arXiv subjects
Publications and source records attributed to S. Habib Mazharimousavi.
The quartic self-interacting conformal scalar field is used to construct a thin-shell wormhole satisfying all energy conditions. Accompanying the scalar field is the extremal Reissner-Nordström black hole with a positive cosmological constant. New junction conditions apt for the higher-order terms are introduced in the Gaussian normal coordinates. Our approach may provide a guideline towards getting rid of exotic matter in TSWs.
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.
We investigate the infinite discontinuity points of stability diagram in thin-shell wormholes. The square of the speed of sound $β_{0}^{2}$,\ which is expressed in terms of pressure and energy density at equilibrium on the throat, arises with a divergent amplitude. As this is physically non-acceptable, \ we revise the equation of state, such that by fine-tuning of the pressure at static equilibrium, which is at our disposal, eliminates such a singularity. The efficacy of the method is shown in Schwarzschild, extremal Reissner-Nordström, and dilaton thin-shell wormholes.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.