arXiv subjects

Ali Övgün

Publications and source records attributed to Ali Övgün.

At least 127 records · Page 7Linked to original sources

arXiv2022

Quasiperiodic oscillations, weak field lensing and shadow cast around black holes in Symmergent gravity

In this work, we perform a systematic study of the symmergent gravity in a black hole environment. The symmergent gravity, an emergent gravity model in which gravity emerges in a way restoring the gauge symmetries and stabilizing the Higgs boson mass, possesses a quadratic-curvature term with a loop-induced coupling proportional to the boson-fermion number difference. We investigated boson-fermion number difference and black hole parameters in symmergent gravity by utilizing the values of various observables. In this regard, we investigated particle dynamics and obtained Keplerian frequencies describing the harmonic oscillations. We determined quasiperiodic oscillations about such orbits by utilizing relativistic precession, warped disk, and epicyclic resonant models. We studied weak deflection angle and planetary perihelion shift and determined bounds on symmergent gravity parameters. We computed shadow radius for both static and co-moving observers near and far from the symmergent black hole and revealed the sensitivity of these observers to the model parameters. We conclude the work by giving an overall discussion of the bounds, and giving future prospects concerning other possible analyses of the symmergent gravity.

gr-qc↗

arXiv2022

Weak Deflection Angle and Greybody Bound of Magnetized Regular Black Hole

In this paper, we examine the weak deflection angle and greybody bound for magnetized regular black hole.$~$ For this purpose, we apply the Gauss-Bonnet theorem on the black hole and obtain the deflection angle in plasma and non-plasma mediums.$~$ Moreover, we investigate graphically the effect of impact parameter on the deflection angle for regular black hole in both mediums. We examine that deflection angle goes to infinity when the impact parameter approaches to zero. We also observe that deflection angle shows negative behaviour at $q=0.6$ and $q=2.09$ but at $0.6<q<2.09$, angle shows the positive behaviour. Furthermore, we study the rigorous bound phenomenon of the greybody factor in the background for magnetized regular black hole. Later, we analyze the graphical behaviour of greybody bound with respect to different values of $ω$ and observe that at small values of $ω$, bound is increasing but for large values, bound is decreasing. Afterthat we examine that when we put $G=1$, $l=0$ and $q=0$, then all results for magnetized regular black hole solution reduces into results of the Schwarzschild black hole solution.

gr-qc↗

arXiv2022

Effect of the magnetic charge on weak deflection angle and greybody bound of the black hole in Einstein-Gauss-Bonnet gravity

The objective of this paper is to analyze the weak deflection angle of Einstein-Gauss-Bonnet gravity in the presence of plasma medium. To attain our results, we implement the Gibbons and Werner approach and use the Gauss-Bonnet theorem to Einstein gravity to acquire the resulting deflection angle of photon's ray in the weak field limit. Moreover, we illustrate the behavior of plasma medium and non-plasma mediums on the deflection of photon's ray in the framework of Einstein-Gauss-Bonnet gravity. Similarly, we observe the graphical influences of deflection angle on Einstein-Gauss-Bonnet gravity with the consideration of both plasma and non-plasma mediums. Later, we observe the rigorous bounds phenomenon of the greybody factor in contact with Einstein-Gauss-Bonnet gravity and calculate the outcomes, analyze graphically for specific values of parameters.

gr-qc↗

arXiv2022

Dark matter effect on the weak deflection angle by black holes at the center of Milky Way and M87 galaxies

In this paper, we investigated the effect of dark matter on the weak deflection angle by black holes at the galactic center. We consider three known dark matter density profiles such as the Cold Dark Matter (CDM), Scalar Field Dark Matter (SFDM), and the Universal Rotation Curve (URC) from the Burkert profile. To achieve this goal, we used how the positional angles are measured by Ishihara et al. method based on Gauss-Bonnet theorem (GBT) on the optical metric. With the help of the non-asymptotic form of the GBT, the longitudinal angle difference is also calculated. First, we find the emergence of apparent divergent terms on the said profiles, which indicates that the spacetime describing the black hole-dark matter combination is non-asymptotic. We showed that these apparent divergent terms vanish when the distance of the source and receiver are astronomically distant from the black hole. Using the current observational data in the Milky Way and M87 galaxies, we find interesting behaviors of how the weak deflection angle varies with the impact parameter, which gives us some hint on how dark matter interacts with the null particles for each dark matter density profile. We conclude that since these deviations are evident near the dark matter core radius, the weak deflection angle offers a better alternative for dark matter detection than using the deviation from the black hole shadow. With the DM profiles explored in this study, we find that SFDM is the most difficult to detect, while the easiest is the URC profile. With the dark matter profiles explored in this study, we find that the variation of the values for weak deflection angle strongly depends on the dark matter mass on a particular profile.

