Searcharxiv⌕ Search

arXiv subjects

M. Halilsoy

Publications and source records attributed to M. Halilsoy.

At least 19 recordsLinked to original sources
arXiv2026

Quantum Probe to the Higher Dimensional Yang-Mills Singularity

We investigate the quantum nature of naked curvature singularities in Einstein-Yang-Mills (EYM) theory using the Horowitz-Marolf (HM) criterion, which assesses quantum singularities via the evolution of quantum scalar fields. Focusing on timelike singularities in spacetime dimension $ D \geq 5 $, we analyze both pure Yang-Mills and Einstein-Maxwell-Yang-Mills (EMYM) solutions. We then incorporate higher curvature corrections through Gauss-Bonnet (GB) terms. From positivity requirement the expression under square root that arises in GB may create a secondary singularity that shall be scrutinized carefully. Our analysis reveals that while EYM and EMYM spacetimes remain quantum mechanically singular, the inclusion of GB corrections can, in general, render the singularity quantum mechanically regular for specific values of the mass parameter $m$, which is related to the YM charge $Q$ in $ D = 5 $ space-time dimensions. Contrary, for space-time dimension $D \geq 6$, although the outer (secondary) singularity may be healed quantum mechanically for certain values of the mass parameter $m$, the central singularity remains quantum mechanically singular.

gr-qc↗
arXiv2025

Branched Spacetime Geometries from Harmonic $S^2$ Maps: Energy Matching and Its Limitations

We investigate a topological construction of general-relativistic geometries based on harmonic self-maps of the two-sphere, $S^{2}\rightarrow S^{2}$. Such maps are classified by an integer degree $k$. For $k>1$, however, the pullback angular metric is not globally smooth: it defines a branched covering with conical excesses at the north and south poles. We calculate the corresponding distributional curvature and show that the branch points extend over the $(t,r)$ sector as codimension-two defects with negative signed tension. For a branched Schwarzschild geometry, we introduce a smooth Reissner-Nordstr\"{o}m (RN)-like radial deformation and define its finite bulk Killing energy relative to the $Q_{k}=0$ geometry in the same topological sector. An explicit phenomenological matching prescription relates this bulk energy to the scaled excess harmonic-map energy and determines $Q_{k}$. Identifying the matching scale with the outer horizon then yields a discrete sequence approaching an extremal limit. We also examine the associated horizon geometry, conditional entropy, regular curvature scalars, weak-field limit, and horizonless zero-mass sector. The same positive matching prescription does not extend universally. For the Simpson-Visser radial ansatz, the relative bulk Killing energy is negative. In the de Sitter case, the reduction $\Lambda \rightarrow \Lambda /k$ reproduces the excess map energy only through a common-volume reference subtraction, not through the direct energy difference between the physical sectors. The construction therefore provides a classical framework for branched topological sectors while also identifying the assumptions and limitations of the associated energy-matching prescription.

gr-qc↗
arXiv2025

Directional Quantum Singularities in Curzon Spacetime

The scalar quantum probe method developed by Horowitz and Marolf is applied to the cylindrically symmetric Curzon solution. The main cause for choosing the Curzon solution is that it is the best known example that exhibits directional singularity. Interestingly the singularity at $r=0$, for the uncharged Curzon spacetime, which is classically very strong with a divergence rate of the order $\frac{1}{r^{10}}$ becomes regular when examined using scalar quantum field. The charged Curzon spacetime, however, due to the emergence of a second singularity off the $r=0$ singularity does not regularize quantum mechanically. All three different charged versions, i.e. electric, magnetic and dyonic share the same feature.

gr-qc↗
arXiv2024

Extremal Black Holes in Cosmology from Colliding Light Beams

From the duality between the interaction region of colliding waves and black holes (BHs) it is known that certain BH solutions can be obtained from colliding gravity coupled electromagnetic (em) waves. In the limit of vanishing gravity waves, we show that extremal Reissner-Nordstrom (RN) BH can form. We show also that direct collision of pure em waves creates the near horizon geometry (NHG) of the same BH, supporting our point. Due to the quantum restrictions such BHs can have horizon radii in the range $R>10^{8}m$. If such BHs from pure light do exist this may open a new frontier in our understanding of cosmological BHs at large. The generation of impulsive gravitational waves in the process may be instrumental in the detection of a BH created from pure light.

