arXiv subjects

S. Danial Forghani

Publications and source records attributed to S. Danial Forghani.

15 recordsLinked to original sources

arXiv2022

Harmonic Thin-Shell Wormhole Supported by Metric-Dependent Equation of State

Abstract This short paper investigates the harmonic behavior of a general-relativistic thin-shell wormhole supported by a particular type of exotic fluid. The exotic fluid obeys a never-studied-before metric-dependent equation of state. This equation of state is tailored such that the wormhole's throat undergoes a harmonic-like behavior before damping to an equilibrium radius. Eventually, the mirror-symmetric Schwarzschild thin-shell wormhole is studied in this framework as an example.

gr-qc↗

arXiv2022

A Closed Universe: de Sitter Cosmic Gate

A new cosmological object in analogy with the concept of a wormhole in general relativity is introduced. As wormholes connect two distant points through a tunnel in spacetime, this new object connects two spacetime through a large mouth which is referred to as a "Cosmic Gate". In this context, two identical copies of the regular part of the de Sitter spacetime are cut through a timelike hyperplane. Then, they are glued at their identical boundaries (the only boundary) to form a complete spacetime. The stability of the cosmic gate against a linear radial perturbation is studied as well. Finally, the radial geodesics of the spacetime for a timelike and a null particle are presented.

gr-qc↗

arXiv2020

A Geometrical Model for the Evolution of Spherical Planetary Nebulae Based on Thin-Shell Formalism

A spherical planetary nebula is described as a geometric model. The nebula itself is considered as a thin-shell which visualized as a boundary of two spacetimes. The inner and outer curvature tensors of the thin-shell are found in order to get an expression of the energy-momentum tensor on the thin-shell. The energy density and pressure expressions are derived using the energy-momentum tensor. The time evolution of the radius of the thin-shell is obtained in terms of the energy density. The model is tested by using a simple power function for decreasing energy density and the evolution pattern of the planetary nebula is attained.

gr-qc↗

arXiv2020

Higher Dimensional Particle Model in Pure Lovelock Gravity

In this paper, based on the thin-shell formalism, we introduce a classical model for particles in the framework of $n+1-$dimensional $\left[\frac{n}{2}\right]$-order pure Lovelock gravity. In particular, we construct a spherically symmetric particle of radius $a$ whose inside is a flat Minkowski spacetime while its outside is charged pLG solution. Knowing that in $n+1-$dimensional spherically symmetric Einstein gravity ($R$-gravity) such a particle model cannot be constructed, as we have discussed first, provides the main motivation for this study. In fact, it is the richness of Lovelock parameters that provides such a particle construction possible. On the thin-shell, the energy-momentum components are chosen to vanish, yet their normal derivatives are non-zero.

physics.gen-ph↗

arXiv2020

Thin-shell Wormholes with Ordinary Matter in Pure Gauss-Bonnet Gravity

In this paper, we introduce higher dimensional thin-shell wormholes in pure Gauss-Bonnet gravity. The focus is on thin-shell wormholes constructed by $N\geq5$-dimensional spherically symmetric vacuum solutions. The results suggest that, under certain conditions, it is possible to have thin-shell wormholes that both satisfy the weak energy condition and be stable against radial perturbations.

gr-qc↗

arXiv2020

Higher Dimensional Particle Model in Third-Order Lovelock Gravity

By using the formalism of thin-shells, we construct a geometrical model of a particle in third-order Lovelock gravity. This particular theory which is valid at least in 7 dimensions, provides enough degrees of freedom and grounds towards such a construction. The particle consists of a flat interior and a non-black hole exterior spacetimes whose mass, charge, and radius are determined from the junction conditions, in terms of the parameters of the theory.

hep-th↗

arXiv2020

Behavior of a Free Quantum Particle in the Poincaré Upper Half-Plane Geometry

Inspired by the recent work of Filho et al., a Hermitian momentum operator is introduced in a general curved space with diagonal metric. The modified Hamiltonian associated with this new momentum is calculated and discussed. Furthermore, granting the validity of the Heisenberg equation in a curved space, the Ehrenfest theorem is generalized and interpreted with the new position-dependent differential operator in a curved space. The modified Hamiltonian leads to a modified time-independent Schrödinger equation, which is solved explicitly for a free particle in the Poincaré upper half-plane geometry. It is shown that a "free particle" does not behave as it is totally free due to curved background geometry.

hep-th↗

arXiv2020

Thin-Shell Wormhole Satisfying Energy Conditions

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.

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

Discontinuity Problem in the Linear Stability Analysis of Thin-Shell Wormholes

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.

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

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↗

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

Generalized Monge gauge

Monge gauge in differential geometry is generalized. The original Monge gauge is based on a surface defined as a height function $h(x,y)$ above a flat reference plane. The total curvature and the Gaussian curvature are found in terms of the height function. Getting benefits from our mathematical knowledge of general relativity, we shall extend the Monge gauge toward more complicated surfaces. Here in this study we consider the height function above a curved surface namely a sphere of radius $R$. The proposed height function is a function of $θ$ and $φ$ on a closed interval. We find the first, second fundamental forms and the total and Gaussian curvatures in terms of the new height function. Some specific limits are discussed and two illustrative examples are given.

physics.gen-ph↗