Advances in High Energy Physics

Volume 2017, Article ID 1215254, 9 pages

https://doi.org/10.1155/2017/1215254

## Stable Dyonic Thin-Shell Wormholes in Low-Energy String Theory

^{1}Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4950, Valparaíso, Chile^{2}Physics Department, Arts and Sciences Faculty, Eastern Mediterranean University, Famagusta, Northern Cyprus, Mersin 10, Turkey^{3}Physics Department, State University of Tetovo, Ilinden Street nn, 1200 Tetovo, Macedonia^{4}Institute of Physics, Faculty of Natural Sciences and Mathematics, Ss. Cyril and Methodius University of Skopje, Arhimedova 3, 1000 Skopje, Macedonia

Correspondence should be addressed to Kimet Jusufi; km.ude.etinu@ifusuj.temik

Received 22 June 2017; Revised 15 September 2017; Accepted 3 October 2017; Published 1 November 2017

Academic Editor: George Siopsis

Copyright © 2017 Ali Övgün and Kimet Jusufi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

Considerable attention has been devoted to the wormhole physics in the past 30 years by exploring the possibilities of finding traversable wormholes without the need for exotic matter. In particular, the thin-shell wormhole formalism has been widely investigated by exploiting the cut-and-paste technique to merge two space-time regions and to research the stability of these wormholes developed by Visser. This method helps us to minimize the amount of the exotic matter. In this paper, we construct a four-dimensional, spherically symmetric, dyonic thin-shell wormhole with electric charge , magnetic charge , and dilaton charge , in the context of Einstein-Maxwell-dilaton theory. We have applied Darmois-Israel formalism and the cut-and-paste method by joining together two identical space-time solutions. We carry out the dyonic thin-shell wormhole stability analyses by using a linear barotropic gas, Chaplygin gas, and logarithmic gas for the exotic matter. It is shown that, by choosing suitable parameter values as well as equation of state parameter, under specific conditions, we obtain a stable dyonic thin-shell wormhole solution. Finally, we argue that the stability domain of the dyonic thin-shell wormhole can be increased in terms of electric charge, magnetic charge, and dilaton charge.

#### 1. Introduction

Wormholes are exotic objects predicted by Einstein’s theory of gravity which act as a space-time tunnel by connecting two different regions of the universe. Though the idea of wormholes is not new [1, 2], the interest in wormholes was recently reborn by the seminal work of Morris and Thorne [3] who studied traversable wormholes. There are, however, several problematic issues related to the possible existence of wormholes; in particular, it was shown that the existence of wormholes requires the violation of energy conditions [4, 5]. Another major problem is related to the stability analysis of wormholes. On the other hand, Visser attempted to minimize the existence of the exotic matter by constructing infinitesimally small thin-shell wormholes [6–9]. Visser’s method is based on the cut-and-paste technique by joining together two identical space-time solutions and making use of the Darmois-Israel formalism [10] to compute the surface stress-energy tensor components. Finally, these results can be used to study the wormhole dynamics with the help of Lanczos equations.

This method was applied to construct a number of thin-shell wormholes (TSW), including charged TSW [11, 12], TSW with a cosmological constant [13], TSW in dilaton gravity [14], TSW from the regular Hayward black hole [15], TSW in higher-dimensional Einstein-Maxwell theory [16, 17], rotating TSW [18, 19], quantum corrected TSW in Bohmian quantum mechanics [20], primordial wormholes induced from Grand Unified Theories (GUTs) [21, 22], canonical acoustic TSW, charged TSW with dilaton field, TSW with a Chaplygin gas, traversable wormholes in the anti-de Sitter space-time, TSW with a negative cosmological constant, wormholes in mimetic gravity, TSW from charged black string, cylindrical TSW, and many other interesting papers [23–58], while the stability analysis is investigated by different models, for example, linear perturbations [9] and specific equations of state (EoS) such as linear barotropic gas (LBG), Chaplygin gas (CG), and logarithmic gas (LogG) for the exotic matter [14, 59–62].

Recently, Goulart found a four-dimensional, spherically symmetric, dyonic black hole and charged wormhole solution in the low-energy effective actions of string theory or supergravity theory [63, 64]. Furthermore, in [65], a time-dependent spherically symmetric black hole solution in the context of low-energy string theory was investigated. The solution found by Goulart is of particular interest since it can be written in terms of five independent parameters: the electric charge , the magnetic charge , the value of the dilation of infinity , and two integration constants, and . Inspired by this work, we aim to use this solution and construct a four-dimensional TSW wormhole in the context of Einstein-Maxwell-dilaton (EMD) theory and then investigate the role of electric charge , magnetic charge , and dilaton charge on the stability domain of the wormhole.

The structure of this paper is as follows. In Section 2, we review briefly the dyonic black hole solutions. In Section 3, using Visser’s cut-and-paste technique, we construct a dyonic thin-shell wormhole (DTSW). In Section 4, we check the stability conditions for different types of gases such as LBG, CG, and LogG for the exotic matter. In Section 5, we comment on our results.

