Cosmological Consequences of New Dark Energy Models in Einstein-Aether Gravity

In this paper, we reconstruct various solutions for the accelerated universe in the Einstein-Aether theory of gravity. For this purpose, we obtain the effective density and pressure for Einstein-Aether theory. We reconstruct the Einstein-Aether models by comparing its energy density with various newly proposed holographic dark energy models such as Tsallis, R$\acute{e}$nyi and Sharma-Mittal. For this reconstruction, we use two forms of scale factor, power-law and exponential forms. The cosmological analysis of underlying scenario has been done by exploring different cosmological parameters. This includes equation of state parameter, squared speed of sound and evolutionary equation of state parameter via graphical representation. We obtain some favorable results for some values of model parameters.


Introduction
Nowadays, it is believed that our universe undergoes an accelerated expansion with the passage of cosmic time. This cosmic expansion has confirmed through various observational schemes such as Supernova type Ia (SNIa) [1]- [4] and Cosmic Microwave Background (CMB) [5]- [9]. The source behind the expansion of the universe is a mysterious force called dark energy (DE) and its nature is still ambiguous [10]- [13]. The current Planck data shows that there is 68.3% DE of the total energy contents of the universe. The first candidate for describing DE phenomenon is cosmological constant but it has fine tuning and cosmic coincidence problems. Due to this reason, different DE models as well as theories of gravity with modifications have been suggested. The dynamical DE models include a family of Chaplygin gas as well as holographic DE models, scalar field models such as K-essence, phantom, quintessence, ghost etc [14,15,16].
One of the DE model is holographic DE (HDE) model which becomes a favorable technique now-a-days to study the DE mystery. This model is established in the framework of holographic principle which corresponds to the area instead of volume for the scaling of number of degrees of freedom of a system. This model is an interesting effort in exploring the nature of DE in the framework of quantum gravity. In addition, HDE model gives the relationship between the energy density of quantum fields in vacuum (as the DE candidate) to the cutoffs (infrared and ultraviolet). Cohen et al. [17] provided a very useful result about the expression of HDE model density which is based on the vacuum energy of the system. The black hole mass should not overcome by the maximum amount of the vacuum energy. Taking into account the nature of spacetime along with long term gravity, various entropy formalism have been used to discuss the gravitational and cosmological setups [18,19,20,21]. Recently, some new HDE models are proposed like Tsallis HDE (THDE) [19], Rényi HDE model (RHDE) [20] and Sharma-Mittal HDE (SMHDE) [21].
The examples of theories with modification setups include f (R), f (T ), f (R, T ), f (G) etc where R shows the Ricci scalar representing the curvature, T means the torsion scalar, T is the trace of the energy-momentum tensor and G goes as the invariant of Gauss-Bonnet [22]- [33]. For recent reviews in terms of DE problem including modified gravity theories, see, for instance [34,35,36,37,38,39,40]. Einstein-Aether theory is one of the modified theory of gravity [41,42] and accelerated expansion phenomenon of the universe has also been investigated in this theory [43]. Meng et al. have also discussed the current cosmic acceleration through DE models in this gravity [44,45]. Recently, Pasqua et al. [46] have made versatile study on cosmic acceleration through various cosmological models in the presence of HDE models.
In the present work, we will develop the Einstein-Aether gravity models in the presence of modified HDE models and well-known scale factors. For these models of modified gravity, we will extract various cosmological parameters. In the next section, we will brief review of Einstein-Aether theory. In section 3, we present the basic cosmological parameters as well as well-known scale factors. We will discuss the cosmological parameters for modfied HDE models in sections 4, 5 and 6. In the last, we will summarize our results.

Einstein-Aether Theory
As our universe is full with many of the natural occurring phenomenons. One of them is transfer of light from one place to another and second is how gravity acts. To explain these kinds of phenomenons, many of the physicists were used the concept of Aether in many of the theories. In modern physics, Aether indicates a physical medium that is spread homogeneously at each point of the universe. Hence, it was considered that it is a medium in space that helps light to travel in a vacuum. According to this concept, a particular static frame reference is provided by Aether and everything has absolute relative velocity in this frame. That is suitable for Newtonian dynamics extremely well. But, when Einstein performed different experiments on optics in his theory of relativity, then Einstein rejected this ambiguity. When CMB was introduced, many of the people took it a modern form of Aether. Gasperini has popularized Einstein-Aether theories [47]. This theory is said to be covariant modification of general relativity in which unit time like vector field(aether) breaks the Lorentz Invariance (LI) to examine the gravitational and cosmological effects of dynamical preferred frame [48]. Following is the action of Einstein-Aether theory [49,50].
where L EA represents the Lagrangian density for the vector field and L m indicates Lagrangian density of matter field. Further, g, R and G indicate determinant of the metric tensor g µν , Ricci scalar and gravitational constant respectively. The Lagrangian density for vector field can be written as where λ represents a Lagrangian multiplier, dimensionless constants are denoted by c i , M referred as coupling constant parameter and A a is a tensor of rank one, that is a vector. The function F (K) is any arbitrary function of K. We obtain the Einstein field equations from Eq.(1) for Einstein-Aether theory as follows where shows energy momentum-tensor for vector field and T m ab indicates energy-momentum tensor for mater field. These tensors are given as T m ab = (ρ + p)u a ub + pg ab , where p and ρ represent energy density and pressure of the matter respectively. Furthermore, u a expresses the four-velocity vector of the fluid and given as u a = (1, 0, 0, 0) and A a is time-like unitary vector and is defined as A a = (1, 0, 0, 0). Moreover Y ab is defined as where indices (a b) show the symmetry. The Friedmann equations modified by the Einstein-Aether gravity are given as follows Here K becomes K = 3ǫH 2 M 2 , where ǫ is a constant parameter. The energy density of Einstein-Aether theory is denoted by ρ EA and called effective energy density while the effective pressure in Einstein-Aether gravity is given by p EA . So, we can rewrite Eqs. (10) and (11) as where The EoS parameter for Einstein-Aether can be obtained by using Eqs. (39) and (15), which is given by