gr-qc↗
arXiv2024

Charged particle geodesics and closed timelike curves in an electromagnetic universe

The spinning electromagnetic universe, known also as the Rotating Bertotti-Robinson(RBR) spacetime is considered as a model to represent our cosmos. The model derives from different physical considerations, such as colliding waves, throat region, and near horizon geometry of the Kerr-Newman black hole. Our interest is whether such a singularity-free spinning cosmology gives rise to a natural direction of flow, a 'chirality' for charged particles. Homochiral structures are known to be crucial for biology to start. Our concern here is cosmology rather than biology, but as in biology, the stable structures in cosmology may also rely on homochiral elements. We show the occurrence of closed timelike curves a 'la' G{\"o}del. Such curves, however, seem possible only at localized cell structures, not at large scales, but according to our prescription of near horizon geometry, they arise in the vicinity of any charged, spinning black hole.

gr-qc↗
arXiv2023

Generating spacetimes from colliding sources

Certain well-known spacetimes of general relativity (GR) are generated from the collision of suitable null-sources coupled with gravitational waves. This is a classical process underlying the full nonlinearity of GR that may be considered alternative to the quantum creativity at a large scale. Schwarzschild, de Sitter, anti de Sitter and the $\gamma $-metrics are given as examples.

gr-qc↗
arXiv2020

Colliding waves in a model of nonlinear electrodynamics

Bell-Szekeres (BS) solution for colliding electromagnetic waves in Einstein-Maxwell (EM) theory describes also colliding waves in nonlinear electrodynamics (NED) with an emergent cosmological constant. Our NED model covers the first leading orders to the well-known Heisenberg-Euler (HE) type in a particular gauge of pure magnetic field. Prior to the problem of collision we obtain dyonic solution for the considered NED theory in a conformally flat spacetime which has both electric and magnetic fields with constant invariants. Our sole finding is that null currents inevitably arise in the process of collision of plane waves in the HE type NED theory.

gr-qc↗
arXiv2020

Gravitational Lensing in Rotating and Twisting Universes

Gravitational lensing caused by the gravitational field of massive objects has been studied and acknowledged for a long period of time. In this paper, however, we propose a different mechanism where the bending of light stems from the non-linear interaction of gravitational, electromagnetic and axion waves that creates the high curvature zone in the space-time fabric. The striking distinction in the present study is that in contrast to the convex lensing in the gravitational field of a massive object, hyperbolic nature of the high curvature zone of the background space-time may give rise to concave lensing. Expectedly detection of this kind of lensing becomes possible through satellite detectors.

gr-qc↗
arXiv2019

Thin-Shells and Thin-Shell Wormholes in New Massive Gravity

Within 2+1-dimensional cosmological new massive gravity, we consider thin-shell and thin-shell wormhole construction. For this, we introduce first, the junction conditions apt for the fourth order terms in the action of the theory. Then, by employing some specific static solutions in new massive gravity, we study the characteristics of associated thin-shells and thin-shell wormholes. Our finding suggests that, firstly, there cannot exist any thin-shells regarding our chosen solutions of cosmological new massive gravity, and secondly, the constructed thin-shell wormhole does not need to be symmetric. More importantly, the thin-shell wormhole, if ever forms, possesses null energy density and null angular pressure on its throat which preferable to their negative-valued counterparts.

gr-qc↗
arXiv2019

Interpolating the Schwarzschild and de-Sitter metrics

The binary potential technique of interpolation (by M. Riesz, Acta Math. 81, 1 (1949)) is applied to some well-known metrics of general relativity. These include Schwarzschild, de Sitter and 2+1-dimensional BTZ spacetimes. In particular, the Schwarzschild-de Sitter solution is analyzed in some detail with a finite range parameter. Reasoning by the high level of non-linearity and absence of a superposition law necessitates search for alternative approaches. We propose the method of interpolation between different spacetimes as one such possibility paving the way toward controlling the two-metric system by a common parameter.

gr-qc↗
arXiv2019

Einstein-non-linear Maxwell-Yukawa black hole

Within the context of nonlinear electromagnetism we consider the Yukawa extension of a Reissner-Nordström black hole. Exact solution is given which modifies certain characteristics of the latter. Some thermodynamical aspects are given for comparison. The model may be considered as a useful agent to describe a short-ranged, charged, massive interaction.

