The phase of accelerating expansion of the universe is indicated by Type Ia Supernovae (SNeIa) , , the Sloan Digital Sky Survey (SDSS) (Seljack et al.  ) and Wilkinson Microwave Anisotropy Probe (WMAP) . The astrophysical observation, also indicates some evidence that cosmic media not a perfect fluid . The mysterious term, known as dark energy, occupying about 73% of dark energy of the universe, is considered to be responsible for its acceleration. The remaining configuration of energy of the universe is composed of 23% of dark matter and 4% of baryonic matter. The easiest candidate for DE is cosmological constant (Λ) , which is used to explain the observed cosmological investigations.
During the evolution of the universe, EoS parameter can be used to differentiate several dark energy models in standard cosmological model. To fit different properties of dark energy many candidates have been proposed such as tachyon ; k-essence ; phantom , ; quintom ; ghost condensate ; Brane world models ; interacting dark energy models ; chaplygin gas models ; holographic dark energy models ; phantom scalar field . Cosmological models like coupled dark energy, holographic dark energy, F(R,T) gravity, F(T) gravity, F(R) gravity, scalar field theory and ΛCDM cosmological models represent the increasing expansion with phantom/quintessence nature with cosmography tests have been studied by Bamba et al. . In the framework of different matrices, exact general relativistic cosmological models in context of DE in presence of anisotropic fluid have been investigated (Yadav and Yadav ; Kumar and Yadav ; Reddy et al. ; Kumar and Akarsu ; Naidu et al. ; Akarsu and Kiling, ; Adhav et al. ; Katore et al.  ). In the literature, anisotropic Bianchi type-I, III & V dark energy models in presence of usual perfect fluid have also been investigated (Yadav et al. ; Akarsu and Kiling ; Singh and Chaubey ; Pradhan ; Yadav and Shah ; Saha and Yadav  ).
According to some basic quantum gravitational principle, nature of dark energy known as holographic dark energy can be studied. This principle  also states that the degree of freedom in a bounded system must be finite and doesn’t scale by its volume but with its boundary era. Here means vacuum energy density. With the help of cosmology, we consider as DE density. Hooft  consider this holographic principle in context of black hole physics, which is another alternative to the solution of DE problem. In purview of dark energy problem, however, holographic principle put forwards a relation between Hubble parameter and Holographic dark energy density as . Present accelerated expansion of the universe doesn’t given by this value. The form of holographic density is given as in which H denote Hubble parameter, and are constant. This was imposed by Granda and Olivers , which must satisfy current observational data.For the purpose of reconstruction of holographic scalar field models of the dark energy, numbers of works have been done.
An important role in the early stage of the universe was played by anisotropy thereby making it necessary to study the homogeneous as well as the anisotropic cosmological models. Therefore the models based on Bianchi types universe are assumed to be spatially homogeneous models of cosmology which are in general anisotropic. The correspondences between the holographic dark energy and each one of phantom, tachyon, chaplygin gas and generalized chaplygin gas in FRW universe have been investigated by Setare    . Cosmological dynamics of interacting holographic dark energy model have been pointed out by Setare and Venegus . Bianchi type space-time having holographic dark energy’s evolution with the help of constant deceleration parameter has been shown by Sarkar and Mahanta . For instant, Sarkar    have studied holographic dark energy model in Bianchi typesspace-time with linearly varying deceleration parameter in general relativity. Also Adhav et al.  , Kiran et al.  and Reddy et al.  have investigated holographic DE in Bianchi models.
