arXiv subjects

M. Halilsoy

Publications and source records attributed to M. Halilsoy.

At least 37 records · Page 2Linked to original sources

arXiv2018

Repeated crossing of two concentric spherical thin-shells with charge

Interaction / collision of two concentric spherical thin-shells of linear fluid resulting in collapse has been considered recently. We show that addition of finely tuned electric charges on the shells apart from the cosmological constant serves to delay the collapse indefinitely, yielding an ever colliding system of two concentric fluid shells. Given the finely tuned charges this provides an example of a perpetual two-body motion in general relativity.

gr-qc↗

arXiv2018

Asymmetric Thin-Shell Wormholes

Spacetime wormholes in isotropic spacetimes are represented traditionally by embedding diagrams which were symmetric paraboloids. This mirror symmetry, however, can be broken by considering different sources on different sides of the throat. This gives rise to an asymmetric thin-shell wormhole, whose stability is studied here in the framework of the linear stability analysis. Having constructed a general formulation, using a variable equation of state and related junction conditions, the results are tested for some examples of diverse geometries such as the cosmic string, Schwarzschild, Reissner-Nordstr% öm and Minkowski spacetimes. Based on our chosen spacetimes as examples, our finding suggests that symmetry is an important factor to make a wormhole more stable. Furthermore, the parameter $γ$, which corresponds to the radius dependency of the pressure on the wormholes's throat, can affect the stability in a great extent.

gr-qc↗

arXiv2017

Stability of spherically symmetric timelike thin-shells in general relativity with a variable equation of state

We study spherically symmetric timelike thin-shells in $3+1-$dimensional bulk spacetime with a variable equation of state for the fluid presented on the shell. In such a fluid the angular pressure $p$ is a function of both surface energy density $σ$ and the radius $R$ of the thin-shell. Explicit cases of the thin shells connecting two non-identical cloud of strings spacetimes and a flat Minkowski spacetime to the Schwarzschild metric are investigated.

gr-qc↗

arXiv2017

Particle Acceleration by Static Black Holes in a Model of $f(R)$ Gravity

Particle collisions are considered within the context of $f(R)$ gravity described by $f(R)=R+2α\sqrt{R}$, where $R$ stands for the Ricci scalar and $α$ is a non-zero constant. The center of mass (CM) energy of head-on colliding particles moving in opposite radial directions near the naked singularity/horizon are considered. Collision of particles in the same direction near the event horizon yields finite energy while the energy of oppositely moving particles grows unbounded. Addition of a cosmological constant does not change the feature. Collision of a massless outgoing photon with an infalling particle and collision of two oppositely moving photons following null-geodesics are also taken into account.

gr-qc↗

arXiv2017

Note on a thin-shell wormhole in extremal Reissner-Nordström geometry

We show that the cold horizon of the extremal Reissner-Nordström can be considered as the throat of a thin-shell wormhole with zero total exotic matter and positive angular pressure. Such a wormhole is physical and stable against radial perturbations provided an appropriate perfect fluid exists at the throat.

gr-qc↗

arXiv2017

Note on an elementary particle model with Bertotti-Robinson core

Spherically symmetric classical model of an elementary particle or a black hole spacetime without central singularity had been constructed by O. B. Zaslavskii in PRD 70(2004)104017. In this model an extremal Reissner-Nordstr% öm (RN) black hole and a Bertotti-Robinson (BR) spacetime are glued at the horizon such that the inner/core spacetime is the regular BR while outside is the extremal RN. In this note we investigate the stability of such a particle / regular black hole against linear radial perturbations. The model turns out to be stable against such perturbations with a linear equation of state after the perturbation.

