S. Danial Forghani | Final International University (original) (raw)

Papers by S. Danial Forghani

arXiv (Cornell University), Feb 21, 2020

In this paper, based on the thin-shell formalism, we introduce a classical model for particles in... more In this paper, based on the thin-shell formalism, we introduce a classical model for particles in the framework of n + 1−dimensional n 2-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.

This short paper investigates the harmonic behavior of a general-relativistic thin-shell wormhole... more 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 metricdependent 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 mirrorsymmetric Schwarzschild thin-shell wormhole is studied in this framework as an example.

Physics Letters B, Nov 1, 2022

A new cosmological object in analogy with the concept of a wormhole in general relativity is intr... more 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 a 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, and 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.

arXiv (Cornell University), Dec 12, 2018

Within 2+1-dimensional Cosmological New Massive Gravity Theory we consider thin-shell wormhole co... more Within 2+1-dimensional Cosmological New Massive Gravity Theory we consider thin-shell wormhole construction. For this, we establish first the junction conditions apt for the fourth order terms in the action of the theory. By employing a specific three-parameter, static solution in Massive Gravity we obtain such thin-shell wormholes. Interestingly, these are thermodynamically stable and contain only normal, i.e. non-exotic matter.

arXiv (Cornell University), Oct 19, 2020

A spherical planetary nebula is described as a geometric model. The nebula itself is considered a... more 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.

SSRN Electronic Journal

Journal of Cosmology and Astroparticle Physics, 2019

The European Physical Journal Plus, 2019

International Journal of Modern Physics D, 2019

The thermodynamic stability of a thin-shell wormhole in a Schwarzschild bulk is considered. From ... more 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. Our method applies only to static thin-shell wormholes and in this sense it may be considered as supplementary to the classical method of stability. The scope of applications of the method is not limited by the Schwarzschild wormhole.

The European Physical Journal Plus, 2018

The European Physical Journal C, 2018

International Journal of Geometric Methods in Modern Physics, 2017

Monge Gauge (MG) in differential geometry is generalized. The original MG is based on a surface d... more Monge Gauge (MG) in differential geometry is generalized. The original MG is based on a surface defined as a height function [Formula: see text] 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 MG toward more complicated surfaces. Here, in this study, we consider the height function above a curved surface, namely a sphere of radius [Formula: see text]. The proposed height function is a function of [Formula: see text] and [Formula: see text] 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.

The European Physical Journal C, 2019

Inspired by the recent work of Filho et al., a Hermitian momentum operator is introduced in a gen... more 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 Schr\"odinger equation, which is solved explicitly for a free particle in the Poincar\'e upper half-plane geometry. It is shown that a "free particle" does not behave as it is totally free due to curved background geometry.

The concept of thin-shell wormhole (TSW) was introduced by Visser in 1989 [1, 2] in the hope of k... more The concept of thin-shell wormhole (TSW) was introduced by Visser in 1989 [1, 2] in the hope of keeping the idea of wormholes alive by con ning the exotic matter to a thin shell, called the throat of the TSW. Exotic matter, which inevitably emerges in the theories of wormholes, is an unwanted type of matter that violates the known energy conditions such as the weak energy condition (WEC) [3]. Pre-Visser's theories had the exotic matter distributed on the certain parts of the spacetime, if not all over it. However, Visser's so called cut-and-paste procedure allows us to con ne such a notorious matter on a very limited part of the space, the TSW itself. Moreover, the cut-and-paste procedure has the advantage that can be applied to a vast variety of spacetimes [4{15], while before Visser only some certain spacetimes had the structure of a wormhole [16]. It is also worth mentioning that while TSWs are categorized as traversable wormholes not all the wormholes are considered to b...

arXiv: High Energy Physics - Theory, 2019

By using the formalism of thin-shells, we construct a geometrical model of particle in third-orde... more By using the formalism of thin-shells, we construct a geometrical model of 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.

By using the formalism of thin-shells, in the first part of this study, we construct a static geo... more By using the formalism of thin-shells, in the first part of this study, we construct a static geometrical model of a particle in Maxwell-augmented third-order Lovelock gravity. This particular theory which is valid at least in seven 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. In the second part of the paper, we construct higher dimensional generic spherically symmetric particle models in pure Einstein, pure Gauss-Bonnet and pure third order Lovelock gravities