Advances in High Energy Physics

Volume 2015 (2015), Article ID 608252, 20 pages

http://dx.doi.org/10.1155/2015/608252

## Dynamics of Mixed Dark Energy Domination in Teleparallel Gravity and Phase-Space Analysis

^{1}Department of Physics, Sinop University, Korucuk, 57000 Sinop, Turkey^{2}Department of Statistics, Sinop University, Korucuk, 57000 Sinop, Turkey

Received 16 October 2015; Revised 26 November 2015; Accepted 29 November 2015

Academic Editor: Frank Filthaut

Copyright © 2015 Emre Dil and Erdinç Kolay. 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 consider a novel dark energy model to investigate whether it will provide an expanding universe phase. Here we propose a mixed dark energy domination which is constituted by tachyon, quintessence, and phantom scalar fields nonminimally coupled to gravity, in the absence of background dark matter and baryonic matter, in the framework of teleparallel gravity. We perform the phase-space analysis of the model by numerical methods and find the late-time accelerated attractor solutions implying the acceleration phase of universe.

#### 1. Introduction

The universe is known to be experiencing an accelerating expansion by astrophysical observations such as Supernova Ia [1, 2], large scale structure [3, 4], the baryon acoustic oscillations [5], and cosmic microwave background radiation [6–9].

In order to explain the late-time accelerated expansion of universe, an unknown form of energy, called dark energy, is proposed. This unknown component of energy possesses some interesting properties; for instance, it is not clustered but spread all over the universe and its pressure is negative for driving the current acceleration of the universe. Observations show that the dark energy occupies 70% of our universe.

What is the constitution of the dark energy? One candidate for the answer of this question is the cosmological constant having a constant energy density filling the space homogeneously [10–13]. But cosmological constant is not well accepted since the cosmological problem [14] and the age problem [15]. For this reason, many other dark energy models have been proposed instead of the cosmological constant. Other candidates for dark energy constitution are quintessence, phantom, and tachyon fields. We can briefly classify the dark energy models in terms of the most powerful quantity of dark energy; its equation of state parameter , where and are the pressure and energy density of the dark energy, respectively. For cosmological constant boundary , but for quintessence the parameter , for phantom , and for nonminimally coupled tachyon with gravity both and [16–18].

The scenario of crossing the cosmological constant boundary is referred to as a “Quintom” scenario. The explicit construction of Quintom scenario has a difficulty, due to a no-go theorem. The equation of state parameter of a scalar field cannot cross the cosmological constant boundary according to no-go theorem, if the dark energy described by the scalar field is minimally coupled to gravity in Friedmann-Robertson-Walker (FRW) geometry. The requirement for crossing the cosmological constant boundary is that the dark energy should be nonminimally coupled to gravity; namely, it should interact with the gravity [19–24]. There are also models in which possible coupling between dark energy and dark matter can occur [25, 26]. In this paper, we consider a mixed dark energy model constituted by tachyon, quintessence, and phantom scalar fields nonminimally coupled to gravity.

The mixed dark energy model in this study is considered in the framework of teleparallel gravity instead of classical gravity. The teleparallel gravity is the equivalent form of the classical gravity, but in place of torsion-less Levi-Civita connection, curvature-less Weitzenbock one is used. The Lagrangian of teleparallel gravity contains torsion scalar constructed by the contraction of torsion tensor, in contrast to the Einstein-Hilbert action of classical gravity in which contraction of the curvature tensor is used. In teleparallel gravity, the dynamical variable is a set of four tetrad fields constructing the bases for the tangent space at each point of space-time [27–29]. The teleparallel gravity Lagrangian with only torsion scalar corresponds to the matter-dominated universe; namely, it does not accelerate. Therefore, to obtain a universe with an accelerating expansion, we can either replace with a function , the so-called gravity (teleparallel analogue of gravity) [30–32], or add an unknown form of energy, so-called dark energy, to the teleparallel gravity Lagrangian that allows also nonminimal coupling between dark energy and gravity to overcome the no-go theorem. The interesting feature of theories is the existence of second or higher order derivatives in equations. Therefore, we prefer the second choice; adding extra scalar fields of the unknown energy forms as dark energy.

As different dark energy models, interacting teleparallel dark energy studies have been introduced in the literature; for instance, Geng et al. [33, 34] consider a quintessence scalar field with a nonminimal coupling between quintessence and gravity in the context of teleparallel gravity. The dynamics of this model has been studied in [35–37]. Tachyonic teleparallel dark energy is a generalization of the teleparallel quintessence dark energy by introducing a noncanonical tachyon scalar field in place of the canonical quintessence field [18, 38–40].

In this study, the main motivation is that we consider a more general dark energy model including three kinds of dark energy models, instead of taking one dark energy model as in [16, 18, 38–40]. In order to explain the expansion of universe by adding scalar fields as the dark energy constituents, there has never been assumed a cosmological model including three kinds of dark energy models. We assume tachyon, quintessence, and phantom fields as a mixed dark energy model which is nonminimally coupled to gravity in the framework of teleparallel gravity. We make the dynamical analysis of the model in FRW space-time. Later on, we translate the evolution equations into an autonomous dynamical system. After that the phase-space analysis of the model and the cosmological implications of the critical (or fixed) points of the model will be studied from the stability behavior of the critical points. Finally, we will make a brief summary of the results.

