arXiv subjects

M. Halilsoy

Publications and source records attributed to M. Halilsoy.

At least 55 records · Page 3Linked to original sources

arXiv2016

Cloud of strings as source in $2+1$-dimensional $f( R) = R^n$ gravity

We present three parameters exact solutions with possible black holes in $% 2+1-$dimensional $f\left( R\right) =R^{n}$ modified gravity coupled minimally to a cloud of strings. These three parameters are $n,$ the cloud of string coupling constant $ξ$ and an integration constant $C$. Although in general one has to consider each set of parameters separately; for $n$ an even integer greater than one we give a unified picture providing black holes. For $n\geq 1$ we analyze null / timelike geodesic within the context of particle confinement.

gr-qc↗

arXiv2015

Hypocycloidal throat for 2+1-dimensional thin-shell wormholes

Recently we have shown that for $2+1-$dimensional thin-shell wormholes a non-circular throat may lead to a physical wormhole in the sense that the energy conditions are satisfied. By the same token, herein we consider angular dependent throat geometry embedded in a $2+1-$dimensional flat spacetime in polar coordinates. It is shown that a generic, natural example of throat geometry is provided remarkably by a hypocycloid. That is, two flat $2+1-$dimensions are glued together along a hypocycloid. The energy required in each hypocycloid increases with the frequency of the roller circle inside the large one.

gr-qc↗

arXiv2015

Einstein-Born-Infeld black holes with a scalar hair in three-dimensions

We present black hole solutions in $2+1-$dimensional Einstein's theory of gravity coupled with Born-Infeld nonlinear electrodynamic and a massless self-interacting scalar field. The model has five free parameters: mass $M$, cosmological constant $\ell $, electric $q$ and scalar $r_{0}$ charges and Born-Infeld parameter $β$. To attain exact solution for such a highly non-linear system we adjust, i.e. finely tune, the parameters of the theory with the integration constants. In the limit $β\rightarrow 0$ we recover the results of Einstein-Maxwell-Scalar theory, obtained before. The self interacting potential admits finite minima apt for the vacuum contribution. Hawking temperature of the model is investigated versus properly tuned parameters. By employing this tuned-solution as basis, we obtain also a dynamic solution which in the proper limit admits the known solution in Einstein gravity coupled with self-interacting scalar field. Finally we establish the equations of a general scalar-tensor field coupled to nonlinear electrodynamics field\ in $2+1-$dimensions without searching for exact solutions.

gr-qc↗

arXiv2015

2+1-dimensional wormhole from a doublet of scalar fields

We present a class of exact solutions in the framework of $2+1-$dimensional Einstein gravity coupled minimally to a doublet of scalar fields. Our solution can be interpreted upon tuning of parameters as an asymptotically flat wormhole as well as a particle model in $2+1-$dimensions.

gr-qc↗

arXiv2015

Screening of the Reissner-Nordström charge by a thin-shell of dust matter

A concentric charged thin-shell encircling a Reissner-Nordström black hole screens the clectric / magnetic charge completely to match with an external Schwarzschild black hole. The negative mass thin-shell is shown to be stable against radial perturbations. It is shown further that by reversing the roles of inside Reissner-Nordström and outside Schwarzschild geometries the mass of the appropriate shell becomes positive.

gr-qc↗

arXiv2015

Einstein-Maxwell gravity coupled to a scalar field in 2+1-dimensions

We consider Einstein-Maxwell-self-interacting scalar field theory described by a potential $V\left( ϕ\right) $ in $2+1-$dimensions. The self-interaction potential is chosen to be a highly non-linear double-Liouville type. Exact solutions, including chargeless black holes and singularity-free non-black hole solutions are obtained in this model.

gr-qc↗

arXiv2015

A topological metric in 2+1-dimensions

Real-valued triplet of scalar fields as source gives rise to a metric which tilts the scalar, not the light cone, in 2+1-dimensions. The topological metric is static, regular and it is characterized by an integer $κ=\pm 1,\pm 2,...$. The problem is formulated as a harmonic map of Riemannian manifolds in which the integer $κ$ equals to the degree of the map.

physics.gen-ph↗

arXiv2015

3+1-dimensional thin-shell wormhole with deformed throat can be supported by normal matter

From physics standpoint exotic matter problem is a major difficulty in thin-shell wormholes (TSWs) with spherical / cylindrical throat topologies. We aim to circumvent this handicap by considering angular dependent throats in $3+1-$dimensions. By considering the throat of the TSW to be deformed spherical, i.e., a function of $θ$ and $φ$, we present general conditions which are to be satisfied by the shape of the throat in order to have the wormhole supported by matter with positive density in the static reference frame. We provide particular solutions / examples to the constraint conditions.

physics.gen-ph↗

arXiv2015

Modified Rindler acceleration as a nonlinear electromagnetic effect

The model proposed originally by Mannheim and Kazanas for fitting the shapes of galactic rotation curves has recently been considered by Grumiller to describe gravity of a central object at large distances. Herein we employ the same geometry within the context of nonlinear electrodynamics (NED). Pure electrical NED model is shown to generate the novel Rindler acceleration term in the metric which explains anomalous behaviors of test particles / satellites. Remarkably a pure magnetic model of NED yields flat rotation curves that may account for the missing dark matter. Weak and Strong Energy conditions are satisfied in such models of NED.

