arXiv subjects

Ali Övgün

Publications and source records attributed to Ali Övgün.

At least 73 records · Page 4Linked to original sources

arXiv2025

Weak Gravitational Lensing in Ricci-Coupled Kalb-Ramond Bumblebee Gravity: Global Monopole and Axion-Plasmon Medium Effects

In this paper, we study the influence of the axion-plasmon medium, as proposed in [10.1103/PhysRevLett.120.181803]\cite{Tercas:2018gxv}, on the optical properties of black holes in a Lorentz-violating spacetime containing a global monopole. Our primary aim is to provide a test for detecting the effects of a fixed axion-plasmon background within the framework of Ricci-coupled Kalb-Ramond bumblebee gravity. By extending the conventional Einstein-bumblebee model through a nonminimal coupling between the Kalb-Ramond field and the Ricci tensor, we demonstrate that the combined presence of a global monopole and Lorentz-violating effects induces significant modifications to the classical Schwarzschild lensing signature. Employing the Gauss-Bonnet theorem within an optical geometry approach, we derive an analytical expression for the deflection angle that incorporates both linear and quadratic contributions from the Lorentz-violating parameter and the monopole charge. Furthermore, we investigate how the axion-plasmon coupling alters light propagation, affecting key observable gravitational deflection angle. Our results indicate that these optical characteristics are notably sensitive to the axion-plasmon parameters, thereby offering promising observational signatures for probing new physics beyond standard general relativity.

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arXiv2025

Shadow and quasinormal modes of the rotating Einstein-Euler-Heisenberg black holes

The Einstein-Euler-Heisenberg (EEH) black hole model is an extension of classical black hole solutions in general relativity, incorporating quantum electrodynamics (QED) effects via the Euler-Heisenberg Lagrangian. The Euler-Heisenberg Lagrangian describes the nonlinear corrections to Maxwell's equations due to virtual electron-positron pair production in a strong electromagnetic field. When this Lagrangian is coupled with Einstein's field equations, it leads to modified black hole solutions that take into account these quantum corrections. In this paper, we investigate the impact of the black hole charge $Q_e$ on the properties of the rotating and electrically charged Einstein-Euler-Heisenberg black holes (EEH). To this aim, we analyzed and discussed findings as to how the black hole charge $Q_e$ affects certain black hole properties such as null regions, shadow cast and its observables, and quasinormal modes (QNMs) relative to the Kerr and Kerr-Newman cases. We find that the presence of a screened charge due to the associated QED effects in this screened Maxwell theory might noticeably alter the properties of black holes, offering insights into the interplay between gravity and quantum field effects.

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arXiv2025

Black hole in the Dekel-Zhao dark matter profile

Motivated by the work of Cardoso et al. [Phys. Rev. D 105 (2022) 6, L061501, https://doi.org/10.1103/PhysRevD.105.L061501] on black holes in galaxies, we derive a new black hole solution surrounded by a Dekel-Zhao (DZ) dark matter profile. The derived metric, influenced by DZ profile parameters, exhibits two distinct regimes: for $r \ll r_{\rm c}$ (\textcolor{black}{$r_{\rm c}$ is a characteristic radius}), exponential corrections dominate, producing significant deviations from the Schwarzschild solution near dense cores, while for $r \gg r_{\rm c}$, these corrections vanish, restoring the Schwarzschild metric at large distances. These findings ensure consistency with general relativity in vacuum. \textcolor{black}{The black hole shadow and deflection angle are analyzed, demonstrating that the shadow radius increases with black hole mass ($M_{\rm BH}$), while higher central densities ($ρ_{\rm ch}$) result in smaller shadows, reflecting the environmental impact of dense dark matter halos.} Photon dynamics reveal how DZ profiles modify critical impact parameters and effective potentials, with gravitational lensing effects highly sensitive to the characteristic radius ($r_{\rm c}$). Smaller $r_{\rm c}$ values lead to larger deflection angles due to stronger gravitational effects near compact cores. This work highlights the significance dark matter profiles in shaping black hole observables, providing a theoretical foundation for future observational studies and advancing the understanding of dark matter-black hole interactions in astrophysical and cosmological contexts.

