arXiv subjects

Özgür Delice

Publications and source records attributed to Özgür Delice.

15 recordsLinked to original sources

arXiv2026

Straight and Wiggly Cosmic Strings in Horndeski Theory

In this article, the behavior of a straight cosmic string is studied for the linearized version of Horndeski theory in cases where the scalar field is massless or massive. Several physical properties of such solutions are discussed in detail regarding the effects of the scalar field of this theory. The mass of the scalar field induces a screening effect such that, in the massive theory, the string solution approaches to the general relativistic one. We also consider wiggly cosmic strings, obtain the solutions for both massless and massive cases, discuss their properties and observe similar screening effects.

gr-qc↗

arXiv2026

Gravitational waves in massive Horndeski theory with a potential

We investigate gravitational waves with an arbitrary potential within the framework of linearized Horndeski theory. We show that the minimum of the potential can play the role of an effective cosmological constant in this theory, which is usually neglected in previous studies of this subject. We first determine the background geometry in this setup by solving the weak field scalar and tensorial equations of linearized Horndeski theory. The solutions of linearized weak-field wave equations, in an appropriate gauge, are then obtained perturbatively to study the propagation and interactions of gravitational waves in this background. We compare our results with different realizations of the cosmological constant in Horndeski theory to compare the role of an arbitrary scalar potential with those of vacuum energy density and a linear potential. The results show that the background curvature arising from the minimum of the scalar potential effectively mimics a cosmological constant, producing distinct redshifts in the frequency and wave number that distinguish the tensor waves from massive scalar ones. We also find that the way the cosmological constant is introduced directly influences the speed and polarization of the scalar wave.

gr-qc↗

arXiv2025

Innermost stable circular orbits around a Reissner-Nordström-global monopole spacetime in a homogeneous magnetic field

We investigate the dynamics of charged particles in the spacetime of a global monopole swallowed by a Reissner-Nordström (RN) black hole in the presence of an external, weak, asymptotically homogeneous magnetic field. We carefully analyze and deduce the conditions to have such a magnetic field around this black hole and show that this is indeed possible in the small but nontrivial charge and monopole term limit. We obtain the general equations of motion and analyze them for special cases of circular orbits, focusing on the innermost stable circular orbit (ISCO) of this configuration. The richness of the parameters and complicated forms of the resulting equations of motion necessitate a numerical approach. Hence, we have presented our results with numerous graphs, which help to understand the evolution of ISCO as a function of the external test magnetic field and the monopole term, depending on the parameters of the black hole, such as its electric charge, as well as the properties of the test particle, such as its specific charge, angular momentum, and energy. We have also analyzed the effective potential that these fields generate and deduced results for the aforementioned values of the external and internal parameters of the spacetime.

gr-qc↗

arXiv2021

Gravitational waves in Brans-Dicke Theory with a cosmological constant

Weak field gravitational wave solutions are investigated in Brans-Dicke (BD) theory in the presence of a cosmological constant. In this setting the background geometry is not flat but asymptotically de-Sitter. We investigate the linearised field equations, and their gravitational wave solutions in a certain gauge choice. We will show that this theory leads to massless scalar waves as in original BD theory and in contrast to massive BD theory. The effects of these waves on free particles and their polarization properties are studied extensively and effects of the cosmological constant is analyzed in these phenomena in detail. The energy flux of these waves are also discussed in this background. By analyzing this flux, we obtain a critical distance where the waves cannot propagate further, which extends Cosmic no Hair Conjecture (CNC) to BD theory with a cosmological constant.

gr-qc↗

arXiv2020

Bending of Light from Reissner-Nordström-de Sitter-Monopole Black Hole

We study light deflection from a Reissner-Nordström-de Sitter black hole in the gravitational monopole background. We first calculate the orbit equation and the contribution of the monopole and black hole parameters to the deflection angle up to second-order for the vanishing cosmological constant case using the Rindler-Ishak method. We also obtain the contribution of the cosmological constant to light deflection in this geometry in the weak field limit using the same method.

gr-qc↗

arXiv2020

Superradiance of a Global Monopole in Reissner-Nordström(-AdS) Space-time

In this article, the behaviour of a charged and massive scalar field around a global monopole swallowed by a Reissner-Nordström-Anti-de Sitter (RN-AdS) black hole is investigated by considering the Klein-Gordon equation in this geometry. The superradiance phenomenon and instability behaviour of the black hole against charged scalar perturbations are studied for both an RN-AdS case and also for an RN black hole surrounded by a reflective mirror, i.e., the black hole bomb case. The effects of the monopole on these cases are discussed analytically and also with the help of several graphs in detail. The monopole charge affects the superradiance threshold frequency and also effects the instability time scale for both cases. The existence of global monopole makes these black holes more stable against superradiance instability.