gr-qc↗

arXiv2022

Weak deflection angle of Kazakov-Solodukhin black hole in plasma medium using Gauss-Bonnet theorem and its greybody bonding

In this paper, we study light rays in a Kazakov-Solodukhin black hole. To this end, we use the optical geometry of the Kazakov-Solodukhin black hole within the Gauss-bonnet theorem. We first show the effect of the deformation parameter $a$ on the Gaussian optical curvature, and then we use the modern method popularized by Gibbons and Werner to calculate the weak deflection angle of light. Our calculations of deflection angle show how gravitational lensing is affected by the deformation parameter $a$. Moreover, we demonstrate the effect of a plasma medium on weak gravitational lensing by the Kazakov-Solodukhin black hole. We discuss that the increasing the deformation parameter $a$, will increase the weak deflection angle of the black hole. Our analysis also uncloak how one may find a observational evidence for a deformation parameter on the deflection angle. In addition, we studied the rigorous bounds of the Kazakov-Solodukhin black hole for the grey body factor and also studied the graphical behaviour of the bounds by fixing $l=0$ and $M=1$ and observed that increasing the deformation parameter $a$ will decrease the rigorous bound $T_b$ of the black hole.

gr-qc↗

arXiv2022

Nonlinear electrodynamics effects on the black hole shadow, deflection angle, quasinormal modes and greybody factors

In this paper, we discuss the effects of nonlinear electrodynamics (NED) on non-rotating black holes, parametrized by the field coupling parameter $β$ and magnetic charge parameter $P$ in detail. Particularly, we survey physical properties of the magnetically charged black hole, thermodynamic properties, observational appearance, quasinormal modes and absorption cross sections. We then show that the black hole gets colder with increasing charge. Investigating the heat capacity, we see that the black hole is thermally stable, which is amplified by introduction of a generalized uncertainty principle (GUP) with a quantum gravity parameter $λ$. Then we compute the deflection angle at the weak field limit, by the Gauss-Bonnet theorem and the geodesic equation, showing that the magnetic charge has a contribution at the first order. By ray-tracing we simulate the observational appearance of a NED black hole with thin disk and spherical accretion. We find that the parameter $P$ has a very strong effect on the shadow radius. We consider quasinormal modes under massless scalar perturbations of the black hole and the greybody factor. We find that the charge introduces a slight difference in the fundamental frequency and that the greybody factor of the NED black hole is strongly steepened by the introduction of increasing charge. To present observational constrains, we show that the magnetic charge of the M87* black hole is between $0\leq P\leq0.024$ in units of M, in agreement with the idea that real astrophysical black holes are mostly neutral. We also find that LIGO/VIRGO and LISA could detect NED black hole perturbations from BHs with masses between $5M_\odot$ and $8.0\cdot 10^8\,M_\odot$. We finally show that for black holes with masses detected with LIGO so far, charged NED black holes would deviate from Schwarzschild by $5\sim 10$ Hz in their fundamental frequencies.

gr-qc↗

arXiv2021

Shadow and weak deflection angle of extended uncertainty principle black hole surrounded with dark matter

In this paper, we discuss the possible effects of dark matter on a Schwarzschild black hole with the correction of extended uncertainty principle (EUP), such as the parameter $α$ and the large fundamental length scale $L_*$. In particular, we surround the EUP black hole of mass $m$ with a static spherical shell of dark matter described by the parameters mass $M$, inner radius $r_s$, and thickness $Δr_s$. In this study, we find that there is no deviation in the event horizon, which readily implies that the black hole temperature due to the Hawking radiation is independent of any dark matter concentration. In addition, we show some effects of the EUP parameter on the innermost stable circular orbit (ISCO) radius of time-like particles, photon sphere, shadow radius, and weak deflection angle. It is found that time-like orbits are affected by deviation of low values of mass $M$. A greater dark matter density is needed to have remarkable effects on the null orbits. Using the analytic expression for the shadow radius and the approximation $Δr_s>>r_s$, it is revealed that $L_*$ should not be lower than $2m$. To broaden the scope of this study, we also calculate the analytic expression for the weak deflection angle using the Ishihara et al. method \cite{Ishihara_2016}. As a result, we show that $Δr_s$ is improved by a factor of $(1+4αm^2/L_*^2)$ due to the EUP correction parameters. The calculated shadow radius and weak deflection angle are then compared using the estimated values of the galactic mass from Sgr A*, M87, and UGC 7232, as well as the mass of the supermassive black hole at their center.

