Abstract
This paper is devoted to study the scalar field dark energy models by taking its different aspects in the framework of gravity. We consider flat FRW universe to construct the equation of state parameter governed by gravity. The stability of the model is discussed with the help of squared speed of sound parameter. It is found that models show quintessence behavior of the universe in stable as well as unstable modes. We also develop the correspondence of model with some scalar field dark energy models like quintessence, tachyonic field, k-essence, dilaton, hessence, and DBI-essence. The nature of scalar fields and corresponding scalar potentials is being analyzed in gravity graphically which show consistency with the present day observations about accelerated phenomenon.
1. Introduction
It is strongly believed that the universe is experiencing an accelerated expansion nowadays. The type Ia Supernovae [1, 2], large scale structure [3], and cosmic microwave background [4] observations have provided main evidence for this cosmic acceleration. This acceleration is caused by some unknown matter having positive energy density and negative pressure is dubbed as “dark energy” (DE). The combined analysis of cosmological observations suggests that the universe is spatially flat and consists of about 70% DE, 30% dust matter, and negligible radiations. Recent WMAP data analysis [5] also has given the confirmation of this acceleration.
The most appealing and simplest candidate for DE is the cosmological constant [6–8] which has the equation of state (EoS) parameter . Over the past decade, there have been many theoretical models for mimicking DE behavior, such as CDM, containing a mixture of cosmological constant and cold dark matter (CDM). However, two problems arise from this scenario, namely, the “fine-tuning” and “cosmic coincidence” problems [9]. In order to solve these two problems, many dynamical DE theoretical models have been proposed [10–14].
The dynamical DE scenario is often realized by some scalar field mechanism which suggests that the energy formed with negative pressure is provided by a scalar field evolving down a proper potential. The scalar field or quintessence (a scalar field slowly evolving down its potential) [15, 16] is one of the most favored candidate of DE. Provided that the evolution of the field is slow enough, the kinetic energy density is less than the potential energy density, giving rise to the negative pressure responsible for the cosmic acceleration. A lot of scalar field DE models have been studied [17], including k-essence [10], dilaton [18], DBI-essence [19, 20], hessence [21], tachyon [11], phantom [22], ghost condensate [23, 24], and quintom [12, 25, 26]. In addition, other proposals on DE include scenarios of interacting DE models [27, 28], brane-world models [29], and Chaplygin gas models [13].
Another approach to explore the accelerated expansion of the universe is the modified theories of gravity. In this scenario, cosmic acceleration of the universe would not arise from mysterious DE as a substance but rather from the dynamics of modified gravity. There are several models of modified gravity which include DGP brane, gravity, gravity, gravity, Gauss-Bonnet gravity, Horava-Lifshitz gravity, and Brans-Dicke gravity [30–41]. The gravity is a theory based on an action integral whose kernel is a function of the Ricci scalar and terms involving its derivatives. Cuzinatto et al. [42] have explored the cosmological consequences of a particular model in the context of this gravity theory. They tested the model against some of the observational data available. A more recent example is the (nonlocal) higher derivative cosmology presented in [43–46].
Recently the reconstruction procedure or correspondences between various DE models become very challenging subject in cosmological phenomena. The correspondence between different DE models, reconstruction of DE/gravity, and their cosmological implications have been discussed by several authors [47–60]. In the present work, we assume gravity model in FRW universe. We discuss the correspondence of gravity with some scalar field dark energy models like quintessence, tachyon, dilaton, k-essence, hessence, and DBI-essence models. The nature of scalar fields and the potentials is analyzed by graphical behavior of their scalar field and potential functions. The scheme of the paper is as follows. In Section 2, we briefly provide a review of gravity and discuss behavior of EoS and stability parameters. Section 3 devotes the analysis of correspondence between this gravity model and some scalar field models. The last section summarizes the obtained results.
2. Brief Review of Gravity
The action of gravity is given by [42]
where , is the coupling constant, is the covariant derivative, and is the matter Lagrangian density. An important feature of is the presence of ghosts [46]. However, here this is not a serious issue because (1) should be considered as an effective action of a fundamental theory of gravity, which make it ghost-free theory. The extended theory of gravity such as String-inspired nonlocal higher derivative gravitational model draws the attention of the researchers. The ghost-free action of this model is given by (the derivation has been discussed in detail by Biswas et al. [45]) where denotes the mass scale and term In order to maintain consistency with the action as low energy limiting case, the only suitable contribution of (3) is the first term of the series. That is, the corresponding term is , . This leads to the equivalence of both actions (1) and (2) up to surface term and yields a relation; that is, Taking the variation of action (1) with respect to , the field equations turn out to be [42] where Here and is the Ricci tensor.