gr-qc↗

arXiv2018

Linearized modified gravity theories with a cosmological term: advance of perihelion and deflection of light

Two different ways of generalizing Einstein's general theory of relativity with a cosmological constant to Brans-Dicke type scalar-tensor theories are investigated in the linearized field approximation. In the first case a cosmological constant term is coupled to a scalar field linearly whereas in the second case an arbitrary potential plays the role of a variable cosmological term. We see that the former configuration leads to a massless scalar field whereas the latter leads to a massive scalar field. General solutions of these linearized field equations for both cases are obtained corresponding to a static point mass. Geodesics of these solutions are also presented and solar system effects such as the advance of the perihelion, deflection of light rays and gravitational redshift were discussed. In general relativity cosmological constant has no role on these phenomena. We see that for the Brans-Dicke theory the cosmological constant has also no effect on these phenomena. This is because solar system observations require very large values of the Brans-Dicke parameter and the correction terms to these phenomena becomes identical to GR for these large values of this parameter. This result is also observed for the theory with arbitrary potential if the mass of the scalar field is very light. For a very heavy scalar field, however, there is no such limit on the value of this parameter and there are ranges of this parameter where these contributions may become relevant in these scales. Galactic and intergalactic dynamics is also discussed for these theories at the latter part of the paper with similar conclusions.

gr-qc↗

arXiv2016

Modified Einstein-Gauss-Bonnet gravity: Riemann-Cartan and Pseudo-Riemannian cases

A modified Einstein-Gauss-Bonnet gravity in four dimensions where the quadratic Gauss-Bonnet term is coupled to a scalar field is considered. The field equations of the model are obtained by variational methods by making use of the constrained-first order formalism covering both pseudo-Riemannian and non-Riemannian cases. In the pseudo-Riemannian case, the Lagrange multiplier forms, which impose the vanishing torsion constraint, are eliminated in favor of the remaining fields and the resulting metric field equations are expressed in terms of the double-dual curvature 2-form. In the non-Riemannian case with torsion, the field equations are expressed in terms of the pseudo-Riemannian quantities by a perturbative scheme valid for a weak coupling constant. It is shown that, for both cases, the model admits a maximally symmetric de-Sitter solution with nontrivial scalar field. Minimal coupling of a Dirac spinor to the Gauss-Bonnet modified gravity is also discussed briefly.

gr-qc↗

arXiv2015

On Stationary Axially Symmetric Solutions in Brans-Dicke Theory

Stationary, axially symmetric Brans-Dicke-Maxwell solutions are reexamined in the framework of the Brans-Dicke (BD) theory. We see that, employing a particular parametrization of the standard axially symmetric metric simplifies the procedure of obtaining the Ernst equations for axially symmetric electrovacuum space-times for this theory. This analysis also permits us to construct a two parameter extension in both Jordan and Einstein frames of an old solution generating technique frequently used to construct axially symmetric solutions for BD theory from a seed solution of general relativity. As applications of this technique, several known and new solutions are constructed including a general axially symmetric BD-Maxwell solution of Plebanski-Demianski with vanishing cosmological constant, i.e. the Kinnersley solution and general magnetized Kerr-Newman--type solutions. Some physical properties and the circular motion of test particles for a particular subclass of Kinnersley solution, i.e., a Kerr-Newman-NUT--type solution for BD theory, are also investigated in some detail.

gr-qc↗

arXiv2015

Superradiance Instability of Small Rotating AdS Black Holes in Arbitrary Dimensions

We investigate the stability of $D$ dimensional singly rotating Myers-Perry-AdS black holes under superradiance against scalar field perturbations. It is well known that small four dimensional rotating or charged Anti-de Sitter (AdS) black holes are unstable against superradiance instability of a scalar field. Recent works extended the existence of this instability to five dimensional rotating charged AdS black holes or static charged AdS black holes in arbitrary dimensions. In this work we analytically prove that, rotating small AdS black holes in arbitrary dimensions also show superradiance instability irrespective of the value of the (positive) angular momentum quantum number. To do this we solve the Klein-Gordon equation in the slow rotation, low frequency limit. By using the asymptotic matching technique, we are able to calculate the real and imaginary parts of the correction terms to the frequency of the scalar field due to the presence of the black hole, confirming the presence of superradiance instability. We see that, unlike in the case of static AdS black holes, the analytical method is valid for rotating AdS black holes for any value of angular momentum number and space-time dimensions. For comparison we derive the corresponding correction terms for Myers-Perry black holes in the black hole bomb formalism in Appendix and see that the results are in agreement.