gr-qc↗

arXiv2021

Schwarzschild-like black hole with a topological defect in bumblebee gravity

In this paper, we derive an exact black hole spacetime metric in the Einstein-Hilbert-Bumblebee (EHB) gravity around global monopole field. We study the horizon, temperature, and the photon sphere of the black hole. Using the null geodesics equation, we obtain the shadow cast by the Schwarzschild-like black hole with a topological defect in bumblebee gravity. Interestingly, the radius of shadow of the black hole increases with increase in the global monopole parameter. We also visualize the shadows and energy emission rates for different values of parameters. Moreover, using the Gauss-Bonnet theorem, we calculate the deflection angle in weak field limits and we discuss the possibility of testing the effect of global monopole field and bumblebee field, on a weak deflection angle. We find that the global monopole parameter and also Lorentz symmetry breaking parameter has an increasing effect on the deflection angle.

gr-qc↗

arXiv2021

Deriving Weak Deflection Angle by Black Holes or Wormholes using Gauss-Bonnet Theorem

In this review, various researches on finding the bending angle of light deflected by a massive gravitating object which regard the Gauss-Bonnet theorem as the premise have been revised. Primarily, the Gibbons and Werner method is studied apropos of the gravitational lensing phenomenon in the weak field limits. Some exclusive instances are deliberated while calculating the deflection angle, beginning with the finite-distance corrections on non-asymptotically flat spacetimes. Effects of plasma medium is then inspected to observe its contribution to the deflection angle. Finally, the Jacobi metric is explored as an alternative method, only to arrive at similar results. All of the cases are probed in three constructs, one as a generic statement of explanation, one for black holes, and one for wormholes, so as to gain a perspective on every kind of influence.

gr-qc↗

arXiv2021

Black Hole Shadow in Symmergent Gravity

Symmergent gravity is the $R+R^2$ gravity theory which emerges in a way restoring gauge symmetries broken explicitly by the ultraviolet cutoff in effective field theories. To test symmergent gravity we construct novel black hole solutions in four dimensions, and study their shadow in the vacuum as well as plasma medium. Our detailed analyses show that the horizon radius, Hawking temperature, Bekenstein-Hawking entropy, shadow angular radius, and photon deflection angle are sensitive probes of the symmergent gravity and particle spectrum of the underlying quantum field theory.

gr-qc↗

arXiv2021

Weak Deflection angle and Shadow by Tidal Charged Black Hole

In this article, we calculate the deflection angle of tidal charged black hole (TCBH) in weak field limits. First we obtain the Gaussian optical curvature and then apply the Gauss-Bonnet theorem on it. With the help of Gibbons-Werner method, we are able to calculate the light's deflection angle by TCBH in weak field limits. After calculating the deflection angle of light, we check the graphical behavior of TCBH. Moreover, we further find the light's deflection angle in the presence of plasma medium and also check the graphical behavior in the presence of plasma medium. Moreover, we investigate the shadow of TCBH. For calculating the shadow, we first find the null geodesics around the TCBH and then find its shadow radius. We also obtain TCBH's shadow in the plasma medium. Hence, we discuss the shadow of the TCBH using the $M87^{*}$ parameters announced by the Event Horizon Telescope.

gr-qc↗

arXiv2021

Evolving topologically deformed wormholes supported in the dark matter halo

In this paper, we construct an evolving wormhole in the dark matter halo. This work is relevant since matter has two components: (i) cosmological part (only time-dependent) and (ii) wormhole part (only space-dependent). In order to implement this, we use the Chaplygin gas as an equation of state for the cosmic part and Navarro-Frenk-White dark matter density profile as well as Thomas-Fermi (TF) profile in order to form a dark wormhole. The flare-out condition of wormhole is also satisfied by violating the null energy condition (NEC) for some specific values of quantities. Furthermore, we reveal more interesting results regarding how an topologically deformation parameter $α$ affects the evolving wormhole sourced with some dark matter models based on the physically motivated shape function.