For FRW universe, the Friedmann equations in the scenario of model turn out to be Here and are defined in terms of Hubble parameter as follows: where denotes the fifth order time derivative of Hubble parameter. The equation of continuity of CDM takes the following form: The term represents the present value of cold matter density. We can extract the EoS parameter; that is, , where It follows that For the reconstructed scenario, we consider the solution of the field equation in exact power law form given by where is the present value of scale factor and is a constant. The term denotes the finite future singularity time and the scale factor (12) is used to check type II (sudden singularity) or type IV (corresponds to ) for positive values of . We plot effective EoS parameter versus time as shown in Figure 1(a) for different values of such that . We choose some typical values of constants in this respect; that is, , , and . This parameter shows increasing behavior and transition from phantom to quintessence phase. For , becomes nearly constant at which represents the quintessence era. As we increase the value of , this parameter again expresses transition from phantom to quintessence with the passage of time but converges to a value that nearly corresponds to vacuum era of the universe.
(a)
(b)
To check the stability of the model, we plot squared speed of sound versus time as shown in Figure 1(b). The sign of squared speed of sound has a significant role in this regard. A positive value indicates the stability of model whereas instability stands for the negative value of . In the plot, we obtain two regions corresponding to the sign as and . For the region , it is found that represents positively decreasing behavior which gives the stability of the model. In the second region, squared speed of sound represents the instability of the model by experiencing negative behavior for all values of .
3. Correspondence between and Scalar Field Models
Here, we investigate the correspondence of gravity with some of the scalar field models such as quintessence, tachyon, k-essence, dilaton, hessence, and DBI-essence models. The scalar field models have been developed through particle physics as well as string theory and nowadays these are used as a DE candidate.
3.1. Quintessence
The quintessence scalar field model has been proposed to resolve the fine-tuning problem taking EoS as time dependent instead of constant. This model can explain current cosmic acceleration by giving negative pressure when potential dominates the kinetic term. It is minimally coupled with gravity and used as a useful tool for discussing early inflation as well as the late time acceleration. The action of this model is described as [14–16] where and denotes the potential of the field. Varying this action with respect to metric tensor , we obtain the corresponding energy-momentum tensor as follows: This gives the energy density and pressure for FRW universe as follows: These expressions yield Replacing and by the energy density and pressure of model, we study the behavior of the resulting scalar field and potential function.
The evolution trajectories of scalar field and potential function versus time for quintessence model are shown in Figure 2 taking the same values of constants as for effective EoS parameter. In left plot (a), represents increasing behavior and becomes more steeper as time goes on, particularly for greater value of . The increasing value of scalar field indicates that kinetic energy is decreasing. The potential function versus time in plot (b) expresses decreasing behavior. The plot of quintessence potential in terms of scalar field is shown in Figure 3 representing increasing behavior. The gradually decreasing kinetic energy while potential remains positive for quintessence model represents accelerated expansion of the universe.
(a) Scalar field
(b) Potential function
3.2. Tachyon
The tachyon model is motivated from string theory and used as a possible candidate to explain DE scenario. It is argued that rolling tachyon condensates possess useful cosmological consequences. The EoS of this rolling tachyon is smoothly interpolated between and which is an interesting feature of this model. This has led a flurry of attempts to construct more viable cosmological models for tachyon which serves as a suitable candidate of inflation at high energy. It is realized that tachyon can be used as a source of DE depending on different forms of its potential. The effective Lagrangian for tachyon is [11, 14] Varying this action with respect to the metric tensor, we have leading to the energy density and pressure as and EoS parameter becomes Taking into account (19) and (20), we have which can be replaced by and in order to check the behavior of scalar field and potential function of tachyon model.
We plot scalar field and potential function of tachyon model as shown in Figure 4. The evolution of this model is much similar to the quintessence model. The scalar field represents increasing behavior versus time and indicates more steeper behavior for . This leads to the decreasing kinetic energy, approximately zero value. This represents the vacuum behavior of the universe depicted by (20). The corresponding potential function expresses decreasing but positive behavior with respect to . Its decreasing behavior from maxima gives inverse proportionality to the scalar field for the later times. This type of behavior corresponds to scaling solutions in the brane-world cosmology [61, 62].