gr-qc↗

arXiv2015

Brans-Dicke-Maxwell Solutions for Higher Dimensional Static Cylindrical Symmetric Spacetime

In this paper, Brans-Dicke-Maxwell type vacuum solutions are considered for a static cylindrically symmetric spacetime in arbitrary dimensions. Exact solutions are obtained by directly solving the field equations for the case where an azimuthal magnetic field is present. Other configurations such as axial magnetic field case can be obtained by suitably relabeling the coordinates. We have also considered conformally related "Einstein frame" to relate the solutions we have obtained with the dilaton-Maxwell type solutions that exist in the literature. We see that for a special case the general solution we present reduces to Dilaton-Melvin spacetime. The General Relativistic limit of these solutions are also discussed and we found that this limit is different from the four dimensional case.

gr-qc↗

arXiv2014

Generalized Sparling-Thirring form in the Brans-Dicke theory

The definition of the Sparling-Thirring form is extended to the Brans-Dicke theory. By writing the Brans-Dicke field equations in a formally Maxwell-like form, a superpotential and a corresponding pseudo energy-momentum form are defined. The general energy expression provided by the superpotential in the Jordan frame is discussed in relation to the corresponding expression in the Einstein frame. In order to substantiate its formal definition, the generalized Sparling-Thirring form is used to calculate the energy for the spherically symmetric vacuum solution in the Brans-Dicke theory.

gr-qc↗

arXiv2013

Multi-Scalar-Tensor Equivalents for Modified Gravitational Actions

A general scheme of constructing scalar-tensor equivalents to modified gravitational actions are studied using the algebra of exterior differential forms and the first order formalism that allows an independent connection and coframe. By introducing appropriate constraints on the connection, pseudo-Riemannian cases as well as non-Riemannian cases are discussed for various gravitational models. The issue of the dynamical degree of freedom for the resulting scalar fields is discussed at the level of the field equations. Explicit scalar-tensor equivalents for gravitational models based on $f(R)$ models, the quadratic curvature lagrangians and the models involving the gradients of the scalar curvature are presented. In particular, explicit ST equivalence for gravitational lagrangians popular in some cosmological models are constructed.

gr-qc↗

arXiv2013

Higher Dimensional Cylindrical or Kasner Type Electrovacuum Solutions

We consider a D dimensional Kasner type diagonal spacetime where metric functions depend only on a single coordinate and electromagnetic field shares the symmetries of spacetime. These solutions can describe static cylindrical or cosmological Einstein-Maxwell vacuum spacetimes. We mainly focus on electrovacuum solutions and four different types of solutions are obtained in which one of them has no four dimensional counterpart. We also consider the properties of the general solution corresponding to the exterior field of a charged line mass and discuss its several properties. Although it resembles the same form with four dimensional one, there is a difference on the range of the solutions for fixed signs of the parameters. General magnetic field vacuum solution are also briefly discussed, which reduces to Bonnor-Melvin magnetic universe for a special choice of the parameters. The Kasner forms of the general solution are also presented for the cylindrical or cosmological cases.

gr-qc↗

arXiv2009

Superradiant Instability of Five-Dimensional Rotating Charged AdS Black Holes

We study the instability of small AdS black holes with two independent rotation parameters in minimal five-dimensional gauged supergravity to massless scalar perturbations. We analytically solve the Klein-Gordon equation for low-frequency perturbations in two regions of the spacetime of these black holes: namely, in the region close to the horizon and in the far-region. By matching the solutions in an intermediate region, we calculate the frequency spectrum of quasinormal modes. We show that in the regime of superradiance only the modes of even orbital quantum number undergo negative damping, resulting in exponential growth of the amplitude. That is, the black holes become unstable to these modes. Meanwhile, the modes of odd orbital quantum number do not undergo any damping, oscillating with frequency-shifts. This is in contrast with the case of four-dimensional small Kerr-AdS black holes which exhibit the instability to all modes of scalar perturbations in the regime of superradiance.

hep-th↗