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A. Övgün

Publications and source records attributed to A. Övgün.

At least 19 recordsLinked to original sources

Can finite-density QCD matter support a regular black hole core?

We investigate whether finite-density QCD matter can provide the stress-energy needed to regularize the center of a spherically symmetric black hole collapse. In a generalized Vaidya geometry, the field equations impose a vacuum-like radial pressure; we therefore model the QCD sector phenomenologically by identifying the transverse pressure with the effective QCD pressure. This is neither an isotropic equilibrium treatment nor a first principles simulation of nonequilibrium collapse. Local energy-momentum conservation is used to reconstruct radial temperature, density, and mass profiles for two QCD-inspired equations of state: a finite-chemical-potential chiral model and a finite-density mean-field quark-gluon-plasma model. Although the chiral model admits an exact Lambert-function solution, its physical high-temperature branch has a non-integrable central density. The mean-field model also yields singular temperature and density profiles. Neither produces the cubic near-center mass scaling required for finite curvature. More generally, regularity under the transverse null energy condition uniquely demands a vacuum-like relation between transverse pressure and energy density, which neither QCD closure approaches. Thus, finite quark chemical potential alters the phase-transition thermodynamics but does not generate a self-regularizing core in this framework. Positive-energy mixtures of baryonic matter and radiation also fail to provide the required stress. A smooth matching to an inner de Sitter-like component illustrates the additional short-distance physics needed.

astro-ph.HE↗

Signatures of a de Sitter-core black hole in ringing, transmission and optical appearance

We investigate a static, asymptotically flat black hole whose central region approaches a de Sitter vacuum. The geometry is controlled by the ADM mass $M_0$ and a core scale $R$, and is generated by an exponentially decaying anisotropic source. After clarifying the stress tensor and the horizon structure, we study scalar-field perturbations, greybody transmission, Hawking emission and the optical image produced by an optically thin infalling flow. The horizon analysis shows that two horizons exist below the critical value $R/M_0\simeq0.7768$, where they merge into an extremal configuration. Increasing the core scale lowers the peak of the scalar effective potential and shifts the quasinormal spectrum away from the Schwarzschild value, with the largest fractional deviations occurring for low multipoles and higher overtones within the WKB domain of validity. The greybody bound indicates stronger filtering as the core becomes more extended, while the QNM--greybody correspondence gives a complementary estimate of the transmission probability in the eikonal regime. The Hawking temperature decreases as the extremal configuration is approached, suppressing the emission rate and shifting its maximum to lower frequencies. For a simple optically thin infalling flow, ray tracing shows a small decrease in the apparent shadow scale and in the peak intensity as the core scale is increased.

gr-qc↗

Radial oscillations of pulsating neutron stars: The UCIa equation-of-state case

Radial oscillations provide a clean dynamical test of the high-density stiffness of neutron-star equations of state. We study spherically symmetric pulsations of nonrotating relativistic stars built from cold, charge-neutral, $β$-equilibrated pure nucleonic matter described within relativistic mean-field theory. As a baseline we adopt the UCIa parameter set [Astron. Astro-phys. 689, A242 (2024)], and we implement high-density stiffening via the $σ$-cut scheme by adding a regulator potential $U_{\rm cut}(σ)$ [Phys. Rev. C 92, no.5, 052801 (2015), Phys. Rev. C 106, no.5, 055806 (2022)]. For representative choices $f_s=0$ (no cutoff) and $f_s=0.58$ (stiffened), we solve the Tolman-Oppenheimer-Volkoff and tidal perturbation equations to obtain equilibrium sequences, mass-radius relations, and tidal deformabilities. We then derive and solve the linear general-relativistic radial pulsation equations to compute the eigenfrequencies and eigenfunctions of the fundamental and overtone modes. The $σ$-cutoff suppresses the growth of the scalar field at supranuclear density, increases the pressure, and shifts the maximum mass, radii, and $Λ_{1.4}$ accordingly, while systematically raising the radial-mode frequencies at fixed mass. Using the sign change of $ω_0^2$ as a stability criterion, we identify stiffened models that remain radially stable up to the observed $\sim 2M_\odot$ mass scale and are consistent with current multimessenger constraints, demonstrating how radial spectra complement static EoS tests.