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arXiv2025

Weak field deflection angle and analytical parameter estimation of the Lorentz-violating Bumblebee parameter through the black hole shadow using EHT data

We explored how the Lorentz symmetry breaking parameter $\ell$ affects the Reissner-Nordstöm BH solution in the context of weak field deflection angle, and the black hole shadow. We aim to derive the general expression for the weak deflection angle using the non-asymptotic version of the Gauss-Bonnet theorem, and we presented a way to simplify the calculations under the assumption that the distance of the source and the receiver are the same. \textcolor{black}{Through the Solar System test, $\ell$ is constrained from around $-10^{-9}$ orders of magnitude to $0$, implying challenging detection of $\ell$ through the deflection of light rays from the Sun.} We also studied the black hole shadow in an analytic way, where we applied the EHT results under the far approximation in obtaining an estimate expression for $\ell$. Using the realistic values of the black hole mass and observer distance for Sgr. A* and M87*, it was shown that $M/r_{\rm o} \neq 1$ is satisfied, implying the relevance and potential promise of the spontaneous Lorentz symmetry breaking parameter's role on the shadow radius uncertainties as measured by the EHT. \textcolor{black}{We find constraints for $\ell$ to be negatively valued, where the upper and lower bounds are $\sim -1.94$ and $\sim -2.04$, respectively.

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arXiv2025

Exploring light deflection and black hole shadows in Rastall theory with plasma effects

In this article, we examine the gravitational deflection of particles in curved spacetime immersed in perfect fluid in the context of Rastall theory. We propose an infinite region approach to Gibbons-Werner to avoid singularity, given that the integral region is generally infinite. In the Rastall theory framework, the black hole solutions in the dust field are studied. Additionally, we check the deflection angle from this spacetime under the influence of plasma. Furthermore, we analytically compute plasma's impact on a black hole shadow using a ray-tracing approach and Hamiltonian equation. Hence, the light ray motion equations are independent of the plasma's velocity. It is assumed that plasma is a dispersive medium, pressureless and non-magnetised, and the plasma particle density corresponds to particle accumulation. The supermassive black hole's shadow and emitted energy are explored when plasma falls radially from infinity onto the black hole.

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arXiv2025

Spacetime-curvature induced uncertainty principle: linking the large-structure global effects to the local black hole physics

This paper links the advanced formulation of the Generalized Uncertainty Principle, termed the Asymptotic Generalized Extended Uncertainty Principle (AGEUP), to the corpuscular framework to derive the AGEUP-inspired black hole metric. The former incorporates spacetime curvature effects to explore black hole dynamics under quantum gravitational corrections, while the latter is a view that black holes are Bose-Einstein condensates of weakly interacting gravitons. In a particular case, the phenomenological union between the AGEUP with cosmological constant $Λ$ to the corpuscular framework enabled a black hole metric that has a scaled mass, which depends on $Λ$ and the Planck length $l_{\rm Pl}$. Interesting implications occur, such as the maximum limit for mass $M$ where $Λ$ ceases to influence the black hole. Another is the derived value of the modulation factor of the EUP term, $α$, if the large-scale fundamental length is defined solely as the cosmological horizon. Additional analyses were done through the shadow and deflection angle phenomena, deriving constraints on the quantum gravity modulation parameter $β$. Constraints from the Event Horizon Telescope (EHT) and Very Long Baseline Interferometry (VLBI) are discussed as avenues for verifying AGEUP-related deviations in black hole shadow radius and deflection angles, offering potential observational evidence of quantum gravitational effects at astrophysical scales. The findings suggest that AGEUP could be instrumental in providing hints on the quantum gravity nature of black holes, particularly in high-energy astrophysical contexts. By linking local black hole physics with large-scale curvature effects, AGEUP paves the way for further research at the intersection of quantum gravity and cosmology, with implications for observational astrophysics and the fundamental structure of spacetime.

