Research Article | Open Access
Stability of the Universe Model Coupled with Phantom and Tachyon Fields
In this paper, we study the phase space analysis of locally rotationally symmetric Bianchi type I universe model by taking interactions between dark matter and scalar field models. We define normalized dimensionless variables to develop an autonomous system of equations. We also find the corresponding critical points in order to study the dynamics of the system. The dynamical analysis indicates that all the critical points correspond to accelerated cosmic expansion for tachyon coupled field. We observe that positive values of provide more stable future attractors as compared to its negative values. We also analyze the behavior of power-law scale factor which shows different cosmological phases. It is found that the region for decelerated expansion gets larger for the phantom coupled matter by increasing while this region decreases for tachyon coupled field.
Recent observations suggest that our universe is expanding at an accelerating rate [1–3]. These observational probes indicate two cosmic phases, i.e., the cosmos phase before radiation and ultimately the current cosmic era. Many substantial attempts have been taken to explore the facts behind the current cosmic acceleration. An unusual form of energy having large negative pressure was proposed by the physicists, known as dark energy (DE). This energy dominates over the matter content undergoing current cosmic expansion.
There have been a massive literature devoted to study the ambiguous nature of DE. The cosmological constant () is taken as the elementary candidate having two conventional problems, i.e., fine-tuning and cosmic coincidence. Several alternative dynamical models can be considered as a substitute of like quintessence [4, 5], phantom model , tachyon field , and k-essence  that also show the expanding nature of cosmos. The extension of general barotropic equation of state (EoS) to some exotic canditates like Chaplygin gas  and its generalization  also represents the DE candidates.
The astonishing idea of scalar field, with an EoS parameter other than , has been proved to be useful to interpret the universe evolution due to its applications in different cosmological problems like cosmic acceleration as well as coincidence problems. The scalar field candidates also yield the inflationary era of cosmic evolution. We can adopt scalar field models (in particular, phantom and tachyon) as an alternative of DE candidates which interact with DM to resolve the coincidence problem. Researchers have paid an extensive attention to the tachyon cosmology in which the tachyon is fundamentally attributed by string theory [11, 12]. Gibbons studied the cosmological impact of tachyon field turning down to its ground phase . The tachyon matter may execute inflationary era and eventually a new form of DM at late times . A phantom field was also presented as an alternative of DE that constitutes large negative pressure with EoS parameter and plays an important role for accelerated expansion of the universe [15–18]. One of the significant features of the phantom model reveals that the universe will end with a big-rip (future singularity). The interaction of DE (phantom or tachyon) and DM may alleviate the coincidence problem [19, 20].
A phase space represents various states including position as well as momentum corresponding to each point of a dynamical system. This analysis yields dynamical characterization of a cosmological model by reducing complexity of the system of equations. It has always been helpful to study different stages of evolution by converting the system of equations to an autonomous one. This investigates the influence of initial data on the stability of a system by checking whether the respective system will remain stable or not .
Copeland et al.  discussed phase space analysis of the inflationary model that was unable to solve density problem. Guo et al.  studied stability of FRW universe model filled with barotropic fluid and phantom scalar field. They found that phantom dominated solution is a stable late-time attractor. Guo et al.  used this analysis to study the dynamical behavior of cosmos by interacting phantom energy with DM. Chen et al.  discussed a detailed phase space analysis of various phantom cosmological models for which the DE sector interacts with the DM. Acquaviva and Beesham  explored this analysis for FRW model and found that the nonlinear extension of viscous cosmic model provides accelerated expansion of the universe. We studied the influence of nonlinear electrodynamics on stability of accelerated expansion in the background of FRW cosmology . Shahalam et al.  presented phase space analysis for various combinations of the coupling between phantom and tachyon fields taking FRW universe. We also discussed phase space analysis of FRW universe model by considering a power-law model for bulk viscosity coefficient .
Bianchi universe models have also been discussed in literature to explore primordial anisotropy and some large angle anomalies identified by CMBR which give rise to the negligence of statistical cosmic isotropy . Belinkskii and Khalatnikov  applied phase plane approach to Bianchi type I (BI) model and studied the effects of shear and bulk viscosity. Coley and Dunn  used this technique to investigate the dynamics of Bianchi type V model by taking a viscous fluid. Burd and Coley  discussed the influence of shear as well as bulk viscosity on stability of Bianchi universe models. Goliath and Ellis  studied dynamical evolution of Bianchi universe model by including the cosmological constant. Sharif and his collaborators discussed phase space analysis of Bianchi I universe in Brans-Dicke gravity with chameleon scalar field  as well as gravity taking nonphantom, vacuum, and phantom phases . Chaubey and Raushan  studied phase space analysis of LRS BI model in the presence of scalar field.
This paper aims to investigate the stability of LRS BI universe model by taking linear interactions of phantom and tachyon fields coupled with DM through phase space analysis. The plan of the paper is as follows. In Section 2, we provide some basic formalism for the evolution equations. An autonomous system of equations is developed by defining normalized dimensionless variables. Section 3 deals with the dynamical analysis of the universe model by taking three different forms of interactions between phantom energy and DM. We also study the phase space analysis of tachyon field coupled with DM in Section 4. Section 5 deals with the formulation of power-law scale factor. Finally, we summarize our findings in the last section.