nucl-th↗

Absorption, Scattering, Geodesics, Shadows and Lensing Phenomena of Black Holes in Effective Quantum Gravity

In this work, we investigate the signatures of black holes within an effective quantum gravity framework recently proposed in the literature [1] . We begin by outlining the general setup, highlighting the two distinct models under consideration. This includes a discussion of their general properties, interpretations, and the structure of the event and inner horizons. We then examine the behavior of light in this context, analyzing geodesics, the photon sphere, and shadow formation. To validate our results, we estimate lower bounds for the shadow radius based on observational data from the Event Horizon Telescope (EHT). Subsequently, we derive the partial radial wave equation for scalar perturbations, enabling us to study the absorption cross section in both low and high frequency regimes. Additionally, we evaluate the greybody factors and provide bounds for both bosonic and fermionic fields. Finally, we present a detailed analysis of gravitational lensing in both the weak and strong deflection limits. For the weak deflection regime, the Gauss Bonnet theorem is employed, while for the strong deflection limit, the Tsukamoto approach is utilized.

gr-qc↗

White Paper and Roadmap for Quantum Gravity Phenomenology in the Multi-Messenger Era

The unification of quantum mechanics and general relativity has long been elusive. Only recently have empirical predictions of various possible theories of quantum gravity been put to test, where a clear signal of quantum properties of gravity is still missing. The dawn of multi-messenger high-energy astrophysics has been tremendously beneficial, as it allows us to study particles with much higher energies and travelling much longer distances than possible in terrestrial experiments, but more progress is needed on several fronts. A thorough appraisal of current strategies and experimental frameworks, regarding quantum gravity phenomenology, is provided here. Our aim is twofold: a description of tentative multimessenger explorations, plus a focus on future detection experiments. As the outlook of the network of researchers that formed through the COST Action CA18108 ``Quantum gravity phenomenology in the multi-messenger approach (QG-MM)'', in this work we give an overview of the desiderata that future theoretical frameworks, observational facilities, and data-sharing policies should satisfy in order to advance the cause of quantum gravity phenomenology.

gr-qc↗

Hawking Radiation via Gauss-Bonnet Theorem

In this paper, we apply the topological method to the various black holes to derive their Hawking temperature. We show that the this method can easily be employed to compute the Hawking temperature of black holes having spherically symmetric topology. Therefore, we conclude that the topological method provides a consistent formula to achieve the Hawking temperature.

hep-th↗

Black hole with confining electric potential in scalar-tensor description of regularized 4-dimensional Einstein-Gauss-Bonnet gravity

In this paper, we analytically present an exact confining charged black hole solution to the scalar-tensor description of regularized 4-dimensional Einstein-Gauss-Bonnet gravity (4DEGBG) coupled to non-linear gauge theory containing $\sqrt{-F^{2}}$ term. Part of the importance of using a non-trivial $4D$ theory of Gauss-Bonnet gravity is that it is proven the absence of the Ostrogradsky instability was formulated in [D. Glavan and C. Lin, Phys. Rev. Lett. 124, 081301 (2020)] and then regularized 4DEGBG theory was obtained in [Hennigar et al. JHEP 07, 027 (2020)] and [Fernandes et al. Phys. Rev. D 102, no.2, 024025 (2020)] as well as this theory gains many attention because of its important finding for black hole physics. We therefore also study some properties of this solution such as temperature, specific heat, shadow and quasinormal modes. Our results here show that a confining charge $f$ gives a significant contribution to the shadow of the black hole as well as quasinormal modes frequencies.