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arXiv2025

Modified particle dynamics and thermodynamics in a traversable wormhole in bumblebee gravity

In this work, we analyze various phenomena influenced by the gravitational field in a bumblebee gravity solution, with a particular emphasis on a traversable wormhole for massless particle modes. Specifically, we calculate the index of refraction, group velocity, time delay, modified distances, and interparticle potential, demonstrating the possibility of photon-photon interactions due to the wormhole geometry. For the latter aspect, we also extend the analysis to massive particle modes, resulting in a ``combination'' of modified Yukawa- and Couloumb-like potentials. These calculations are shown to be dependent on the wormhole's parameters, particularly the wormhole throat. In addition to these analyses, the Hawking temperature is derived using the trapping horizon method, yielding negative values. Furthermore, we derive the thermodynamic properties of photon-like modes by incorporating the modified dispersion relation arising from the wormhole geometry, focusing on non-interacting particle modes. Remarkably, all calculations are conducted in a fully analytical framework.

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arXiv2025

Properties of an axisymmetric Lorentzian non-commutative black hole

In this work, we start by examining a spherically symmetric black hole within the framework of non-commutative geometry and apply a modified Newman-Janis method to obtain a new rotating solution. We then investigate its consequences, focusing on the horizon structure, ergospheres, and the black hole's angular velocity. Following this, a detailed thermodynamic analysis is performed, covering surface gravity, Hawking temperature, entropy, and heat capacity. We also study geodesic motion, with particular emphasis on null geodesics and their associated radial accelerations. Additionally, the photon sphere and the resulting black hole shadows are explored. Finally, we compute the quasinormal modes for scalar perturbations using the 6th-order WKB approximation.

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arXiv2024

Quasinormal Modes and Greybody Factors of Charged Symmergent Black Hole

In this paper, we investigate quasinormal modes (QNMs) and greybody factors within the framework of Symmergent gravity, an emergent gravity model with an $R + R^2$ curvature sector. Building on our previous work on static spherically-symmetric solutions [Class. Quant. Grav. 40 (2023) 19, 195003], we explore the effects of the key parameters, including the quadratic curvature coupling parameter $c_{\rm O}$ and the vacuum energy parameter $α$. For both scalar and electromagnetic perturbations, an increase in $α$ leads to a nearly linear rise in both oscillation frequencies and damping rates. The other parameter $c_{\rm O}$ affects the QNMs spectrum nonlinearly. Additionally, the charge $Q$ of the black hole introduces nonlinear behavior, where higher charges amplify the black hole's electromagnetic field, resulting in increased oscillation frequencies and faster stabilization. These findings enhance our understanding of charged black hole stability and gravitational wave astrophysics. Further, the analysis of greybody factors reveals that increasing $α$, $c_{\rm O}$, and $Q$ reduces the absorption of radiation, with electromagnetic perturbations reaching maximum absorption at slightly lower frequencies compared to scalar perturbations.

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arXiv2024

Exact Regular Black Hole Solutions with de Sitter Cores and Hagedorn Fluid

In this paper, we present three exact solutions to the Einstein field equations, each illustrating different black hole models. The first solution introduces a black hole with a variable equation of state, $P = k(r)ρ$, which can represent both singular and regular black holes depending on the parameters $M_0$ and $w_0$. The second solution features a black hole with Hagedorn fluid, relevant to the late stages of black hole formation, and reveals similarities to the first solution by also describing both singular and regular black holes in a specific case. Furthermore, we investigate the shadows cast by these black hole solutions to constrain their parameters. Recognizing that real astrophysical black holes are dynamic, we developed a third, dynamical solution that addresses gravitational collapse and suggests the potential formation of naked singularities. This indicates that a black hole can transition from regular to singular and back to regular during its evolution.

