Advances in High Energy Physics

Volume 2015 (2015), Article ID 283273, 12 pages

http://dx.doi.org/10.1155/2015/283273

## Dynamics of Generalized Tachyon Field in Teleparallel Gravity

^{1}Department of Physics, Islamic Azad University, Babol Branch, Babol, Iran^{2}Department of Basic Sciences, Babol University of Technology, Babol 47148-71167, Iran

Received 11 October 2014; Accepted 15 January 2015

Academic Editor: Chao-Qiang Geng

Copyright © 2015 Behnaz Fazlpour and Ali Banijamali. 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

We study dynamics of generalized tachyon scalar field in the framework of teleparallel gravity. This model is an extension of tachyonic teleparallel dark energy model which has been proposed by Banijamali and Fazlpour (2012). In contrast with tachyonic teleparallel dark energy model that has no scaling attractors, here we find some scaling attractors which means that the cosmological coincidence problem can be alleviated. Scaling attractors are presented for both interacting and noninteracting dark energy and dark matter cases.

#### 1. Introduction

The usual proposal to explain the late-time accelerated expansion of our universe is an unknown energy component, dubbed as dark energy. The natural choice and most attractive candidate for dark energy is the cosmological constant, but it is not well accepted because of the cosmological constant problem [1, 2] as well as the age problem [3]. Thus, many dynamical dark energy models as alternative possibilities have been proposed. Quintessence, phantom, k-essence, quintom, and tachyon field are the most familiar dark energy models in the literature (for reviews on dark energy models see [4, 5]). The tachyon field arising in the context of string theory [4, 5] and its application in cosmology both as a source of early inflation and late-time cosmic acceleration have been extensively studied [6–9].

The so-called “teleparallel equivalent of general relativity” or teleparallel gravity was first constructed by Einstein [10–13]. In this formulation one uses the curvature-less Weitzenbock connection instead of the torsion-less Levi-Civita connection. The relevant Lagrangian in teleparallel gravity is the torsion scalar which is constructed by contraction of the torsion tensor. We recall that the Einstein-Hilbert Lagrangian is constructed by contraction of the curvature tensor. Since teleparallel gravity with torsion scalar as Lagrangian density is completely equivalent to a matter-dominated universe in the framework of general relativity, it cannot be accelerated. Thus one should generalize teleparallel gravity either by replacing with an arbitrary function, the so-called gravity [14–16], or by adding dark energy into teleparallel gravity allowing also a nonminimal coupling between dark energy and gravity. Note that both approaches are inspired by the similar modifications of general relativity, that is, gravity [17, 18] and nonminimally coupled dark energy models in the framework of general relativity [19–26].

Recently Geng et al. [27, 28] have included a nonminimal coupling between quintessence and gravity in the context of teleparallel gravity. This theory has been called “teleparallel dark energy” and its dynamics was studied in [29–31]. Tachyonic teleparallel dark energy is a generalization of teleparallel dark energy by inserting a noncanonical scalar field instead of quintessence in the action [32]. Phase-space analysis of this model has been investigated in [33]. On the other hand, there is no physical argument to exclude the interaction between dark energy and dark matter. The interaction between these completely different components of our universe has the same important consequences such as addressing the coincidence problem [34]. There are also observational evidences of the interaction in dark sector [35–40]. In [35–38] Bertolami et al. have shown that this interaction might imply a violation of the equivalence principle. By using optical, X-ray, and weak lensing data from the relaxed galaxy clusters, Abdalla et al. [39, 40] have found the signature of interaction between dark energy and dark matter. An interacting scenario provides solution to a number of cosmological problems in both canonical scalar field models [41] and nonminimally coupled scalar field models such as Brans-Dick field [42, 43].

In this paper, we consider generalized tachyon field as responsible for dark energy in the framework of teleparallel gravity. We will be interested in performing a dynamical analysis of such a model in FRW space time. In such a study we investigate our model for both interacting and noninteracting cases. We consider the nonminimal coupling function of the form and choose in (2). The basic equations are presented in Section 2. In Section 3 the evolution equations are translated in the language of the autonomous dynamical system by suitable transformation of the basic variables. Section 3.1 deals with phase-space analysis as well as the cosmological implications of the equilibrium points of the model in noninteracting dark energy dark matter case. In Section 3.2 an interaction between dark energy and dark matter has been considered and critical points and their behavior have been extracted. Section 4 is devoted to a short summary of our results.