gr-qc↗

Effect of null aether field on weak deflection angle of black holes

We study the light rays in a static and spherically symmetric gravitational field of null aether theory (NAT). To this end, we employ the Gauss-Bonnet theorem to compute the deflection angle by a NAT black hole in the weak limit approximation. Using the optical metrics of the NAT black hole, we first obtain the Gaussian curvature and then calculate the leading terms of the deflection angle. Our calculations show how gravitational lensing is affected by the NAT field. We also show once again that the bending of light stems from a global and topological effect.

gr-qc↗

Effect of the quintessential dark energy on weak deflection angle by Kerr-Newmann Black hole

In this work, we study the weak gravitational lensing in the background of Kerr-Newman black hole with quintessential dark energy. Initially, we compute the deflection angle of light by charged black hole with quintessential dark energy by utilizing the Gauss-Bnnet theorem. Firstly, we suppose the light rays on the equatorial plane in the axisymmetric spacetime. In doing so, we first find the corresponding optical metrics and then calculate the Gaussian optical curvature to utilize in Gauss-Bonnet theorem. Consequently, we calculate the deflection angle of light for rotating charged black hole with quintessence. Additionally, we also find the deflection angle of light for Kerr-Newman black hole with quintessential dark energy. In order to verify our results, we derive deflection angle by using null geodesic equations which reduces to the deflection angle of Kerr solution with the reduction of specific parameters. Furthermore, we analyze the graphical behavior of deflection angle $Θ$ w.r.t to impact parameter $b$. Our graphical analysis retrieves various results regarding to the deflection angle by the Kerr-Newman black hole with quintessential dark energy.

gr-qc↗

Effect of the dilaton field and plasma medium on deflection angle by black holes in Einstein-Maxwell-dilaton-axion theory

In this paper, we argue that one can calculate the weak deflection angle of light in the background of Einstein-Maxwell-Dilaton-Axion black hole using the Gauss-Bonnet theorem. To support this, the optical geometry of the black hole with the Gibbons-Werner method are used to obtain the deflection angle of light in the weak field limits. Moreover, we investigate the effect of a plasma medium on deflection of light for a given black hole. Because of dilaton and axion are one of the candidate of the dark matter, it can give us a hint on observation of dark matter which is supported by the black hole. Hence we demonstrate the observational viability via showing the effect of the dark matter on the weak deflection angle of light.

gr-qc↗

Weak gravitational lensing of regular black holes with cosmic strings using the Gauss-Bonnet theorem

In this paper, we investigate light bending in the spacetime of regular black holes with cosmic strings in weak field limits. To do so, we apply the Gauss-Bonnet theorem to the optical geometry of the black hole; and, using the Gibbons-Werner method, we obtain the deflection angle of light in the weak field limits which shows that the bending of light is a global and topological effect. Afterwards, we demonstrate the effect of a plasma medium on the deflection of light by RBCS. We discuss that increasing cosmic string parameter $μ$ and mass $M_0$ will increase the bending angle.

gr-qc↗

Eruptive Massive Vector Particles of 5-Dimensional Kerr-Gödel Spacetime

In this paper, we investigate Hawking radiation of massive spin-1 particles from 5-dimensional Kerr-Gödel spacetime. By applying the WKB approximation and the Hamilton-Jacobi ansatz to the relativistic Proca equation, we obtain the quantum tunneling rate of the massive vector particles. Using the obtained tunneling rate, we show how one impeccably computes the Hawking temperature of the 5-dimensional Kerr-Gödel spacetime.