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arXiv2024

Influence of GUP corrected Casimir energy on zero tidal force wormholes in modified teleparallel gravity with matter coupling

In recent times, the study of the Casimir effect in quantum field theory has garnered increasing attention because of its potential to be an ideal source of exotic matter needed for stabilizing traversable wormholes. It has been confirmed through experimental evidence that this phenomenon involves fluctuations in the vacuum field, leading to a negative energy density. Motivated by the above, we have investigated Casimir wormholes with corrections from the Generalized Uncertainty Principle (GUP) within the framework of matter-coupled teleparallel gravity. Our analysis includes three well-known GUP models: the Kempf, Mangano, and Mann (KMM) model, the Detournay, Gabriel, and Spindel (DGS) model, and a third model called Model II. For a broader analysis, we have considered two well-known model functions for the teleparallel theory: a linear $f(T,\mathcal{T})=αT+β\mathcal{T}$ and a quadratic model $f(T,\mathcal{T})=ηT^2+χ\mathcal{T}$. The shape function solutions corresponding to both models are examined in the absence of tidal forces in spacetime. We also demonstrate the crucial role played by the parameters of the $f(T,\mathcal{T})$ models in the violation of the energy conditions. With the increasing interest in detecting gravitational waves from astrophysical objects, we have thoroughly discussed the perturbation of the wormhole solutions in the scalar, electromagnetic, axial gravitational, and Dirac field backgrounds. We employ the $3^{rd}$ order WKB expansion to find the complex frequencies associated with the quasinormal modes of energy dissipation. Additionally, we also calculate the active mass and total gravitational energy for the wormhole geometry. The amount of exotic matter involved in sustaining these wormholes is also found in this paper. Furthermore, the physical stability of such Casimir wormholes is examined using the Tolman-Oppenheimer-Volkoff equation.

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arXiv2024

Topological dyonic black holes of massive gravity with generalized quasitopological electromagnetism

In this paper we investigate new dyonic black holes of massive gravity sourced by generalized quasitopological electromagnetism in arbitrary dimensions. We begin by deriving the exact solution to the field equations defining these black holes and look at how graviton's mass, dimensionality parameter, and quasitopological electromagnetic field affect the horizon structure of anti-de Sitter dyonic black holes. We also explore the asymptotic behaviour of the curvature invariants at both the origin and infinity to analyze the geometric structure of the resultant black holes. We also compute the conserved and thermodynamic quantities of these dyonic black holes with the help of established techniques and known formulas. After investigating the relevancy of first law, we look at how various parameters influence the local thermodynamic stability of resultant black hole solution. We also examine how thermal fluctuations affect the local stability of dyonic black holes in massive gravity. Finally, we study the shadow cast of the black hole.

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arXiv2024

An Effective Model for the Quantum Schwarzschild Black Hole: Weak Deflection Angle, Quasinormal Modes and Bounding of Greybody Factor

In this paper, we thoroughly explore two crucial aspects of a quantum Schwarzschild black solution within four-dimensional space-time: i) the weak deflection angle, ii) the rigorous greybody factor and, iii) the Dirac quasinormal modes}. Our investigation involves employing the Gauss-Bonnet theorem to precisely compute the deflection angle and establishing its correlation with the Einstein ring. Additionally, we derive the rigorous bounds for greybody factors through the utilization of general bounds for reflection and transmission coefficients in the context of Schrodinger-like one-dimensional potential scattering. We also compute the corresponding Dirac quasinormal modes using the WKB approximation. We reduce the Dirac equation to a Schrodinger-like differential equation and solve it with appropriate boundary conditions to obtain the quasinormal frequencies. To visually underscore the quantum effect, we present figures that illustrate the impact of varying the parameter $r_0$, or more specifically, in terms of the parameter $α$. This comprehensive examination enhances our understanding of the quantum characteristics inherent in the Schwarzschild black solution, shedding light on both the deflection angle and greybody factors in a four-dimensional space-time framework.

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arXiv2024

Effects of non-commutative geometry on black hole properties

In this study, we investigate the signatures of a non-commutative black hole solution. Initially, we calculate the thermodynamic properties of the system, including entropy, heat capacity, and Hawking radiation. For the latter quantity, we employ two distinct methods: surface gravity and the topological approach. Additionally, we examine the emission rate and remnant mass within this context. Remarkably, the lifetime of the black hole, after reaching its final state due to the evaporation process, is expressed analytically up to a grey-body factor. We estimate the lifetime for specific initial and final mass configurations. Also, we analyze the tensorial quasinormal modes using the 6th-order WKB method. Finally, we study the deflection angle, i.e., gravitational lensing, in both the weak and strong deflection limits.