gr-qc↗
arXiv2019

Global Monopole metric in 2+1-dimensions

In order to obtain the geometry of a global monopole without cosmological constant and electric charge in $2+1-$ dimensions we make use of the broken $% O(2)$ symmetry. In the absence of exact solution we determine the series solutions for both the metric and monopole functions in a consistent manner that satisfy all equations in appropriate powers. The new expansion elements are of the form $\frac{1}{r^{n}}\left( \ln r\right) ^{m},$ for the radial distance $r$ and positive integers $m$ and $n$ constrained by $m\leq n$. To the lowest order of expansion we find that in analogy with the negative cosmological constant the geometry of the global monopole acts repulsively, i.e., in the absence of a cosmological constant the global monopole plays at large distances the role of a negative cosmological constant.

gr-qc↗
arXiv2018

Thermodynamic Stability of a Schwarzschild Thin-Shell Wormhole

The thermodynamic stability of a thin-shell wormhole in a Schwarzschild bulk is considered. From the first law, entropy function is found which satisfies the local intrinsic stability conditions. Heat capacity emerges as a well-defined regular function justifying the stability of a Schwarzschild thin-shell wormhole. The scope of applications of the method is not limited by the Schwarzschild wormhole.

gr-qc↗
arXiv2018

Absence of Buckling in Nerve Fiber

In this study we give a geometrical model which employs the smoothness of nerve fibers as differentiable curves. We show that a nerve fiber may encounter large curvature due to the possible helicial bending and hence it could cause the fiber to buckle. However, its membrane structure provides a mechanism, entirely geometrical to avoid it. To overcome the challenge of emerging helix we project it into a plane.

physics.gen-ph↗
arXiv2018

Regularization of the Reissner-Nordström black hole

An inner de Sitter region is glued smoothly and consistently with an outer Reissner-Nordström (RN) spacetime on a spherical thin-shell. Mass and charge of the outer RN spacetime are defined by the de Sitter and shell parameters. Radius of the shell plays the role of a cut-off which by virtue of regular de Sitter inside removes the singularity at $r=0.$ The topology of inner de Sitter with the radius of the thin-shell becomes compact. For stability the perturbed shell is shown to satisfy a modified polytropic equation of state which has vanishing mass and pressure on the unperturbed shell as dictated by the junction conditions.

physics.gen-ph↗
arXiv2018

The Stability of Asymmetric Cylindrical Thin-Shell Wormholes

In continuation of a preceding work on introducing asymmetric thin-shell wormholes as an emerging class of traversable wormholes within the context, this time cylindrically symmetric spacetimes are exploited to construct such wormholes. Having established a generic formulation, first the Linet-Tian metric generally, and then the cosmic string metric and a black string metric in greater details are studied as constructing blocks of cylindrical asymmetric thin-shell wormholes. The corresponding wormholes are investigated within the linearized stability analysis framework to firstly, demonstrate that they can exist from the mechanical stability point of view, and secondly, indicate the correlation between the stability and symmetry in each case, if there is any at all. From here, we have extracted a pattern for the way stability changes with the asymmetry degree for the two examples; however, it was observed that the symmetric state is not the most neither the less stable state. There are also some side results: It was learned that any cylindrical thin-shell wormhole made of two cosmic string universes cannot be supported by a barotropic equation of state. Furthermore, as another side outcome, it was perceived that the radius dependency of the so-called variable equation of state, which is used all over this article, has a great impact on the mechanical stability of the cylindrical asymmetric thin-shell wormholes studied in this brief.

gr-qc↗
arXiv2018

Fate of a Thin-Shell Wormhole Powered by Morris-Thorne Wormhole

Asymmetric thin-shell wormholes from two traversable Morris-Thorne wormhole spacetimes, with identical shape but different redshift functions, are constructed. Energy density of the thin-shell wormhole derives its power from a Morris-Thorne wormhole which is already exotic. By choice, the weak energy condition for the thin-shell wormhole is satisfied. A linear barotropic equation of state is assumed to hold after the radial perturbations. The fate of our thin-shell wormhole, after the perturbation, is striking: the asymmetric thin-shell wormhole is destined either to collapse to the original Morris-Thorne wormhole or expand indefinitely along with the radius of the throat. In case it collapses to the original wormhole, the result is an asymmetric Morris-Thorne wormhole. Although this asymmetry does not reflect into the embedding diagram of the wormhole, passing across the throat, the wormhole adventurer feels a different redshift function.

gr-qc↗