gr-qc↗

arXiv2017

Magnetic Morris-Thorne wormhole in 2+1-dimensions

In the context of $2+1-$dimensional gravity coupled to a particular nonlinear electrodynamics (NED), we obtain a class of traversable / Morris-Thorne type wormhole solutions. The problem is reduced to a single function dependence in which the shape function acts as generator to the wormholes. The field ansatz is pure magnetic and the nonlinear Lagrangian is $\sqrt{F_{μν}F^{μν}}$ i.e. the square root of the Maxwell Lagrangian. In $2+1-$dimensions the source-free pure magnetic non-linear Maxwell equation with square-root Lagrangian is trivially satisfied. The exotic energy density is found explicitly and the flare-out conditions are emphasized.

gr-qc↗

arXiv2017

Classical and quantum analysis of an Einstein-Scalar solution in $2+1$ dimensions

The classical and quantum properties of a new solution obtained in $2+1$% -dimensional gravity coupled with a real scalar field is analyzed in detail. The considered new solution is a one-parameter generalization of a previously known solution. We investigate the solution classically by calculating exact null and null circular geodesics which correspond to a typical particle probe of the correspondsing black hole spacetime. The solution admitting naked singularity is investigated within the context of quantum mechanics. The timelike naked singularity is probed with spin-zero and spin-half quantum particles. We show that the curvature singularity in the new solution is stronger and hence in general the spacetime remains quantum singular with respect to a quantum particle probe.

gr-qc↗

arXiv2017

Morse simulation of the Global Monopole equation in flat spacetime

We show that a Morse type potential simulates an analytic solution for the highly non-linear global monopole field equation in three and higher dimensional flat spacetimes. Owing to the fact that in the flat space limit the similar equation remains intact we wish to borrow the curved space terminology of global monopole also in flat spacetime. This may provide a compelling example that can be used effectively in different non-linear theories such as flat space $ϕ^{4},$ as well as in curved spacetimes.

physics.gen-ph↗

arXiv2017

Thin-shell wormholes in 2+1-dimensional Einstein-Scalar Theory

We present an infinite class of one-parameter scalar field extensions to the BTZ black hole in 2+1-dimensions. By virtue of the scalar charge the thin-shell wormhole supported by a linear fluid at the throat becomes stable against linear perturbations. More interestingly, we provide an example of thin-shell wormhole which is strictly stable in the sense that it is confined in between two classically intransmissible potential barriers.

gr-qc↗

arXiv2017

Cycloid Experiment for freshmen physics labs

We establish an instructive experiment to investigate the minimum time curve traveled by a small billiard ball rolling in a grooved track under gravity. Our intention is to popularize the concept of \textit{minimum time curve} anew, and to propose it as a feasible physics experiment both for freshmen and sophomore classes. We observed that even the non-physics major students did enjoy such a cycloid experiment.

physics.ed-ph↗

arXiv2016

Particle Collision near 1+1- Dimensional Horava-Lifshitz Black Hole and Naked Singularity

The unbounded center of mass (CM) energy of oppositely moving colliding particles near horizon emerge also in 1+1- dimensional Horava-Lifshitz gravity. This theory has imprints of renormalizable quantum gravity characteristics in accordance with the method of simple power counting. Surpris- ingly the result obtained is not valid for a 1- dimensional Compton- like process between an outgoing photon and an infalling massless/ massive particle. It is possible to achieve unbounded CM energy due to collision between infalling photons and particles. The source of outgoing particles may be at- tributed to an explosive process just outside the horizon for a black hole and the naturally repulsive character for the case of a naked singularity. It is found that absence of angular momenta in 1+1- dimensions does not yield unbounded energy for collisions in the vicinity of naked singularities.

gr-qc↗

arXiv2016

Black p-Branes versus black holes in non-asymptotically flat Einstein-Yang-Mills theory