#### 2. Basic Equations

Our model is described by the following action as a generalization of tachyon teleparallel dark energy model [32]: where ( are the orthonormal components of the tetrad), while is the Lagrangian of teleparallelism with as the torsion scalar (for an introductory review of teleparallelism see [12]). shows nonminimal coupling of generalized tachyon field with gravity in the framework of teleparallel gravity and . The second part in is the Lagrangian density of the generalized tachyon field which has been studied in [44, 45]. is the nonminimal coupling function, is a dimensionless constant measuring the nonminimal coupling, and is the matter Lagrangian. For our model reduced to tachyonic teleparallel dark energy discussed in [32]. Here we consider the case for two reasons. The first is that for arbitrary our equations will be very complicated and one can not solve them analytically and the second is that for we will obtain interesting physical results as we will see below.

Furthermore, due to complexity of tachyon dynamics, [46] has proposed an approach based on the redefinition of the tachyon field as follows: In order to obtain a closed autonomous system and perform the phase-space analysis of the model, we apply (3) in (2) for that leads to the following action: In a spatially flat FRW space-time, and a vierbein choice of the form , the corresponding Friedmann equations are given by where is the Hubble parameter and a dot stands for the derivative with respect to the cosmic time . In these equations, and are the matter energy density and pressure, respectively.

The effective energy density and pressure of generalized tachyon dark energy read where .

The equation of motion of the scalar field can be obtained by variation of the action (4) with respect to : with a general interaction coupling term between dark energy and dark matter, , and . In (7) and (8) we have used the useful relation: which simply arises from the calculation of torsion scalar for the FRW metric (5). The scalar field evolution (8) expresses the continuity equation for the field and matter as follows: where is the equation of state parameter of dark energy which is attributed to the scalar field . The barotropic index is defined by with .

Although dynamics of tachyonic teleparallel dark energy has been studied in [33], no scaling attractors has been found. Here we are going to perform a phase-space analysis of generalized tachyonic teleparallel dark energy and as we will see below that some interesting scaling attractors appear in such theory.

#### 3. Cosmological Dynamics

In order to perform phase-space and stability analysis of the model, we introduce the following auxiliary variables: The auxiliary variables allow us to straightforwardly obtain the density parameter of dark energy and dark matter: while the equation of state of the field reads where and From now we concentrate on exponential scalar field potential of the form and the nonminimal coupling function of the form . These choices lead to constant and , respectively.

Another quantities with great physical significance, namely, the total equation of state parameter and the deceleration parameter, are given by Using auxiliary variables (11) the evolution equations (6) and (8) can be recast as a dynamical system of ordinary differential equations: where , , and prime in (18) denotes differentiation with respect to the so-called e-folding time .

The next step is the introduction of interaction term to obtain an autonomous system out of (18). The fixed points for which depend on the choice of the interaction term and two general possibilities will be treated in the sequel. The stability of the system at a fixed point can be obtained from the analysis of the determinant and trace of the perturbation matrix . Such a matrix can be constructed by substituting linear perturbations , , and about the critical point into the autonomous system (18). The matrix of the linearized perturbation equations of the autonomous system is shown in the appendix. Therefore, for each critical point we examine the sign of the real part of the eigenvalues of . According to the usual dynamical system analysis, if the eigenvalues are real and have opposite signs, the corresponding critical point is a saddle point. A fixed point is unstable if the eigenvalues are positive and it is stable for negative real part of the eigenvalues.

In the following subsections we will study the dynamics of generalized tachyon field with different interaction term . Although one can work with a general ( and correspond to dust matter and radiation, resp.) without loss of generality, we assume for simplicity.

##### 3.1. The Case for

The first case clearly means that there is no interaction between dark energy and background matter. In this case, there are two critical points presented in Table 1. From (12) and (14) one can obtain the corresponding values of density parameter and equation of state of dark energy at each point. Also, using (17) we can find the condition required for acceleration at each point. These parameters and conditions have been shown in Table 1. The stability and existence conditions of critical points and are presented in Table 2. We mention that the corresponding eigenvalues of perturbation matrix at critical points and are considerably involved and here we do not present their explicit expressions, but we can find sign of them numerically.