Cosmological Parameters
To understand the geometry of the universe, following are some basic cosmological parameters.

Equation of State Parameter
In order to categorize the different phases of the evolving universe, the EoS parameter is widely used. In particular, the decelerated and accelerated phases which contain DE, DM, radiation dominated eras. This parameter is defined in terms of energy density ρ and pressure p as ω = p ρ .

Squared Speed of Sound
To examine the behavior of DE models, there is another parameter which is known as squared speed of sound. It is denoted by v 2 s and is calculated by the following formula The stability of the model can be checked by it. If its graph is showing negative values then we may say that model is unstable and in case of nonnegative values of the graph, it represents the stable behavior of the model.

ω-ω ′ Plane
There are different DE models which have different properties. To examine their dynamical behavior, we use ω-ω ′ plane, where prime denotes the derivative with respect to ln a and subscript Λ indicates DE scenario. This method was developed by Caldwell and Linder [51] and dividing ω-ω ′ plane into two parts. One is the freezing part in which evolutionary parameter gives negative behavior for negative EoS parameter, i.e., (ω ′ < 0, ω < 0) while for positive behavior of evolutionary parameter corresponding to negative EoS parameter yields thawing part (ω ′ > 0, ω < 0) of the evolving universe.

Scale Factor
The scale factor is the measure that how much the universe has expanded since given time. It is represented by a(t). Since, the latest cosmic observations have shown that universe is accelerating so a(t) > 0. As Einstein-Aether is one of the modified theory which may produce the accelerated expansion of the universe. By using this theory, we can reconstruct various well-known DE models. In order to do this, we take some modified HDE models such as THDE, RHDE and SMHDE models. Since F (K) is a function that the Einstein-Aether theory contains, which can be determined by comparing the densities with the above DE models. For this purpose, we use some wellknown forms of scale factor a(t). We consider two forms of scale factors a(t) in terms of power and exponential terms. These are (i) Power-law form: a(t) = a 0 t m , m > 0, where a 0 is a constant which indicates the value of scale factor at present-day [52]. From this scale factor, we get H,Ḣ, K as follows (ii) Exponential form: a(t) = e αt θ where α is a positive constant and θ lies between 0 and 1. This scale factor gives

Reconstruction from Tsallis Holographic Dark Energy Model
The energy density of THDE model is given by [19] where B is an unknown parameter. Taking into account Hubble radius as IR cutoff L, that is L = 1 H , we have In order to construct a DE model in the framework of Einstein-Aether gravity with THDE model, we compare the densities of both models, (i.e., ρ EA = ρ D ). This yields which results the following form Power-law form of scale factor: Using the expression of F (K) along with Eq. (19) in (39), we obtain the energy density and pressure as Using these expressions of energy density and pressure, we find the values of some cosmological parameters in the following. The EoS parameter takes the following form The derivative of EoS parameter with respect to ln a is given by We plot EoS parameter versus z using the relation t =  Figure 1. All the three trajectories represent the phantom behavior of the universe related to redshift parameter. In Figure 2, we plot ω ′ EA − ωEA plane taking same values of the parameters for −1 ≤ z ≤ 1. For m = 2 and 3, the evolving EoS parameter shows negative behavior with respect to negative EoS parameter which indicates the freezing region of the universe. The trajectory of ω ′ EA for m = 4 represents the positive behavior for negative EoS parameter and expresses the evolving universe in thawing region of the universe.     Figure 3 shows the plot of v 2 s versus z to check the behavior of Einstein-Aether model for THDE and power-law scale factor for same values of parameters. The trajectories represent the negative behavior of the model which indicated the instability of the model. Exponential form of scale factor: Following the same steps for exponential form of scale factor, we get energy density and pressure as Now by using above density and pressure, we obtain the EoS parameter and its derivative for Einstein-Aether gravity as follows The squared speed of sound for second form of scale factor is given by   EoS parameter corresponds to the ΛCDM model ω EA = −1. In Figure 5, the graph is plotted between ω ′ EA and ω EA . The graph represents initially freezing region and then indicates the thawing region of the evolving universe. As we increase the value of α, the trajectories indicate the thawing region only. However, the graph of v 2 s versus z as shown in Figure 6 shows the unstable behavior.