(a) Scalar field
(b) Potential function
3.3. k-Essence
Armendáriz-Picón et al. [63] introduced the concept inflation driven by kinetic energy (termed as k-inflation) for discussing the inflation in the early universe at high energies. This scenario was used for DE purpose by Chiba et al. [64]. Moreover, this analysis was extended by Armendáriz-Picón et al. [65] to a more generalized form of Lagrangian, named as “k-essence.” The quintessence scalar field relies on potential energy in order to explain late time cosmic acceleration of the universe. However, this model takes the modifications of kinetic energy in the scalar fields to address the cosmic expansion. The generalized form of scalar field action is where shows the pressure density as a function of potential and . The energy density and pressure are given by The corresponding EoS parameter yields which represents the fact that the accelerated expansion of the universe is experienced for the interval for . The potential function takes the form
Equating and in (24) and (25), the evolution trajectories of and versus for k-essence scalar field model are given in Figure 5. The plot (a) represents increasing behavior of scalar field with respect to time for all values of . The plot (b) of versus shows compatible behavior of the model for all from early era to later times. This indicates that for approaches to the required interval for accelerated expansion of the universe. Also, the potential function versus and is shown in Figures 6(a)-6(b), expressing positively decreasing behavior.
(a)
(b)
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3.4. Dilaton
It is shown [14] that phantom field with negative kinetic energy plagued a problem of quantum instabilities. To avoid this problem, dilaton model has been originated which is later used to explain DE puzzle. Also, it is realized that this model avoids some quantum instabilities with respect to the phantom field models of DE. This model is found in low energy effective string theory, where dilatonic higher-order corrections were made to the tree-level action. The Lagrangian of dilaton field can be expressed in terms of pressure of scalar field as [14, 18] where and are taken to be positive constants. This Lagrangian (pressure) produces the following energy density: The EoS parameter takes the form
By keeping and in above three equations, we can get kintetic energy corresponding scalar field numerically by keeping same values for other constants while . The behavior of this model is approximately the same as of the k-essence model. Figure 7(a) represents the increasing behavior of scalar field versus time. In plot (b), the kinetic energy term versus time shows increasing behavior with the passage of time. The EoS parameter given in (28) indicates the bound of kinetic energy term such as for accelerated expansion of the universe. In this regard, we observe that the value is inconsistent with this interval while the remaining values of represent compatible results.
(a)
(b)
3.5. Hessence
The noncanonical complex scalar field, named as hessence, has the Lagrangian density as [21] where with and which lead to . These two variables are introduced to describe the hessence scalar field. The hessence comprises the special case of quintom model in terms of and . For FRW spacetime, the energy density and pressure take the form where represents the total conserved charge within the physical volume due to the internal symmetry. Using (30), we obtain the scalar field and potential of hessence scalar filed model as Replacing energy density and potential of hessence scalar field with and in the above equations, we plot scalar field and potential of hessence scalar field model as shown in Figure 8. The scalar field expresses increasing behavior with respect to as shown in plot (a). In plot (b), the potential function represents the same behavior as the remaining scalar field models. Initially, this indicates decreasing behavior with the passage of time.
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(b)
3.6. DBI-Essence
Here we consider Dirac-Born-Infeld- (DBI-) essence scalar field model whose action is given by [19, 20] where and are the warped brane tension and the DBI potential, respectively. The corresponding energy density and pressure yield where is reminiscent from the usual relativistic Lorentz factor. Now, we consider here two particular cases which involve as a constant and vice versa.
Case 1 (). In this case, we assume as constant and where for the sake of simplicity. The becomes Using (33), we obtain the following expressions for DBI-essence for constant : where . Inserting energy density and pressure of model with those of and in (35), we plot and versus as shown in Figure 9. This represents approximately the same behavior as for quintessence scalar field model. The scalar field shows increasing behavior while potential represents decreasing but positive behavior.
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Case 2 (). Here we take not as a constant. For this purpose, we assume where is an arbitrary constant. This case leads to the following expressions: where . In order to plot scalar field and potential of DBI-essence scalar field (with constant), we substitute and in (37). Figure 10 represents the same behavior as for constant ; however, the values of scalar field and potential are quite different in both cases.