The geometric parameters Hubble parameter (H) and deceleration parameter (q) were duly defined for characterizing the expansion history of the universe. From the observation of the cosmic acceleration , it can be stated that DE with negative equation of state (EoS) and likely exist. Leading to the need of higher time derivative of scale factor a, new term is introduced for the deduction of the informational context regarding the dynamical evolution of . Sahni et al.  defined a new parameter called as statefinder parameter as
Note that r also known as cosmic jerk. The geometry describe by the set of quantities extended to include . Different cosmological models corresponding to trajectories in the plane shows quantitatively different behaviors. Fixed point corresponds to a spatially flat ΛCDM (cosmological constant Λ with cold dark matter) scenario. A better way of establishing the “distance” from a given point of departure of dark energy to the fixed point from spatially flat ΛCDM was provide in this model, can be demonstrated from  . For differentiation of variety of dark energy involvingcosmological constant, interacting dark energy model, Brane world model, the chaplygin gas, quintessence, quintom, phantom, use of statefinder parameters may be successful (Sahni et al. ; Alam et al. ; Zimdahl and Pavon ; Zhang  ). With the help of evolution diagram, it is very easy to identify “distance” the flat ΛCDM scenario to the given dark energy model.
At the present paper, the LRS Bianchi type-II cosmological modelwith interacting dark matter (DM) and holographic dark energy (DE) are investigated. Statefinder diagnostic pair i.e. is adopted to separate other existing dark energy models from this model. Some important geometrical and physical features of the models have also been discussed.
The physical parameters of cosmological importance for LRS Bianchi type-II space-time are:
The volume scale factor:
The average scale factor:
The mean Hubble parameter:
The deceleration parameter:
The mean anisotropy parameter:
where be the directional Hubble parameters along axes respectively.
2. Metric and Field Equations
The spatially homogeneous and anisotropic LRS Bianchi type-IImetric can be written as
where and are the scale factors and functions of the cosmic time t only .
The Einstein’s field equations in GR given as ( and )
Also, matter tensor for cold dark matter (pressureless i.e. ) and holographic dark energy are given as
where denote the energy density of dark matter , represent the energy density and is the pressure of holographic dark energy.
The Einstein’s field Equation (2.2) for LRS Bianchi type-II model (2.1) with the help of matter tensor (2.3) to (2.4) can be written as
where overhead dot denotes derivative with respect to cosmic time t.
Conservation equations stating the interaction between the DM and DE as,
represents the state parameter for holographic dark energy and is given by .
The quantity Q indicates for the interacting term. The direction of transfer of energy depends upon the sign of Q. We consider that Q be positive, which stands for energy transfer from dark energy to dark matter. If Q is negative, stands for energy transfer from dark matter to dark energy. Overhead dot denotes the derivative with respect to commoving time.These continuity equations state that interacting term should be a function of a quantity with unit inverse of time i.e. (Hubble parameter is the first and natural choice) multiplied with energy density. The conservation equation and other models stating the interaction between DE and DM were established by Wetterich   (see also Billyard and Coley  ) and were used side by side the holographic DE by Horvat . Expression for interaction between dark energy (DE) and dark matter (DM) can be written phenomenological as (Cai and Wang ; Amendola et al. ; Guo et al.   ).
where represents a coupling constant.
We obtain the energy density of DM from equations (2.8) and (2.9) as
where is a real constant of integration.
Using Equations (2.10) and (2.11), we get the interacting term as
3. Cosmological Solutions
It may be simply observed that the system have three equations as Equations (2.5)-(2.7) having five unknowns , , , and . Therefore to get set of solutions, one extra probable relation among these parameters needed. For this we considered that expansion scalar ( ) proportional to the shear scalar ( ). This gives
where and are the metric potentials and , are constants. Thorne  explained the intension behind considering this condition. The universe’s Hubble parameter is isotropy in present day within ≈30 percent (Kantowski and Sachs ; Kristian and Sachs  ). This has been suggested by the reviews of the velocity-red-shift relation for extra lactic sources. To place accurately, red-shift studies space the limit
on the proportion of shear ( ) to Hubble parameter (H) in neighborhood of galaxy at present day. Usual congruence to the homogeneous expansion persuades that the relation remain invariant, this has been noted by Collins et al. .