Reconstruction from Rényi Holographic Dark Energy Model
The energy density of RHDE model is [21] ρ D = 3C 2 L −2 For Hubble horizon, it takes the form Now we compare the Einstein-Aether model energy density with the RHDE model density (i.e., ρ EA = ρ D ) in order to get reconstructed equation, The solution of this equation is given by Power-law form of scale factor: Inserting all the corresponding values in Eqs. (39) and (15), we get density and pressure of Einstein-Aether gravity model as follows In this case, the EoS parameter takes the form and ω ′ EA is given as follows The expression for squared speed of sound turns out as The plot of EoS parameter is shown in Figure 7 with respect to z. All the trajectories of EoS parameter represent the quintessence phase of the universe. Figure 8 shows the graph of ω EA − ω ′ EA plane for same range of z. The trajectories of ω ′ EA describe the negative behavior for all ω EA < 0 give the freezing region of the universe. To check the stability of the underlying model, Figure 9 shows the unstable behavior of the model. However, for m = 2, we get some stable points for z < −0.475. Exponential form of scale factor: Taking into account second scale factor Eq.(15) along with F (K), we get the following energy density and pressure The EoS parameter is obtained from above energy density and pressure. This parameter with its derivative are given by   Figure 10 represents the graph of EoS parameter versus z for RHDE model taking exponential form of the scale factor. For α = 2, initially the trajectory expresses the transition from decelerated phase to accelerated phase and then crosses the phantom divide line and gives the phantom phase of the universe. For higher values of the α that is for α = 3, 4, the trajectories of EoS parameter represents the quintessence phase. In Figure 11, we plot the graph of evolution parameter of EoS versus EoS parameter which gives the freezing region of the universe. Figure 12 shows the graph of v 2 s for stability analysis of the model. Initially the graph gives the stability and then for decreasing z, the model becomes unstable. As we increase the value of α, the trajectories give more stable points.

Reconstruction from Sharma-Mittal Holographic Dark Eneryg Model
Sharma-Mittal introduced a two parametric entropy and is defined as [20] where r is a new free parameter. The expression of SMHDE model for Hubble horizon is given by By comparing the energy densities of SMHDE model and Einstein-Aether gravity model, we find which leads us to the following solution Power-law form of scale factor: For this scale factor, we obtain The cosmological parameters are given by   We plot EoS parameter for SMHDE model with respect to redshift parameter as shown in Figure 13 for power-law scale factor. For m = 3 and 4, the trajectories represent the transition from quintessence to phantom phase while m = 2 indicates the phantom era throughout for z. The plot of this parameter with its evolution parameter is given in Figure 14 which shows the freezing region of the evolving universe. However, for higher values of m, we may get thawing region (ω ′ EA > 0). Figure 15 gives the graph of squared speed of sound versus redshift. The trajectory for m = 2 shows the stability of the model as redshift parameter decreases while other trajectories describe the unstable behavior of the model. Exponential form of scale factor: Following the same steps, we obtain the following expressions for energy density, pressure and parameters for exponential scale factor. These are πR πt 2 δ + t 2θ α 2 θ 2 2 θ KM 2 t 2 − 4 + 3 1 + 3πδǫ KM 2 R/δ × πt 2 δ + t 2θ α 2 θ 2 2 + 3t 2θ ǫθ 8(−2 + θ) πt 2 αδ + t 2θ α 3 θ 2 2 − 3α 2 × 1 + πt 2−2θ δ α 2 θ 2 R/δ t 4θ α 4 θ 4 (−5 + 2θ) + 2πt 2+2θ α 2 θ 2 (R(3 − 2θ) + δ(−5 + 2θ)) + π 2 t 4 2Rδ(5 − 4θ) + 4R 2 (−1 + θ) + δ 2 (−5 + 2θ)    Figure 18: Plot of v 2 s versus z taking exponential scale factor for SMHDE model. its evolutionary parameter and squared speed of sound to check the stability of reconstructed models for this theory.
The remaining results have been summarized as follows: EoS parameter for power-law scale factor: • THDE ⇒ phantom behavior, • RHDE ⇒ quintessence phase, • SMHDE ⇒ transition from quintessence to phantom phase for m = 3, 4, phantom era for m = 2.
Squared speed of sound for exponential scale factor: • THDE ⇒ unstable, • RHDE ⇒ stability for higher values and instability for lower values, • SMHDE ⇒ unstable.
It is mentioned here that for m = 2 for power-law form of scale factor in the case of SMHDE model, we obtain phantom region with stable behavior in freezing region which leads to most favorable result with current cosmic expansion scenario.