physics.gen-ph↗

arXiv2021

Quasinormal modes of dS and AdS Black Holes: Feedforward neural network method

In this paper, we show how the quasinormal modes (QNMs) arise from the perturbations of massive scalar fields propagating in the curved background by using the artificial neural networks. To this end, we architect a special algorithm for the feedforward neural network method (FNNM) to compute the QNMs complying with the certain types of boundary conditions. To test the reliability of the method, we consider two black hole spacetimes whose QNMs are well-known: $4D$ pure de Sitter (dS) and five-dimensional Schwarzschild anti-de Sitter (AdS) black holes. Using the FNNM, the QNMs of are computed numerically. It is shown that the obtained QNMs via the FNNM are in good agreement with their former QNM results resulting from the other methods. Therefore, our method of finding the QNMs can be used for other curved spacetimes that obey the same boundary conditions.

gr-qc↗

arXiv2021

Traversable wormholes in the extended teleparallel theory of gravity with matter coupling

This study explores the Gaussian and the Lorentzian distributed spherically symmetric wormhole solutions in the $f(τ, T)$ gravity. The basic idea of the Gaussian and Lorentzian noncommutative geometries emerges as the physically acceptable and substantial notion in quantum physics. This idea of the noncommutative geometries with both the Gaussian and Lorentzian distributions becomes more striking when wormhole geometries in the modified theories of gravity are discussed. Here we consider a linear model within $f(τ,T)$ gravity to investigate traversable wormholes. In particular, we discuss the possible cases for the wormhole geometries using the Gaussian and the Lorentzian noncommutative distributions to obtain the exact shape function for them. By incorporating the particular values of the unknown parameters involved, we discuss different properties of the new wormhole geometries explored here. It is noted that the involved matter violates the weak energy condition for both the cases of the noncommutative geometries, whereas there is a possibility for a physically viable wormhole solution. By analyzing the equilibrium condition, it is found that the acquired solutions are stable. Furthermore, we provide the embedded diagrams for wormhole structures under Gaussian and Lorentzian noncommutative frameworks. Moreover, we present the critical analysis on an anisotropic pressure under the Gaussian and the Lorentzian distributions.

gr-qc↗

arXiv2021

Cosmology with variable $G$ and nonlinear electrodynamics

In a bid to resolve lingering problems in cosmology, more focus is being tilted towards cosmological models in which physical constants of nature are not necessarily real constants but vary with cosmic time. In this paper, we study a cosmological model in nonlinear electrodynamics with Newton's gravitational constant $G$, which is not a constant but varies in terms of a power law of the scale factor of the universe. Hence, the evolution of the scale factor $a(t)$ is studied in this model, which depends on a fine-tuning term of nonlinear electrodynamics, $α$. Finally, we verify the stability of the model using the speed of sound.

gr-qc↗

arXiv2021

Weak deflection angle by asymptotically flat black holes in Horndeski theory using Gauss-Bonnet theorem

The principal objective of this project is to investigate the gravitational lensing by asymptotically flat black holes in the framework of Horndeski theory in weak field limits. To achieve this objective, we utilize the Gauss-Bonnet theorem to the optical geometry of asymptotically flat black holes and apply the Gibbons-Werner technique to achieve the deflection angle of photons in weak field limits. Subsequently, we manifest the influence of plasma medium on deflection of photons by asymptotically flat black holes in the context of Horndeski theory. We also examine the graphical impact of deflection angle on asymptotically flat black holes in the background of Horndeski theory in plasma medium as well as non-plasma medium.

gr-qc↗

arXiv2021

Weak deflection angle by Casimir wormhole using Gauss-bonnet theorem and its shadow

In this paper, we calculate the weak deflection angle by Casimir wormhole and its shadow. To do so, we derive the Gaussian optical curvature and use the Gauss-Bonnet theorem. Then we find the deflection angle by Casimir wormhole in weak field limits. Moreover, we obtain the weak deflection angle in the presence of plasma medium and see the effect of the plasma medium on the weak deflection angle. Moreover, we study a shadow of Casimir wormhole and we plot and discuss them. We show the shadow of Casimir wormhole's behavior when changing the value of $a$.

gr-qc↗

arXiv2020

Weak deflection angle of black-bounce traversable wormholes using Gauss-Bonnet theorem in the dark matter medium

In this paper, we first use the optical metrics of black-bounce traversable wormholes to calculate the Gaussian curvature. Then we use the Gauss-Bonnet theorem to obtain the weak deflection angle of light from the black-bounce traversable wormholes. Then we investigate the effect of dark matter medium on weak deflection angle using the Gauss-Bonnet theorem. We show how weak deflection angle of wormhole is affected by the bounce parameter $a$. Using the Gauss-bonnet theorem for calculating weak deflection angle shows us that light bending can be thought as a global and topological effect.

gr-qc↗