From Equations (2.5) and (2.6), we get,
Equation (3.2) with the help of by Equation (3.1) leads to
which reduces to
On simplification Equation (3.3) convert to
where , , .
From Equation (3.4), we get
where be real integrating constant.
Equation (3.5) leads to
The solution of (3.6) is not tenable for . One can choose the value of n such that the above relation can be integrable. We consider the following two cases.
First Model for
When , Equation (3.6) reduces to
which after integration and , leads to
where is constant of integration. Equation (3.13) for , leads to
Therefore, the metric (3) reduces to
The volume scale factor, average scale factor, mean Hubble parameter, deceleration parameter, shear scalar and mean anisotropy parameter obtained as,
Using Equation (3.11) in Equations (2.11) and (2.12), we get energy density of dark matter and interacting term as,
With the help of Equations (3.7), (3.8) and (2.18) in Equation (2.7), we get the energy density of holographic dark energy as,
With the help of Equations (3.7), (3.8) in Equation (2.6), we get the pressure of holographic dark energy as,
Using Equations (3.18) and (3.19), the EoS parameter of holographic dark energy is obtain as,
Statefinder parameters is defined and given as,
, , (3.21)
Second Model for
When , Equation (3.6) reduces to
which after integration leads to
where is constant of integration. Equation (3.1) for , leads to
Therefore, the metric (2.1) reduces to
The volume scale factor, average scale factor, mean Hubble parameter, deceleration parameter, shear scalar and mean anisotropy parameter are obtained as,
Using Equation (3.27) in Equations (2.11) and (2.12), we get energy density of dark matter and interacting term as,
With the help of Equations (3.24), (3.25) and (3.32) in the Equation (3.2.7), the energy density of holographic dark energy obtain as,
With the help of Equations (3.24), (3.25) in the linear combination of Equations (2.5)-(2.6), the pressure of holographic dark energy obtained as,
Using Equations (3.34) and (3.35), the EoS parameter of holographic dark energy is obtain as,
Statefinder parameters is defined as
Here we discussed two models for and . In the model , as time increases spatial volume also increases. As , and as , . Accelerating expansion of the model occurs for the situation . Whereas it is simply observe that model is in state of deceleration is obtained for the situation when . It is important to note that the model (3.10) include periodic function which is responsible for cyclic universe. Also in this model starts from phantom region and it reaches to (i.e. cosmological constant). After some finite time it enter into quintessence region ( ) and again come back into phantom region. Model for , as , and as , i.e. universe expands as time increases. As , , and as time increases , starts from phantom region. For some finite time, it extended to (i.e. cosmological constant), which denotes that our model drive to ΛCDM. After some time, it reaches to quintessence region ( ) for late time as shown in Figure 1. The available observation in cosmology, especially the three year WMAP data (Sprengel et al.  ), the SNeIa data (Riess et al. ; Asteir et al.  ) and the SDSS data (Eisenstein et al.  ) pointed out that the ΛCDM model or the model reducible to ΛCDM are present as a standard model in cosmology, which is similar to present universe. Figure 2 shows that the plane having evolving trajectory for corresponding model is different from other dark energy models.
Figure 1. Evolution of EoS parameter versus cosmic time t.
Figure 2. State finder parameters s versus r.
In this paper, we have studied LRS Bianchi Type-II universe filled with interacting dark matter and holographic dark energy. Here we have discussed two models for 1) ; 2) . In the model , we obtained the cyclic universe and in case of model for , we obtained the expanding universe. The cosmological parameter matches with the present day observations. The model for converges into phantom region whereas the model for converges into quintessence region. The statefinder parameter has been calculated in order to differentiate our model of dark energy with all other models of dark energy. We aspire that the future accurate observation will be capable of determining these statefinder parameters and consequently explore the nature of DE.
The author would like to acknowledge his deep sense of gratitude to the anonymous referee whose valuable comments and suggestion have improved this work.
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