arXiv subjects

Ali Övgün

Publications and source records attributed to Ali Övgün.

At least 55 records · Page 3Linked to original sources

arXiv2025

Axisymmetric black hole in a non-commutative gauge theory: classical and quantum gravity effects

This work explores both classical and quantum aspects of an axisymmetric black hole within a non-commutative gauge theory. The rotating solution is derived using a modified Newman-Janis procedure. The analysis begins with the horizon structure, ergospheres, and angular velocity. The thermodynamic properties are examined through surface gravity, focusing on the Hawking temperature, entropy, and heat capacity. In addition, the remnant mass is calculated. The Hawking radiation is treated as a tunneling process for bosonic and fermionic particles, along with the corresponding particle creation density. Geodesic motion is explored, emphasizing null geodesics, radial accelerations, the photon sphere, and black hole shadows. Finally, the gravitational lensing in the strong deflection limit is investigated.

gr-qc↗

arXiv2025

Analytical approach for calculating shadow of dynamical black hole

We develop a compact and transparent framework for photon dynamics and shadow formation in slowly evolving, spherically symmetric spacetimes. Starting from the Eddington-Finkelstein action, we derive a force-decomposed radial equation in which the radial acceleration splits into an induced term sourced by mass variation, a centrifugal term, and a purely general-relativistic correction. A key result is a gauge-invariant energy-flux relation, $d(E^2)/dv=-Λ/r$, with $Λ\equiv \varepsilon\,\dot M\,\dot v^2$, which controls how time dependence modifies the canonical energy of null geodesics. In the adiabatic regime we obtain an explicit first-order shift of the photon-sphere radius, $r_{ph}(v)=r_0-a_i/(a_g'+a_c')$, and connect it to the observable shadow through the evolving critical impact parameter, $b_{\rm crit}(v)=\sqrt{r_{ph}(v)^2/f(r_{ph}(v))}$. For Vaidya spacetimes this predicts that accretion ($\dot M>0$) expands the photon sphere and increases the shadow angle, whereas mass loss has the opposite effect. Our formulation refines classic force-balance ideas to dynamical settings, provides a constructive link to time-dependent photon surfaces, and yields simple, observer-ready expressions for the evolution of the shadow. The framework offers a baseline for confronting time-variable horizon-scale imaging with dynamical inflow/outflow models.

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arXiv2025

Shedding Light on Gravity: Black Hole Shadows and Lensing Signatures in Lorentz Gauge Theory

Recent advances, including gravitational wave detections and imaging of black hole shadows, have strongly validated general relativity. Nevertheless, ongoing cosmological observations suggest potential limitations of general relativity, spurring interest in modified theories of gravity. This work investigates the Lorentz-gauge formulation of gravity-a novel framework that addresses key conceptual challenges in quantum gravity and cosmology by leveraging the recent black hole solutions presented in Ref. \cite{Koivisto:2024asr}. By analyzing black hole shadow structures and gravitational lensing effects-both weak and strong deflection regimes-we highlight unique observational signatures of Lorentz gauge gravity. Our findings provide valuable tools for future observational tests, potentially distinguishing these modified gravity models from general relativity and advancing our understanding of spacetime geometry and fundamental gravitational interactions.

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arXiv2025

The CosmoVerse White Paper: Addressing observational tensions in cosmology with systematics and fundamental physics

The standard model of cosmology has provided a good phenomenological description of a wide range of observations both at astrophysical and cosmological scales for several decades. This concordance model is constructed by a universal cosmological constant and supported by a matter sector described by the standard model of particle physics and a cold dark matter contribution, as well as very early-time inflationary physics, and underpinned by gravitation through general relativity. There have always been open questions about the soundness of the foundations of the standard model. However, recent years have shown that there may also be questions from the observational sector with the emergence of differences between certain cosmological probes. In this White Paper, we identify the key objectives that need to be addressed over the coming decade together with the core science projects that aim to meet these challenges. These discordances primarily rest on the divergence in the measurement of core cosmological parameters with varying levels of statistical confidence. These possible statistical tensions may be partially accounted for by systematics in various measurements or cosmological probes but there is also a growing indication of potential new physics beyond the standard model. After reviewing the principal probes used in the measurement of cosmological parameters, as well as potential systematics, we discuss the most promising array of potential new physics that may be observable in upcoming surveys. We also discuss the growing set of novel data analysis approaches that go beyond traditional methods to test physical models. [Abridged]