2. General Equations
Bianchi universe models are the simplest extensions of FRW universe with the addition of anisotropic effects. The LRS BI model is described by the line element [38, 39]:where and are the cosmic scale factors. The mean Hubble parameter, in terms of the directional Hubble parameters ( and ), can be defined asWe consider a power-law relation , , where represents a constant anisotropic parameter that gives the deviation of anisotropy from the isotropic model. This leads to the following relation:This assumption can be rationalized by the velocity redshift expression for extragalactic sources through which the Hubble cosmic expansion may achieve isotropy when shear to expansion scalar ratio is constant .
We assume a cosmic fluid by interacting phantom field with matter such that the conservation of energy yieldswhere is the interaction term, dot corresponds to the derivative with respect to time, , , , , , and represent energy densities and pressures of the matter and phantom energy, respectively. It should be noted that the transfer of energy between two components depends on the sign of interaction term. The energy passes from phantom to matter for while yields the opposite. The conservation equations clearly show that . The constraint and Raychaudhuri equations are formulated from the field equations aswhere and . Due to many arbitrary parameters, it seems difficult to find analytical solution of the evolution equation. For this purpose, we define the following normalized dimensionless quantities:that transform the dynamical equations into an autonomous system. Differentiating and with respect to , we haveThe Raychaudhuri and conservation equations through these dimensionless parameters becomewhere is a constant. The constraint equation can be written in the formThe effective EoS for the cosmic fluid and phantom field are given by
3. Dynamics of Interacting Phantom Energy
In this section, we discuss dynamics of LRS BI universe by the phase space analysis taking an interaction between phantom energy and matter. In this context, we impose the condition to solve the system of (10) and (11) and also determine the corresponding critical points . The stability of universe model depends on the behavior of critical points and their eigenvalues. We take three different types of coupling between phantom field and matter as follows.
Firstly, we consider as an interaction model for the universe where both phantom field and DM are coupled. Equation (13) for this coupling takes the formwhere . The autonomous system of equations becomesWe can find the eigenvalues by the Jacobian matrixwhere suffix defines the values at critical points . The fixed point is called a source or unstable point (resp., a sink or stable point) if both eigenvalues consist of positive (resp., negative) real parts. The real parts of the eigenvalues having opposite signs correspond to a saddle point of the system. We evaluate the following critical points in this case. For , the eigenvalues of Jacobian matrix are obtained asWe are interested to explore the influence of physical parameters ( and ) on the stability of cosmos coupled with scalar field. We plot the dynamical characteristics of critical points for in the vicinity of different values of and as shown in Figure 1. We find negative eigenvalues which show the point as a stable future attractor for in the physical phase space except for at which the system becomes undetermined. For , , this point behaves as an unstable node.
The effective potential for the cosmic fluid is defined asWe also evaluate the effective EoS parameter and deceleration parameter asThe dynamical analysis shows accelerated expansion of the universe model for as .
For , the corresponding eigenvalues areThis point shows a stable attractor with decelerated expansion for . This point becomes unstable past attractor for negative values of . We find that gives accelerated expansion while the universe undergoes decelerated expansion for as .
For , we haveFor positive values of , this behaves as a saddle point showing decelerated expansion ultimately followed by accelerated expansion. For , this point is saddle or stable showing accelerated expanding universe model corresponding to or , respectively.
For , we haveThis point behaves as a saddle point for all choices except for and that show stable future attractor. It is mentioned here that all the points lie in accelerated expanding phase of the universe for negative values of as and . We summarize the results for stability of LRS BI universe coupled with phantom energy and matter in Table 1.
For this coupling, (13) leads toSimilarly, the autonomous system of equations takes the formWe follow the same procedure to find the critical points. For , we haveThis point gives a changing behavior of critical points corresponding to different values of and . For , the point is an unstable node by taking (Figure 2). For , the eigenvalues become undetermined; hence, we neglect it. The negative value of gives stable future attractor. It is found that point indicates accelerated expansion of cosmos since for different choices of parameters.
For , the eigenvalues are given byHere, all values of and correspond to saddle or unstable points except and that yield stable future attractor. The universe undergoes decelerated expansion for negative values of while corresponds to the accelerated expansion. For , the eigenvalues are obtained asWe find that point is a saddle node for all choices of and except for and at which it behaves as a stable point. This point yields decelerated expansion for negative values of . For , the eigenvalues areThis point is a saddle node for negative values of while corresponds to stable/unstable attractors depending on parameter . It is found that lies in a region of accelerated expansion for all the choices of parameters. Table 2 shows the dynamical analysis for LRS BI model.
In this section, we assume a combination of the form and for the coupling through which (13) becomesThe evolution and conservation equations are given byFor , the corresponding eigenvalues areIn this case, we examine unstable nodes for by varying that shows decelerated expanding universe except for (Figure 3). On the other hand, the positive values of give accelerated expansion of the universe.
For , the corresponding eigenvalues are given asThis point corresponds to either stable or unstable nodes for or , respectively. It is found that lies in the matter dominated phase for all choices of the parameters. For
, the eigenvalues are
where The behavior of eigenvalues as well as trajectories indicate that point is a saddle node for all choices of the parameters. The summary of the above results is provided in Table 3.
4. Coupled Tachyon Dynamics
Now we study the evolution of LRS BI universe by considering tachyon coupled cosmic fluid. The conservation equations arewhere and . In this case, the evolution equations take the formHere, we define the following dimensionless quantities:such that the dynamical system yieldsWe consider inverse square potential with constant parameter . In this case, we take the only coupling through which (53) and (54) lead toThe effective EoS and deceleration parameters can be written as
For , we find the eigenvalues asWe find that point is saddle/unstable for which becomes stable for negative values of (, ) undergoing accelerated expansion as (Figure 4). We provide the obtained results in Table 4.