gr-qc↗

arXiv2015

2+1-dimensional traversable wormholes supported by positive energy

We revisit the shapes of the throats of wormholes, including thin-shell wormholes (TSWs) in $2+1-$dimensions. In particular, in the case of TSWs this is done in a flat $2+1-$dimensional bulk spacetime by using the standard method of cut-and-paste. Upon departing from a pure time-dependent circular shape i.e., $r=a\left( t\right) $ for the throat, we employ a $% θ-$dependent closed loop of the form $r=R\left( t,θ\right) ,$ and in terms of $R\left( t,θ\right) $ we find the surface energy density $σ$ on the throat. For the specific convex shapes we find that the total energy which supports the wormhole is positive and finite. In addition to that we analyze the general wormhole's throat. By considering a specific equation of $r=R\left( θ\right) $ instead of $r=r_{0}=const.,$ and upon certain choices of functions for $R\left( θ\right) $ we find the total energy of the wormhole to be positive.

gr-qc↗

arXiv2015

Quantum probes of timelike naked singularities in $2+1-$ dimensional power - law spacetimes

The formation of naked singularities in $2+1-$ dimensional power - law spacetimes in linear Einstein-Maxwell and Einstein-scalar theories sourced by azimuthally symmetric electric field and a self-interacting real scalar field respectively, are considered in view of quantum mechanics. Quantum test fields obeying the Klein-Gordon and Dirac equations are used to probe the classical timelike naked singularities developed at $r=0$. We show that when the classically singular spacetimes probed with scalar waves, the considered spacetimes remains singular. However, the spinorial wave probe of the singularity in the metric of a self-interacting real scalar field remains quantum regular. The notable outcome in this study is that the quantum regularity/singularity can not be associated with the energy conditions.

physics.gen-ph↗

arXiv2014

Emergent cosmological constant from colliding electromagnetic waves

In this study we advocate the view that the cosmological constant is of electromagnetic (em) origin, which can be generated from the collision of em shock waves coupled with gravitational shock waves. The wave profiles that participate in the collision have different amplitudes. It is shown that, circular polarization with equal amplitude waves does not generate cosmological constant. We also prove that the generation of the cosmological constant is related to the linear polarization. The addition of cross polarization generates no cosmological constant. Depending on the value of the wave amplitudes, the generated cosmological constant can be positive or negative. We show additionally that, the collision of nonlinear em waves in a particular class of Born-Infeld theory also yields a cosmological constant.

physics.gen-ph↗

arXiv2014

Flare-out conditions in static thin-shell wormholes

We reconsider the generalized flare-out conditions in static wormhole throats given by Hochberg and Visser. We show that due to the presence of matter sources on the throat, these conditions are not applicable to the thin-shell wormholes.

gr-qc↗

arXiv2014

Thin-shell wormholes supported by total normal matter

The Zipoy-Voorhees-Weyl (ZVW) spacetime characterized by mass ($M$) and oblateness ($δ$) is proposed in the construction of viable thin-shell wormholes (TSWs). Departure from spherical / cylindrical symmetry yields positive total energy in spite of the fact that local energy density may take negative values. We show that oblateness of the bumpy sources / black holes can be incorporated as a new degree of freedom that may play role in the resolution of the exotic matter problem in TSWs. Small velocity perturbation reveals, however, that the resulting TSW is unstable.

gr-qc↗

arXiv2014

Counterrotational effects on stability of 2+1-dimensional thin-shell wormholes

The role of angular momentum in a 2+1-dimensional rotating thin-shell wormhole (TSW) is considered. Particular emphasis is made on stability when the shells (rings) are counterrotating. We find that counter-rotating halves make the TSW supported by the equation of state of a linear gas more stable. Under a small velocity dependent perturbation, however, it becomes unstable.

gr-qc↗

arXiv2014

Microscopic thin shell wormholes in magnetic Melvin universe

We construct thin shell wormholes in the magnetic Melvin universe. It is shown that in order to make a TSW in the Melvin spacetime the radius of the throat can not be larger than $\frac{2}{B_{0}}$ in which $B_{0}$ is the magnetic field constant. We also analyze the stability of the constructed wormhole in terms of a linear perturbation around the equilibrium point. In our stability analysis we scan a full set of the Equation of States such as Linear Gas, Chaplygin Gas, Generalized Chaplygin Gas, Modified Generalized Chaplygin Gas and Logarithmic Gas. Finally we extend our study to the wormhole solution in the unified Melvin and Bertotti-Robinson spacetime. In this extension we show that for some specific cases, the local energy density is partially positive but the total energy which supports the wormhole is positive.

gr-qc↗

arXiv2014

Stability of generic cylindrical thin shell wormholes

We revisit the stability analysis of cylindrical thin shell wormholes which have been studied in literature so far. Our approach is more systematic and in parallel to the method which is used in spherically symmetric thin shell wormholes. The stability condition is summarized as the positivity of the second derivative of an effective potential at the equilibrium radius, i.e. $V^{\prime \prime}\left(a_{0}\right) >0$. This may serve as the master equation in all stability problems for the cylindrical thin-shell wormholes.

gr-qc↗

arXiv2014

Thin-shell wormholes from the regular Hayward black hole

We revisit the regular black hole found by Hayward in $4-$dimensional static, spherically symmetric spacetime. To find a possible source for such a spacetime we resort to the non-linear electrodynamics in general relativity. It is found that a magnetic field within this context gives rise to the regular Hayward black hole. By employing such a regular black hole we construct a thin-shell wormhole for the case of various equations of state on the shell. We abbreviate a general equation of state by $p=ψ\left( σ\right) $ where $p$ is the surface pressure which is the function of the mass density ($σ$). In particular, a linear, logarithmic, Chaplygin, etc. forms of equations of state are considered. In each case we study the stability of the thin-shell against linear perturbations. We plot the stability regions by tuning the parameters of the theory. It is observed that the role of the Hayward parameter is to make the TSW more stable. Perturbations of the throat with small velocity condition is also studied. The matter of our TSWs, however, remains to be exotic.

gr-qc↗