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arXiv2024

Traces of quantum gravitational correction at third-order curvature through the black hole shadow and particle deflection at the weak field limit

This study investigates the impact of the quantum-gravity correction at the third-order curvature ($c_6$) on the black hole's shadow and deflection angle on the weak field regime, both involving finite distances of observers. While the calculation of the photonsphere and shadow radius $R_{\rm sh}$ can easily be achieved by the standard Lagrangian for photons, the deflection angle $α$ employs the finite-distance version of the Gauss-Bonnet theorem (GBT). We find that the photonsphere reduces to the classical expression $r_{\rm ph} = 3M$ for both the Planck mass and the theoretical mass limit for BH, thus concealing the information about the applicability of the metric on the quantum and astrophysical grounds. Our calculation of the shadow, however, revealed that $c_6$ is strictly negative and constrains the applicability of the metric to quantum black holes. For instance, the bounds for the mass is $M/l_{\rm Pl} \in [0.192, 4.315]$. We also derived the analytic formula for the observer-dependent shadow, which confirms $c_6$'s influence on quantum black holes even for observers in the asymptotic regions. The influence of such a parameter also strengthens near the quantum black hole. Our analytic calculation of $α$ is shown to be independent of $c_6$ if the finite distance $u \rightarrow 0$, and $c_6$ is not coupled to any time-like geodesic. Finally, the effect of $c_6$ manifests in two ways: if $M^2$ is large enough to offset the small value of $l_{\rm Pl}$ (which is beyond the theoretical mass limit), or if $b$ is comparable to $l_{\rm Pl}$ for a quantum black hole.

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arXiv2024

Imprints of a gravitational wave through the weak field deflection of photons

This paper investigates the novel phenomenon of gravitational lensing experienced by gravitational waves traveling past a Schwarzschild black hole perturbed by a specific, first-order, polar gravitational wave. We utilize the Gauss-Bonnet theorem, uncovering a topological contribution to the deflection of light rays passing near the black hole. We demonstrate that the deflection angle can be determined by analyzing a region entirely outside the light ray's path, leading to a calculation based solely on the parameters of the perturbing wave (Legendre polynomial order, $l$, and frequency, $σ$). This approach offers a unique perspective on gravitational lensing and expands our understanding of black hole interactions with gravitational waves.

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arXiv2024

Analyzing the Influence of Geometrical Deformation on Photon Sphere and Shadow Radius: A New Analytical Approach -- Spherically Symmetric Spacetimes

In this paper, we introduce a new approach to study the behavior of the photon sphere and shadow radius. Our method uses extended gravitational decoupling and reveals two important analytic results. First, the additional matter field alters the photon sphere radius: it increases if $g'(r_{ph}^{(0)})>0$ and decreases if $g'(r_{ph}^{(0)})<0$ (where $g'$ represents the derivative of a specific metric function evaluated at the original photon sphere radius). Second, the presence of the matter field can modify the black hole shadow size. If $g\left(r_{ph}^{(0)}\right)>0$, the shadow shrinks, while it grows for $g\left(r_{ph}^{(0)}\right)<0$. These findings provide a deeper insight into how matter distribution influences the characteristics of black holes and their observable features. Through a systematic framework and various illustrative examples, our investigation not only clarifies these fundamental aspects but also significantly enhances the theoretical framework of black hole astrophysics.

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arXiv2024

General Approach on Shadow Radius and Photon Spheres in Asymptotically Flat Spacetimes and the Impact of Mass-Dependent Variations

Recent observations of black hole shadows have revolutionized our ability to probe gravity in extreme environments. This manuscript presents a novel analytic method to calculate, in leading-order terms, the key parameters of photon sphere and shadow radius. This method offers advantages for complex metrics where traditional approaches are cumbersome. We further explore the impact of black hole mass on the photon sphere radius, providing insights into black hole interactions with their surroundings. Our findings contribute significantly to black hole physics and gravity under extreme conditions. By leveraging future advancements in observations, such as the next-generation Event Horizon Telescope (ngEHT), this work paves the way for even more precise tests of gravity near black holes.

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