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4. Concluding Remarks
In this work, we have discussed the flat FRW universe model in the framework of gravity. We have obtained the modified Friedmann equations and the effective energy density as well as pressure coming from extra terms of the gravity sector. These can be treated as DE provided that the strong energy condition violates. We have assumed the power law solution of scale factor , . The term denotes the finite future singularity time and the scale factor is used to check type II (sudden singularity) or type IV (corresponds to ) for positive values of . We have plotted the effective EoS parameter versus time in Figure 1(a) for different values of and , , and . The EoS parameter shows increasing nature and transition from phantom to quintessence phase. For , the becomes nearly constant at which represents the quintessence era. For increasing values of , the EoS parameter again expresses transition from phantom to quintessence in the course of time, but this converges to vacuum era of the universe. In Figure 1(b), we have plotted squared speed of sound versus time. A positive value indicates the stability of model whereas instability stands for the negative value of . We obtain two regions corresponding to the sign as and . For the region , it is found that squared speed of sound represents positively decreasing behavior which gives the stability of the model for all values of . In the second region, squared speed of sound represents negative behavior which gives the instability (classically unstable) of the model for all values of .
Since scalar field models of DE are effective theories of an underlying theory of DE, we have studied the correspondence between the effective DE coming from gravity with other scalar field DE models like quintessence, tachyon, k-essence, dilaton, hessence, and DBI-essence and constructed the scalar field as well as corresponding scalar potentials which describe the dynamics of the scalar fields graphically. We have taken the same values of the parameters in all cases taking different values of power of scale factor. The physical interpretations of all the DE models are summarized as follows.
The evolution trajectories of scalar field and potential function versus time for the correspondence of with quintessence dark energy model are shown in Figure 2 by taking the same values of constants as for effective EoS parameter . From Figure 2(a), we have observed that the quintessence scalar field represents increasing behavior and becomes more steeper as time goes on, particularly for greater value of . The increasing value of scalar field indicates that kinetic energy is decreasing. The potential function versus time in plot 2(b) expresses decreasing nature. The plot of quintessence potential in terms of scalar field is shown in Figure 3 representing increasing behavior. The gradually decreasing kinetic energy while potential remains positive for quintessence model represents accelerated expansion of the universe.
We have plotted the scalar field and potential function of tachyon model in Figure 4 for the correspondence scenario. The scalar field (Figure 4(a)) represents increasing behavior versus time and indicates more steeper behavior for . This leads to the positively decreasing kinetic energy, which approximately tends to zero (i.e., the vacuum). The corresponding potential function (Figure 4(b)) expresses decreasing but positive behavior with respect to . Its decreasing behavior from maxima gives inverse proportionality to the scalar field for the later times. This type of behavior corresponds to scaling solutions in the brane-world cosmology.
The k-essence model expresses the evolution of scalar field and kinetic energy term versus . The evolution trajectories of and versus for k-essence scalar field model are given in Figure 5. The plot 5(a) represents increasing behavior of scalar field with respect to time for all values of . The plot 5(b) of versus shows compatible behavior of the model for all from early era to later times. This indicates that, for , approaches to the required interval for accelerated expansion of the universe. Also, the potential function versus and is shown in Figures 6(a)-6(b), expressing positively decreasing nature.
The scalar field and kinetic energy term of dilaton model are drawn in Figure 7, keeping the same values for other constants while . Figure 7(a) represents the increasing behavior of scalar field versus time. In the plot 7(b), the kinetic energy term versus time shows increasing behavior in the course of time. The EoS parameter indicates the bound of kinetic energy term such as for accelerated expansion of the universe. In this regard, we observe that the value is inconsistent with this interval while the remaining values of represent compatible results.
For hessence model, scalar field and potential are shown in Figure 8. The scalar field expresses increasing behavior with respect to , which is shown in Figure 8(a). In plot 8(b), the potential function represents the same behavior as the remaining scalar field models. Initially, this indicates decreasing behavior with the passage of time. It is noted that if , the hessence model may generate the scalar field model for quintessence. So quintessence model is the special case of hessence model.
We have studied scalar field and potential functions of DBI-essence model taking constant and variable DBI-parameter, which have been drawn in Figures 9 and 10. The scalar field shows increasing behavior while potential represents decreasing but positive behavior in both cases.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.