astro-ph.CO↗

arXiv2025

Multimodal signatures of asymptotic (A)dS Kalb-Ramond black holes: Constraints through the shadow, weak deflection angle, and topological photon spheres

This study explores novel static, neutral black hole solutions within Kalb-Ramond (KR) gravity in asymptotically (anti-)de Sitter [(A)dS] spacetimes, incorporating spontaneous Lorentz symmetry breaking (LSB) via an antisymmetric tensor field. Focusing on two metric configurations, we derive general analytical expressions for the horizon radius, photon sphere, shadow radius, and weak gravitational deflection angle. The universality of these expressions enables their applicability to a broad class of non-rotating spacetimes beyond KR gravity. By confronting these models with empirical data from the Event Horizon Telescope (EHT) and Solar System experiments, the work yields tight constraints on the Lorentz-violating parameter $\ell$, demonstrating that certain parameterizations - particularly Case B - yield observationally viable and physically consistent outcomes. Additionally, we employ topological methods to analyze black hole thermodynamics and photon sphere stability, uncovering critical geometrical structures in both thermodynamic and optical contexts. These multimodal signatures collectively offer a powerful framework for testing Lorentz-violating extensions of General Relativity (GR) and highlight the potential of topological diagnostics in gravitational physics.

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arXiv2025

Analyzing Deflection Angles and Photon Sphere Dynamics of Magnetically Charged Black Holes in Nonlinear Electrodynamic

In this paper, we investigate the gravitational lensing properties of magnetically charged black holes within the framework of nonlinear electrodynamics. We derive the deflection angle and examine the influence of the nonlinear electrodynamics parameter $ξ$ on light bending. We initially employ a geometric approach based on the Gauss-Bonnet theorem to analyze the gravitational deflection of null and timelike particles. This method encapsulates the global characteristics of the lensing effect in an elegant manner. In the subsequent part of the work, we explore the impact of nonlinear electromagnetic corrections on the black hole shadow. Using numerical techniques, we study the behavior of the photon sphere and demonstrate that a reduction in the photon sphere radius leads to a correspondingly smaller shadow. We compare these results with those for the Schwarzschild and Reissner-Nordström black holes, highlighting the distinctive features introduced by nonlinear electrodynamics. Furthermore, we examine the strong deflection limit for light trajectories near these black holes, focusing on the roles of both the magnetic charge $Q$ and the nonlinear parameter $ξ$. Our analysis reveals that the combined effects of $Q$ and $ξ$ enhance the strong deflection angle, resulting in a more pronounced lensing effect than that predicted by the classical Reissner-Nordström solution. These findings suggest that the nonlinear interactions may provide a potential observational signature for identifying NED black holes.

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arXiv2025

Testing black holes with cosmological constant in Einstein-bumblebee gravity through the black hole shadow using EHT data and deflection angle