#### 2. Dyonic Black Holes in the EMD Theory

In this part, we use the dyonic black hole solutions in the EMD theory found by Goulart [63]. Firstly, we consider the action of the EMD without a dilaton potential and without an axion:where the field strength is given byFurthermore, for constant axion field, the bosonic sector of version of supergravity theory is [41]. It is noted that there are five independent parameters, that is, , and . Accordingly, the space-time of the general spherically symmetric solution is given by the line element [63]wherewithNote that the corresponding electric and magnetic charges are and , respectively. stands for the value of the dilaton at infinity. Furthermore, there are two integration constants, that is, and . On the other hand, and are dependent constants inasmuch as they transform into each other under S-duality (i.e., and ). It is noted that , which is the dilaton coupling, is also invariant. Here, and are the inner and outer horizons, respectively [63].

The Hawking temperature is calculated byand the entropy of the black hole is One can also define the dilaton charge as follows:where, depending on the values of electric/magnetic charge of black hole, it can be positive or negative. Firstly, the four parameters’ dyonic solution is found in [37]. Here, the key point is that there is no boundary condition on and to make this dyonic black hole.

The Ricci scalar is calculated as follows: The domain of is restricted with the causality. The singularity is found at for , or at for .

One of the special cases which we use to construct a DTSW when is that the dilaton charge is a constant such as . Furthermore, we suppose that and [37]. The solution becomes One can find the magnetically charged solutions of [38, 39] by using and also the Schwarzschild solution by setting .

#### 3. Construction of DTSW

Let us now proceed to use the cut-and-paste technique to construct a DTSW using metric (3). Consider two spherically symmetric space-time solutions of the dyonic black hole metric in 4 dimensions and then remove from each four-dimensional manifold the regions described by [9] where is the radius of the throat of the DTSW with an important condition . In other words, should be greater than the event horizon in order to avoid the formation of an event horizon. Next, paste these two manifolds at the boundary hypersurface given by which results with a geodesically complete manifold . According to the Darmois-Israel formalism, we can choose the coordinates on as , while the coordinates on the induced metric are . For the parametric equation on , we can write

Our main goal is to compare various characteristics of EMD theory and dyonic black hole, such as the surface stress-energy tensor and the basic question of stability. For this purpose, we define the dynamical induced metric on that can now be written in terms of the proper time on the shell, where , as follows:

The junction conditions on imply from the Lanczos equations in which is the energy momentum tensor on the thin shell and and are defined as and , respectively. Furthermore, the extrinsic curvature is defined by

We can choose the unit vectors , such that and normal to as follows:

Adopting the orthonormal basis , for metric (3), the extrinsic curvature components are found as [24]where the prime and the dot represent the derivatives with respect to and , respectively. With the definitions of and and the introduction of the surface stress-energy tensor , we have the Einstein equations on the shell (also called the Lanczos equations):which in our case results in a shell of radius with energy density and transverse pressure . Using the above results from the Lanczos equations, one can easily check that the surface density and the surface pressure are given by the following relations [24, 25]:

Note that the energy density is negative at the throat because of the flare-out condition in which the area is minimal at the throat (then increases for close to and ), so we have exotic matter. From the last two equations, we can now write the static configuration of radius ; by setting and , we get

From (21), we see that the surface density is negative (i.e., ); as a consequence of this, the WEC is violated. The amount of exotic matter concentrated at the wormhole is calculated by the following integral:

In the case of a TSW, we have and , where is the Dirac delta function. The above integral can be easily evaluated if we first make use of the Dirac delta function

Substituting the value of energy density in the last equation, for the energy density located on a thin-shell surface in static configuration, we find

To analyze the attractive and repulsive nature of the wormhole, we can calculate the observer’s four-acceleration , where the four-velocity reads . For the radial component of the four-acceleration, we find

One can easily observe that the test particle obeys the equation of motion

We conclude from the last equation that if , we get the geodesic equation, while the wormhole is attractive when and repulsive when .

#### 4. Stability Analysis

In this section, using the formalism developed in Section 3, we calculate the potential and define the stability method for DTSW. From the energy conservation, we have [24]where the area of the wormhole is calculated by . It is noted that the internal energy of the throat is located at the left side of (28) as a first term. Then, the second term represents the work done by the internal forces of the throat; on the other hand, there is a flux term in the right side of the equation. Furthermore, to calculate the equation of dynamics of the wormhole, we use in (19) and find this simple equation:with potentialA Taylor expansion to the second order of the potential around the static solution yields [24]From (30), the first derivative of isand the last equation takes the formThe second derivative of the potential is

Since , replacing the parameter and , we have that , and using (35) again, we obtain

The wormhole is stable if and only if .

##### 4.1. Stability Analysis of DTSW via the LBG

In what follows, we will use three different gas models for the exotic matter to explore the stability analysis: LBG [40], CG [61, 62], and finally LogG [15].

The equation of state of LBG [9, 14, 59, 60] is given by and hence where is a constant parameter. For more useful information as regards the effects of the parameters , , and , we show graphically the DTSW stability in terms of and , as depicted in Figure 1.