectories and interactions compared to conventional general relativity. By comprehensively probing these aspects, we aim to unravel the distinctive features and consequences of Schwarzschild-like black holes in the context of metric-affine bumblebee gravity, offering new insights into the nature of gravitational interactions and their observable signatures.

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arXiv2022

Effects of Variable Equations of State on the Stability of Nonlinear Electrodynamics Thin-Shell Wormholes

This paper explores the role of nonlinear electrodynamics on the stable configuration of thin-shell wormholes formulated from two equivalent geometries of Reissner-Nordström black hole with nonlinear electrodynamics. For this purpose, we use cut and paste approach to eliminate the central singularity and event horizons of the black hole geometry. Then, we explore the stability of the developed model by considering different types of matter distribution located at thin-shell, i.e., barotropic model and variable equations of state (phantomlike variable and Chaplygin variable models). We use linearized radial perturbation to explore the stable characteristics of thin-shell wormholes. It is interesting to mention that Schwarzschild and Reissner-Nordström black holes show the unstable configuration for such type of matter distribution while Reissner-Nordström black hole with nonlinear electrodynamics expresses stable regions. It is found that the presence of nonlinear electrodynamics gives the possibility of a stable structure for barotropic as well as variable models. It is concluded that stable region increases for these models by considering higher negative values of coupling constant $α$ and the real constant $n$.

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arXiv2017

Tunneling of Massive/Massless Bosons From the Apparent Horizon of FRW Universe

In this article we investigate the Hawking radiation of vector particles from the apparent horizon of a Friedmann-Robertson-Walker (FRW) universe in the framework of quantum tunneling method. Furthermore we use Proca equation, a relativistic wave equation for a massive/massless spin-1 particle (massless $γ$ photons, weak massive $W^{\pm}$ and $Z^{0}$ bosons, strong massless gluons and $ρ$ and $ω$ mesons) together with a Painlevé spacetime metric for the FRW universe. We solve the Proca equation via Hamilton-Jacobi (HJ) equation and the WKB approximation method. We recover the same result for the Hawking temperature associated with vector particles as in the case of scalar and Dirac particles tunnelled from outside to the inside of the apparent horizon in a FRW universe.

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arXiv2017

Anisotropic stellar models admitting conformal motion

We address the problem of finding static and spherically symmetric anisotropic compact stars in general relativity that admit conformal motions. The study is framed in the language of f(R) gravity theory in order to expose opportunity for further study in the more general theory. Exact solutions of compact stars are found under the assumption that spherically symmetric spacetimes which admits conformal motion with matter distribution anisotropic in nature. In this work, two cases have been studied for the existence of such solutions: first, we consider the model given by f(R) = R and then f(R) = aR+b. Finally, specific characteristics and physical properties have been explored by analytically along with graphical representations for conformally symmetric compact stars in f(R) gravity.

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