This study probes spacetime solutions within Einstein-Bumblebee gravity, a modified gravitational framework incorporating spontaneous Lorentz symmetry violation through a vector field mechanism. By introducing a cosmological constant into this model, the research scrutinizes thermodynamic properties of black holes in both anti-de Sitter (AdS) and de Sitter (dS) geometries. The investigation demonstrates how Lorentz-violating parameters alter foundational thermodynamic principles, including revisions to the first law of black hole mechanics and shifts in critical phenomena during phase transitions. Notably, the bumblebee coupling parameter emerges as a critical factor governing horizon structure and thermal emission characteristics, with pronounced deviations from general relativity (GR) predictions observed as this parameter increases. The analysis extends to observational signatures by calculating shadow profiles of these modified black holes. Shadow morphology exhibits dual dependence on the cosmological constant and the bumblebee parameter, presenting measurable discrepancies from classical relativity that could be constrained through Event Horizon Telescope (EHT) observational data. Furthermore, using geometric formalisms, the study quantifies light deflection phenomena in weak and strong gravitational regimes. Results reveal that both the cosmological constant and Lorentz-violating parameter induce detectable modifications to lensing angles compared to Schwarzschild or Kerr benchmarks. These deviations, while subtle, underscore the necessity for next-generation astronomical instruments capable of resolving fine-scale spacetime curvature effects.

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arXiv2025

Regular Black Hole Models in the Transition from Baryonic Matter to Quark Matter

In this paper, we investigate gravitational collapse scenarios involving baryonic matter transitioning into quark-gluon plasma under extreme astrophysical conditions, focusing on their implications for the formation of regular black holes. Standard gravitational collapse models inevitably predict central singularities, highlighting the limitations of classical general relativity in extreme density regimes. By introducing a physically motivated, inhomogeneous transition rate between baryonic and quark matter, we demonstrate analytically and numerically that it is possible to construct regular black hole solutions featuring a nonsingular de Sitter-like core. We further analyze the observable consequences of these models, particularly emphasizing modifications to the black hole shadow radius, which provide direct observational constraints accessible through Event Horizon Telescope (EHT) measurements.

gr-qc↗

arXiv2025

Formation of regular black hole from baryonic matter

We present a family of exact, singularity-free solutions describing the collapse of baryonic matter characterized by a barotropic equation of state whose coefficient $α(r,v)$ varies in both radius and time. By matching these interior solutions to the Husain exterior metric, we obtain a self-consistent, dynamical spacetime representing a regular black hole. Although the pressure profile of our models grows with radius and eventually violates the dominant energy condition beyond a critical surface-necessitating an external junction to ensure a globally well-defined spacetime-the interior solution remains non-singular throughout the collapse. We further analyze the optical properties of these regular black holes and find that both the photon sphere radius and the corresponding shadow radius increase monotonically as the local equation of state parameter $α$ is raised. Moreover, the matching interface between the interior and exterior metrics naturally suggests a phase transition in the collapsing fluid, which can postpone the formation of an apparent horizon. Taken together, our results not only highlight novel physical features of horizon formation in regular collapse models but also identify characteristic shadow signatures that could be tested by future observations.

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arXiv2025

Quantum gravity effects on particle creation and evaporation in a non-commutative black hole via mass deformation

In this work, we explore a gravitational non-commutative black hole by gauging the de Sitter SO(4,1) group and employing the Seiberg-Witten map. Specifically, we examine modifications of non-commutativity represented through mass deformation. Initially, we address modifications to Hawking radiation for bosonic particle modes by analyzing the Klein-Gordon equation in curved spacetimes. We compute the Bogoliubov coefficients, showing how $Θ$ introduces a correction to the amplitude associated with particle creation. Additionally, we derive the power spectrum and the Hawking temperature within this framework. We also derive Hawking radiation from a tunneling perspective, leading to expressions for the power spectrum and particle number density. A similar analysis is performed for fermion particles. Remarkably, we obtain an analytical expression for black hole evaporation lifetime and compare our results with recent estimates of non-commutativity in the literature.

gr-qc↗

arXiv2025

Probing Weak-Force Corrections to Black Hole Geometry via Long Range Potentials: Feinberg-Sucher and Ferrer-Nowakowski Potentials

We consider corrections to the Schwarzschild black hole metric arising from exotic long-range forces within quantum field theory frameworks. Specifically, we analyze two models: the Feinberg-Sucher potential for massless neutrinos and Ferrer-Nowakowski potentials for boson-mediated interactions at finite temperatures, yielding metric corrections with $r^{-5}$ and $r^{-3}$ dependencies. Using analytic expansions around the Schwarzschild photon sphere, we find that attractive potential corrections enhance gravitational lensing, enlarging the photon sphere and shadow radius, while repulsive potential corrections induce gravitational screening, reducing these observables. Our results clearly illustrate how different quantum-derived corrections can produce measurable deviations from standard Schwarzschild predictions, providing robust theoretical benchmarks for future astrophysical observations.