#### 2. Dynamics of the Model

Our model consists of three scalar fields as the three-component dark energy domination without background dark matter and baryonic matter. These are the canonical scalar field, quintessence , and two noncanonical scalar fields, tachyon and phantom , and all these scalar fields are nonminimally coupled to gravity. Since we consider only the dark energy dominated sector without the matter content of the universe, the action of the mixed teleparallel dark energy with a nonminimal coupling to the gravity can be written as [16, 38, 39] where and are the orthonormal tetrad components, such thatwhere , run over 0, 1, 2, and 3 for the tangent space at each point of the manifold and , take the values 0, 1, 2, and 3 and are the coordinate indices of the manifold. While is the torsion scalar, it is defined asHere is the torsion tensor constructed by the Weitzenbock connection , such that [41] All the information about the gravitational field is contained in the torsion tensor in teleparallel gravity. The Lagrangian of the theory is set up according to the conditions of invariance under general coordinate transformations, global Lorentz transformations, and the parity operations [42].

Furthermore, in (1) and , , and are the functions responsible for nonminimal coupling between gravity and tachyon, quintessence, and phantom fields, respectively. , , and are the dimensionless coupling constants and , , and are the potentials for tachyon, quintessence, and phantom fields, respectively.

We consider a spatially flat FRW metricand a tetrad field of the form . Then the Friedmann equations for FRW metric read aswhere is the Hubble parameter, is the scale factor, and dot represents the derivative with respect to cosmic time . and are the energy density and the pressure of the corresponding scalar field constituents of the dark energy.

Conservation of energy gives the evolution equations for the dark energy constituents, such asThe total energy density and the pressure of dark energy readwith the total equation of state parameterwhere , , and are the equation of state parameters and , , and are the density parameters for the tachyon, quintessence, and phantom fields, respectively. Then the total density parameter is defined aswhere we assume that three kinds of scalar fields constitute the dark energy with an equal proportion of density parameter such that .

The Lagrangian of the scalar fields is reexpressed from the action in (1), asThen the energy density and pressure values for three scalar fields can be found by the variation of the total Lagrangian in (1) with respect to the tetrad field . After the variation of Lagrangian, there come contributions from the geometric terms so the -component and -component of the stress-energy tensor give the energy density and the pressure, respectively. Accordingly the energy density and pressure values for the tachyon, quintessence, and phantom fields read asrespectively. Here we have used the relation , and prime denotes the derivative of related coupling functions with respect to the related field variables. Now, we can find the equation of motions for three scalar fields from the variation of the field Lagrangians (11) with respect to the field variables , , and , such thatThese equations of motions are for the tachyon, quintessence, and phantom constituents of dark energy, respectively. Here, the prime of potentials denotes the derivative of related field potentials with respect to the related field variables. All these evolution equations in (18) can also be obtained by using the relations (12)–(17) in the continuity equations (7).

We now perform the phase-space analysis of the model in order to investigate the late-time solutions of the universe considered here.

#### 3. Phase-Space and Stability Analysis

We study the properties of the constructed dark energy model by performing the phase-space analysis. Therefore we transform the aforementioned dynamical system into its autonomous form [38, 39, 43–46]. To proceed we introduce the auxiliary variablestogether with and for any quantity , the time derivative is .

We rewrite the density parameters for the fields , , and in the autonomous system by using (10), (12), (14), and (16) with (19)and the total density parameter iswhere . Then the equation of state parameters can be written in the autonomous form by using (12)–(17) in for every scalar field, such aswhere , , and . From (9) and (20) and (22), we obtain in the autonomous system, asWe can express in the autonomous system by using (6) and (23), such thatHere is only a jerk parameter used in other equations of cosmological parameters. Then the deceleration parameter is

Now we transform the equations of motions (6) and (18) into the autonomous system containing the auxiliary variables in (19) and their derivatives with respect to . Thus we obtain , where is the column vector including the auxiliary variables and is the column vector of the autonomous equations. After writing , we find the critical points of , by setting . We expand around , where is the column vector of perturbations of the auxiliary variables, such as , , and for each scalar field. Thus, we expand the perturbation equations up to the first order for each critical point as , where is the matrix of perturbation equations. For each critical point, the eigenvalues of perturbation matrix determine the type and stability of the critical points [47–50].

Particularly, the autonomous form of the cosmological system in (6) and (18) is [51–60] where , , and . Henceforth, we assume the nonminimal coupling functions , , and ; thus , , and are constant. Also the usual assumption in the literature is to take the potentials , , and [43, 61–63]. Such potentials give also constants , , and .

Now we perform the phase-space analysis of the model by finding the critical points of the autonomous system in (26). To obtain these points, we set the left hand sides of (26) to zero. After some calculations, four critical points are found by assuming and as −1 for each critical point. The critical points are listed in Table 1 with the existence conditions.