gr-qc↗

The Bekenstein-Hawking Corpuscular Cascading from the Backreacted Black Hole

Exciting peculiarities of Planck-scale physics have immediate effects on the Bekenstein-Hawking radiation emitted from black holes (BHs). In this paper, using the tunneling formalism, we determine the Bekenstein-Hawking temperature for the vector particles from a backreacted black hole (BBH) constructed from a conformal scalar field surrounded by a BTZ (Banados-Teitelboim-Zanelli) BH. Then, under the effect of the generalized uncertainty principle, we extend our calculations for scalar particles to understand the effects of quantum gravity. Then, we calculate an evaporation time for the BBH, the total number of Bekenstein-Hawking particles, and the quantum corrections of the number. We observe that remnants of the BH evaporation occur and that they affect the Bekenstein-Hawking temperature of the BBH as well as the total number of Bekenstein-Hawking particles.

gr-qc↗

Inflation and Acceleration of the Universe by Nonlinear Magnetic Monopole Fields

Despite impressive phenomenological successes, cosmological models are incomplete without an understanding of what happened at the big bang singularity. Maxwell electrodynamics, considered as a source of the classical Einstein field equations, leads to the singular isotropic Friedmann solutions. In the context of Friedmann-Robertson-Walker (FRW) spacetime, we show that singular behavior does not occur for a class of nonlinear generalizations of the electromagnetic theory for strong fields. A new mathematical model is proposed for which the analytical nonsingular extension of FRW solutions is obtained by using the nonlinear magnetic monopole fields.

gr-qc↗

Hawking Radiation of Mass Generating Particles From Dyonic Reissner Nordström Black Hole

The Hawking radiation is considered as a quantum tunneling process, which can be studied in the framework of the Hamilton-Jacobi method. In this study, we present the wave equation for a mass generating massive and charged scalar particle (boson). In sequel, we analyze the quantum tunneling of these bosons from a generic 4-dimensional spherically symmetric black hole. We apply the Hamilton-Jacobi formalism to derive the radial integral solution for the classically forbidden action which leads to the tunneling probability. To support our arguments, we take the dyonic Reissner-Nordström black hole as a test background. Comparing the tunneling probability obtained with the Boltzmann formula, we succeed to read the standard Hawking temperature of the dyonic Reissner-Nordström black hole.

gr-qc↗

Existence of traversable wormholes in the spherical stellar systems

Potentiality of the presence of traversable wormholes in the outer/inner regions of the halos of galaxies, situated on the Navarro-Frenk-White (NFW) density profile and Universal Rotation Curve (URC) dark matter models have been investigated recently. Since this covers our own galaxy also as a possible home for traversable wormholes it prompts us to further the subject by considering alternative density distributions. From this token herein we make use of the Einasto model to describe the density profiles for the same purpose. Our choice for the latter is based on the fact that theoretical dark matter halos produced in computer simulations are best described by such a profile. For technical reasons we trim the number of parameters in the Einasto profile to a possible minimum. Based on such a model it is shown that traversable wormholes in the outer regions of spiral galaxies are possible while the inner part regions prohibit such formations.

gr-qc↗

On a Particular Thin-shell Wormhole

In this paper, using a black hole with scalar hair, we construct a scalar thin-shell wormhole (TSW) in 2+1 dimensions by applying the Visser's famous cut and paste technique. The surface stress, which is concentrated at the wormhole throat is determined by using the Darmois-Israel formalism. By employing the various gas models, we study the stability analysis of the TSW. The region of stability is changed by tuning the parameters of "l" and "u". It is observed that the obtained particular TSW originated from the black hole with scalar hair could be more stable with particular $l$ parameter, however it still needs exotic matter.

gr-qc↗

Entangled Particles Tunneling From a Schwarzschild Black Hole immersed in an Electromagnetic Universe with GUP

Quantum gravity has exciting peculiarities on the Planck scale.The effect of generalized uncertainty principle (GUP) to the entangled scalar/fermion particles' tunneling from a Schwarzschild black hole immersed in an electromagnetic Universe is investigated by the help of semi-classical tunneling method. The quantum corrected Hawking temperature of this black hole with an external parameter "a" modifies the Hawking temperature for the entangled particles.

gr-qc↗