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arXiv2025

Charged black holes in Kalb-Ramond gravity: Weak Deflection Angle, Shadow cast, Quasinormal Modes and Neutrino annihilation

In this paper, we investigate the phenomenology of electrically charged black holes in a Lorentz-violating gravitational framework mediated by a background Kalb-Ramond (KR) antisymmetric tensor field. Employing the Gauss-Bonnet theorem in a non-asymptotically flat geometry, we derive analytic expressions for the weak deflection angle of light and massive particles, revealing persistent corrections due to the Lorentz-violating parameter $ \ell $. Scalar and Dirac perturbations are also studied using both the WKB approximation and the Poschl-Teller approximation approach to verify the stability of the solution against these types of perturbations. Shadow analysis further uncovers a nontrivial deformation of the photon sphere and critical impact parameter, with KR-induced effects modifying the charge contribution in a manner incompatible with standard Einstein-Maxwell theory. Constraints derived from Event Horizon Telescope data for Sgr A* and M87* validate the model and provide stringent bounds on $ \ell $, establishing the KR framework as an observationally testable extension of General Relativity.

gr-qc↗

arXiv2025

Traversable Wormholes in Einstein-Euler-Heisenberg Gravity: Geometry, Energy Conditions, and Gravitational Lensing

In this study, we investigate traversable wormholes within the framework of Einstein-Euler-Heisenberg (EEH) nonlinear electrodynamics. By employing the Einstein field equations with quantum corrections from the Euler-Heisenberg Lagrangian, we derive wormhole solutions and examine their geometric, physical, and gravitational properties. Two redshift function models are analyzed: one with a constant redshift function and another with a radial-dependent function $Φ=r_{0}/r$. Our analysis demonstrates that the inclusion of quantum corrections significantly influences the wormhole geometry, particularly by mitigating the need for exotic matter. The shape function and energy density are derived and examined in both models, revealing that the energy conditions, including the weak and null energy conditions (WEC and NEC), are generally violated at the wormhole throat. However, satisfaction of the strong energy condition (SEC) is observed, consistent with the nature of traversable wormholes. The Arnowitt-Deser-Misner (ADM) mass of the EEH wormhole is calculated, showing contributions from geometric, electromagnetic, and quantum corrections. The mass decreases with the Euler-Heisenberg correction parameter, indicating that quantum effects contribute significantly to the wormhole mass. Furthermore, we investigate gravitational lensing within the EEH wormhole geometry using the Gauss-Bonnet theorem, revealing that the deflection angle is influenced by both the electric charge and the nonlinear parameter. The nonlinear electrodynamic corrections enhance the gravitational lensing effect, particularly at smaller impact parameters.

gr-qc↗

arXiv2025

Geodesics, accretion disk, gravitational lensing, time delay, and effects on neutrinos induced by a non-commutative black hole

This paper explores gravitational phenomena associated with a non-commutative black hole. Geodesic equations are derived, and a thin accretion disk is analyzed to model the black hole shadow image, considering an optically thin, radiating, and infalling gas. Retrolensing effects are examined to trace photon emission configurations, while gravitational lensing is investigated through weak and strong deflection limits, with lensing equations and observables applied to Sagittarius A*. The study also includes calculations of time delay, energy deposition rate from neutrino annihilation, phase and probability of neutrino oscillation, and neutrino gravitational lensing.