We present a class of non-asymptotically flat (NAF) charged black p-branes (BpB) with p-compact dimensions in higher dimensional Einstein-Yang-Mills theory. Asymptotically the NAF structure manifests itself as an anti-de-sitter spacetime. We determine the total mass / energy enclosed in a thin-shell located outside the event horizon. By comparing the entropies of BpB with those of black holes in same dimensions we derive transition criteria between the two types of black objects. Given certain conditions satisfied our analysis shows that BpB can be considered excited states of black holes. An event horizon $r_{+}$ versus charge square $Q^{2}$ plot \ for the BpB reveals such a transition where $r_{+}$ is related to the horizon radius $r_{h}$ of the black hole (BH) both with the common charge $% Q. $

gr-qc↗

arXiv2016

Black holes from multiplets of scalar fields in 2+1- and 3+1-dimensions

We obtain classes of black hole solutions constructed from multiplets of scalar fields in 2+1 / 3+1 dimensions. The multi-component scalars don't undergo a symmetry breaking so that only the isotropic modulus is effective. The Lagrangian is supplemented by a self-interacting potential which plays significant role in obtaining the exact solutions. In 2+1 / 3+1 dimensions doublet / triplet of scalars is effective which enriches the available black hole spacetimes and creates useful Liouville weighted field theoretic models.

gr-qc↗

arXiv2016

Generation of spherically symmetric metrics in $f\left( R\right) $ gravity

In $D-$dimensional spherically symmetric $f\left( R\right) $ gravity there are three unknown functions to be determined from the fourth order differential equations. It is shown that the system remarkably integrates to relate two functions through the third one to provide reduction to second order equations accompanied with a large class of potential solutions. The third function which acts as the generator of the process is $F\left( R\right) =\frac{df\left( R\right) }{dR}.$ We recall that our generating function has been employed as a scalar field with an accompanying self-interacting potential previously which is entirely different from our approach. Reduction of $f\left( R\right) $ theory into system of equations seems to be efficient enough to generate a solution corresponding to each generating function. As particular examples, besides known ones, we obtain new black hole solutions in any dimension $D$. We further extend our analysis to cover non-zero energy-momentum tensors. Global monopole and Maxwell sources are given as examples.

gr-qc↗

arXiv2016

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↗

arXiv2016

Non-abelian magnetic black strings versus black holes

We present $d+1-$dimensional pure magnetic Yang-Mills (YM) black strings (or $1-$branes) induced by the $d-$dimensional Einstein-Yang-Mills-Dilaton black holes. Born-Infeld version of the YM field makes our starting point which goes to the standard YM field through a limiting procedure. The lifting from black holes to black strings, (with less number of fields) is by adding an extra, compact coordinate. This amounts to the change of horizon topology from $S^{d-2}$ to a product structure. Our black string in $5-$dimensions is a rather special one, with uniform Hawking temperature and non-asymptotically flat structure. As the YM charge becomes large the string gets thinner to tend into a breaking point and transform into a $4-$% dimensional black hole.

gr-qc↗

arXiv2016

Wavy way to the Kerr metric and the quantum nature of its ring singularity

From inherent non-linearity two gravitational waves, unless they are unidirectional, fail to satisfy a superposition law. They collide to develop a new spacetime carrying the imprints of the incoming waves. Same behaviour is valid also for any massless lightlike field. As a result of the violent collision process either a naked singularity or a Cauchy horizon (CH) develops. It was shown by Chandrasekhar and Xanthopoulos (CX) that a particular class of colliding gravitational waves (CGW) spacetime is locally isometric to the Kerr metric for rotating black holes. This relation came to be known as the CX duality. Such a duality can be exploited as an alternative derivation for the Kerr metric as we do herein. Not each case gives rise to a CH but those which do are transient to a black hole state provided stability requirements are met. These classical considerations can be borrowed to shed light on black hole formation in high energy collisions. Their questionable stability and many other sophisticated agenda, we admit that await for a full - fledged quantum gravity. Yet, to add an element of novelty, a quantum probe is sent in the plane $θ=π/2$ to the naked ring singularity of Kerr which develops for the overspinning case $(a>M)$ to test it from a quantum picture. We show that the spatial operator of the reduced Klein-Gordon equation has a unique self-adjoint extension. As a result, the classical Kerr`s ring singularity is healed and becomes quantum regular. Our poetic message of the paper is summarized as Let there be light that collide with might to disperse the night and create holes that are white

gr-qc↗