Advances in High Energy Physics

Volume 2019, Article ID 4026856, 8 pages

https://doi.org/10.1155/2019/4026856

## Phantom-Like Behavior in Modified Teleparallel Gravity

^{1}Department of Physics, Faculty of Basic Sciences, University of Mazandaran, P.O. Box 47416-95447, Babolsar, Iran^{2}Research Institute for Astronomy and Astrophysics of Maragha (RIAAM), P. O. Box 55134-441, Maragha, Iran^{3}Department of Physics, Azarbaijan Shahid Madani University, P.O. Box 53714-161, Tabriz, Iran

Correspondence should be addressed to Raheleh Shojaee; ri.ca.vinuraza@eeajohs.r

Received 18 October 2018; Revised 18 January 2019; Accepted 4 February 2019; Published 20 March 2019

Academic Editor: Sunny Vagnozzi

Copyright © 2019 Sakineh Karimzadeh and Raheleh Shojaee. 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.

#### Abstract

The recent data from Planck2018 shows that the equation of state parameter of effective cosmic fluid today is . This indicates that it is possible for the universe to be in a phantom dominated era today. While a phantom field is essentially capable of explaining this observation, it suffers from some serious problems such as instabilities and violation of the null energy condition. So, it would be interesting to realize this effective phantom behavior without adopting a phantom field. In this paper, we study possible realization of an effective phantom behavior in modified teleparallel gravity. We show that modified teleparallel gravity is able essentially to realize an effective phantom-like behavior in the absence of a phantom field. For this purpose, we choose some observationally viable functions and prove that there are some subspaces of the models parameter space capable of realizing a phantom-like behavior without adopting a phantom field.

#### 1. Introduction

Different cosmological observations of the last two decades such as measurements of luminosity distances of Supernova Type Ia (SNIa) [1, 2], the cosmic microwave background (CMB) temperature anisotropy through the Wilkinson Microwave Anisotropy probe (WMAP) satellite [3–5], large scale structure [6], the integrated Sachs-Wolfe effect [7], and more recently the Planck satellite data [8, 9] show that the universe is currently in a positively accelerating phase of expansion. Since general relativity with ordinary matter and energy components cannot explain such a novel observation, different solutions have been proposed to justify this late time cosmic speed-up. There are generally two approaches to explain this observation by modification of Einstein’s field equations. The first approach attempts to modify the energy momentum (matter) part of the field equations by adding a yet unknown component dubbed dark energy, and the second approach tries to modify the geometric part of the field equations. Scalar fields dark energy models [10–12] belong to the first category, while modified gravity theories such as gravity [13, 14], Gauss-Bonnet and gravity [15, 16], massive gravity [17], Lovelock gravity [18], extra-dimensional gravity [19, 20], Teleparallel Equivalent of General Relativity (TEGR) [21–25], and gravity [26–28] belong to the second category. A simple candidate for dark energy proposal is the cosmological constant [29, 30] and the corresponding cosmological model (the CDM) has very good agreement with recent observational data. But this scenario has not a dynamical behavior and its equation of state parameter stays always at . The origin of cosmological constant is not yet well understood; it needs a huge amount of fine-tuning and it is impossible to realize phantom divide line crossing in the pure CDM model. Because of these shortcomings with cosmological constant, in recent years attention has been drawn towards modified gravity theories and, among the various kinds of modified gravity models, teleparallel gravity (TEGR) and gravity have recently obtained a lot of regard. Teleparallel gravity is completely equivalent to general relativity at the level of equations and in this theory the Lagrangian is written in terms of the torsion scalar . Much similar to the gravity that one replaces by in the Einstein-Hilbert action of general relativity, gravity is an extension of TEGR that replaces by a generic function of torsion as in teleparallel gravity. In this model the Ricci scalar is zero since there is no curvature; instead the torsion field is considered in the framework of Einstein’s other (teleparallel) gravity. In recent years, various aspects of teleparallel and modified teleparallel gravity are studied in details (see [31–37] for some various works in this field). The observational viability of teleparallel and modified teleparallel gravity has been studied in [38, 39].

With an* effective phantom-like behavior* one means that the effective energy density of the cosmic fluid is positive and increases with time, and at the same time the effective equation of state parameter stays less than [40]. Typically for realization of such a behavior phantom fields are considered, while the existence of phantom field causes instabilities and violates the null energy condition [41–43]. Possible realization of phantom-like behavior in some cosmological models such as the DGP braneworld model [44–46] and gravity is studied [47]. Also possible existence of a phantom-like phase in teleparallel and also modified teleparallel gravity theories have been studied in some references such as = [28, 48–51]. In this paper, we study possible realization of the phantom-like behavior in modified teleparallel, , gravity. In order to achieve this goal, we choose three viable models of gravity and, with appropriate selection of the models parameters, we show that phantom-like behavior can be realized without any phantom fields in these models of modified teleparallel gravity. This result is important because unlike phantom fields, gravity respects the null energy condition and, more importantly, matter is always stable in this framework [52]. The paper is organized as follows. In Section 2, we briefly review field equations in gravity. In Section 3, we choose three models of in order to see whether the adopted models have an effective phantom behavior or not. For this purpose, we plot the effective energy density and the equation of state parameter versus the redshift and compare the results with the latest observational data from Planck2018. Finally, in Section 4, we present the summary and conclusion.