gr-qc↗

arXiv2025

Black Hole Solutions in Dark Photon Models with Higher Order Corrections

In this work, we derive new analytic, static, symmetric black hole solutions in theories involving dark photons with minimal and higher-order magnetic dipole interactions. Starting from the effective non-relativistic potential between fermions mediated by a dark photon, we derive explicit corrections to the Schwarzschild geometry induced by dark photon and spin-dependent terms. These corrections alter the metric significantly at short distances, modifying the horizon radius, Hawking temperature, photon sphere, and consequently, the black hole shadow. Employing perturbative expansions, we provide analytic expressions for the deviations from the Schwarzschild solution, highlighting an exponential suppression controlled by the dark photon mass. Our results demonstrate that higher-order magnetic dipole interactions produce distinctive spin-dependent curvature terms, amplifying gravitational effects near the horizon. These findings provide a theoretical foundation for future phenomenological tests of dark photon models through gravitational wave astronomy and black hole imaging, while highlighting dark photons' role as mediators of dark matter interactions that can influence structure formation and direct detection experiments.

gr-qc↗

arXiv2025

Strange Quark Stars and Condensate Dark Stars in Bumblebee Gravity

In this paper, we investigate the properties of relativistic stars made of isotropic matter within the framework of the minimal Standard Model Extension, where a Bumblebee field (BF) coupled to spacetime induces spontaneous Lorentz symmetry breaking. We adopt analytic equations-of-state describing either condensate dark stars or strange quark stars. We solve the structure equations numerically, and we compute the mass-to-radius relationships. The influence of the Bumblebee parameter $\mathbf{l}$ is examined in detail, and an upper bound is obtained using the massive pulsar (PSR) J0740+6620 and the strangely light High Energy Stereoscopic System (HESS) J1731-347 compact object.

gr-qc↗

arXiv2025

New Black Hole Solutions of Second and First Order Formulations of Nonlinear Electrodynamics

Inspired by the so-called Palatini formulation of General Relativity and of its modifications and extensions, we consider an analogous formulation of the dynamics of a self-interacting gauge field which is determined by non-linear extension of Maxwell's theory, usually known as nonlinear electrodynamics. In this first order formalism the field strength and the gauge potential are treated, a priori as independent, and, as such, varied independently in order to produce the field equations. Accordingly we consider within this formalism alternative and generalized non-linear Lagrangian densities, some of them of a new kind which gives up the restriction of equivalence to second order Lagrangians. Several new spherically-symmetric objects are constructed analytically and their main properties are studied. The solutions are obtained in flat spacetime ignoring gravity and for the self-gravitating case with emphasis on black holes. As a background for comparison between the first and second order formalisms, some of the solutions are obtained by the conventional second order formalism, while for others a first order formalism is applied. Among the self-gravitating solutions we find new families of black holes and study their main characteristics. Some of the flat space solutions can regularize the total energy of a point charge and a subset of them exhibit also finite field strength and energy density, although their black hole counterparts are not regular.

gr-qc↗

arXiv2025

Black hole with global monopole charge in self-interacting Kalb-Ramond field

In this study, we explore a static, spherically symmetric black hole solution in the context of a self-interacting Kalb-Ramond field coupled with a global monopole. By incorporating the effects of Lorentz-violating term $\ell$ and the monopole charge $η$ in the KR field, we derive the modified gravitational field equations and analyze the resulting black hole spacetime. The obtained solution exhibits deviations from the Schwarzschild metric with topological defect, as it is influenced by the monopole charge and self-interaction potential. We investigate the thermodynamic properties of the black hole, including its Hawking temperature, entropy, and specific heat, revealing novel stability conditions. Additionally, we perform solar system tests such as perihelion precession, gravitational redshift, light deflection, and time delay of signals to impose constraints on the Lorentz-violating parameter and monopole charge. Our findings suggest that these parameters have to be significantly small, although there are different constraints imposed by individual tests, ranging from $10^{-9}\leq|\ell|\leq 10^{-4}$ and $10^{-9}\leqη\leq 10^{-6}\, \mathrm{m}^{-1}$.

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