#### 2. Field Equations for Modified Teleparallel Gravity

##### 2.1. Field Equations of Gravity

In teleparallel (Einstein’s other) gravity one needs to define four orthogonal vector fields named* tetrads* which form a basis for spacetime manifold. In this framework, the manifold and the Minkowski metrics are related as follows [21, 52]: where the Greek indices run from to in coordinate basis of the manifold while the Latin indices run the same but in the tangent space of the manifold and is the Minkowski metric. The connection in teleparallel gravity, that is, the Weitzenbock connection, is defined as follows: which gives the spacetime a nonzero torsion but zero curvature in contrast to general relativity. By this definition the torsion tensor and its permutations arewhere is called the* Superpotential*. In correspondence with Ricci scalar we define a torsion scalar as So, the gravitational action of teleparallel gravity can be written as follows: where is the determinant of the vierbein which is equal to . Variation of this action with respect to the vierbeins gives the teleparallel field equations as follows: Now similar to modifying the action of general relativity in which is replaced by a general function , one can replace the teleparallel action by a function . Doing this, the resulting modified field equations are where is the energy momentum tensor of matter, , and . We note that we have set the spin connection to zero (as usually it is done in the formulation of gravity). In this case the theory does not have local Lorentz invariance. In what follows we set .

##### 2.2. Cosmological Considerations

Now for cosmological considerations we assume a spatially flat FRW metric as [34, 53] where is the scale factor of the universe. For Lagrangian density, we obtain where is the Hubble parameter. Therefore, the cosmological field equations become and where and are the ordinary matter energy density and pressure, respectively. The conservation equation in this framework is as follows: Now, (12) and (13) can be rewritten as follows: and where and are the energy density and pressure of the* torsion fluid* that are defined as follows, respectively,Finally, the equation of state parameter of the torsion fluid, , is defined as Note that if we consider , then (17) and (19) can be rewritten as follows:respectively. With these preliminaries, in the next section we study possible realization of the phantom-like behavior in some observationally viable models.

#### 3. Phantom-Like Behavior in Gravity

To have an effective phantom-like behavior, the effective energy density must be positive and growing positively with time. Also the equation of state parameter should be less than . It has been shown that modifying the geometric part of the Einstein field equations alone leads to an effective phantom-like behavior while there is no phantom field in the problem (see [44–47]). Now we show that modified teleparallel gravity ( gravity) can realize an effective phantom-like behavior while the null energy condition for the effective cosmic (torsion) fluid, that is, , is respected. We note that as has been shown in [52], matter is always stable in these theories. To see the phantom-like behavior of gravity, we adopt some models of gravity that could be viable through, for instance, cosmological and solar system tests. In this respect we consider a scale factor of the type This scale factor has two adjustable parameters, and , and by assumption . This choice is coming from bouncing cosmological solutions in the context of string theory with quintom matter [54]. Note that this choice belongs to the class of scale factors that are usually adopted in literature. Based on the definition , with this choice of the scale factor and (11), we find We note that it is easy to show that the scale factor (22) with two adjustable parameters, and , is essentially a solution of the field equations. So, in what follows we explore possible realization of the effective phantom-like behavior with adopted functions.

##### 3.1. Phantom-Like Effect with

In this form of , and are two model’s dimensionless parameters, in which only is an independent parameter. As we can see, for , this type of gravity can reduce to cosmology, while for it reduces to the Dvali, Gabadadze, and Porrati (DGP) braneworld model. Inserting into the first Friedmann equation (12) and calculating the result for the present time, we obtain as follows [34]: where and are the dimensionless density parameter of the dust matter and the present Hubble parameter, respectively. We note that, in order to have a positively accelerating expansion of the universe, we need those values of , where [55]. Now for this type of gravity, we find that (remember that )and By applying (20) and (21), we obtain the effective density and equation of state parameter for this model as

Now to see whether this model has an effective phantom-like behavior or not, we plot the evolution of the effective energy density and the equation of state parameter versus the redshift, . In this manner we obtain a suitable amount for the independent parameter to have an effective phantom-like behavior that this amount is about . In this respect, by using the equations (28) and (29), Figures 1 and 2 are plotted. Also we have used , , and in all numerical calculations in the paper. This value is in agreement with the results reported in literature. In Figure 2, as we can see the effective equation of state parameter for the present time () is ; this amount is in favor of the recent data from Planck2018. However, we should pay attention that, in this form of , the effective equation of state parameter crosses the cosmological constant (phantom divide) line, , from phantom-like phase to the quintessence-like phase in future. This behavior is not in favor of the observational data that show transition from the quintessence phase to the phantom phase. Nevertheless, having a positively growing energy density is a favorable capability of this model.