Anisotropic Bianchi Type-III Bulk Viscous Fluid Universe in Lyra Geometry

An anisotropic Bianchi type-III cosmological model is investigated in the presence of a bulk viscous fluid within the framework of Lyra geometry with time-dependent displacement vector. It is shown that the field equations are solvable for any arbitrary function of a scale factor. To get the deterministic model of the universe, we have assumed that (i) a simple power-law form of a scale factor and (ii) the bulk viscosity coefficient are proportional to the energy density of the matter. The exact solutions of the Einstein’s field equations are obtained which represent an expanding, shearing, and decelerating model of the universe. Some physical and kinematical behaviors of the cosmological model are briefly discussed.


Introduction
After Einstein (1916) proposed his theory of general relativity which provided a geometrical description of gravitation, many physicists attempted to generalize the idea of geometrizing the gravitation to include a geometrical description of electromagnetism.One of the first attempts was made by Weyl [1] who proposed a more general theory by formulating a new kind of gauge theory involving metric tensor to geometrize gravitation and electromagnetism.But Weyl theory was criticized due to the nonintegrability of length of vector under parallel displacement.Later, Lyra [2] suggested a modification of Riemannian geometry by introducing a gauge function into the structureless manifold which removed the nonintegrability condition.This modified geometry is known as Lyra geometry.Subsequently, Sen [3] formulated a new scalar-tensor theory of gravitation and constructed an analogue of the Einstein's field equations based on Lyra geometry.He investigated that the static model with finite density in Lyra manifold is similar to the static model in Einstein's general relativity.Halford [4] has shown that the constant displacement vector field in Lyra geometry plays the role of cosmological constant in general relativity.He has also shown that the scalar-tensor treatment based in Lyra geometry predicts the same effects, within observational limits, as in Einstein's theory (Halford,[5]).
Soleng [6] has investigated cosmological models based on Lyra geometry and has shown that the constant gauge vector field either includes a creation field and be identical to Hoyle's creation cosmology (Hoyle,[7], Hoyle, and Narlikar [8,9]) or contains a special vacuum field which together with the gauge vector term may be considered as a cosmological term.In the latter case, solutions are identical to the general relativistic cosmologies with a cosmological term.
Bali and Chandnani [25] have investigated a Bianchi type-III bulk viscous dust filled universe in Lyra geometry under certain physical assumptions.Recently, V. K. Yadav and L. Yadav [26] have presented Bianchi type-III bulk viscous and barotropic perfect fluid cosmological models in Lyra's geometry with the assumption that the coefficient of viscosity of dissipative fluid is a power function of the energy density.In this paper, we investigate a Bianchi type-III universe filled with a bulk viscous fluid within the framework of Lyra geometry with time-dependent displacement vector field without assuming the barotropic equation of state for the matter field.The organization of the paper is as follows.In Section 2, we present the metric and Einstein's field equations.In Section 3, we deal with the solution of the field equations.We first show that the field equations are solvable for any arbitrary scale function.There after, we obtain exact solutions of the field equation by assuming (i) a power-law form of a scale factor and (ii) the bulk viscosity coefficient is directly proportional to the energy density of the matter.In Section 4, we discuss the physical and dynamical behaviors of the universe.Section 5 summarizes the main results presented in the paper.

Metric and Field Equations
The diagonal form of the Bianchi type-III space-time is considered in the form where  is a constant and , , and  are functions of cosmic time .
Einstein's field equations in normal gauge for Lyra's geometry given by Sen [3] are where   is the displacement vector field defined as   = (0, 0, 0, ()).The energy-momentum tensor  ] for bulk viscous fluid is given by where  is the energy density and  ] is the four-velocity vector of the fluid satisfying    ] = 1.The effective pressure  is given by where  is the isotropic pressure,  is the gauge function, and  is the bulk viscosity coefficient.
For the metric (1), the field equations (2) together with (3) and ( 4) lead to where an overhead dot denotes differentiation with respect to time .Here, we have used the geometrized unit in which 8 = 1 and  = 1.

The energy conservation equation 𝑇 ]
;] = 0 leads to The conservation of left-hand side of (2) yields (Bali and Chandnani [25]) The physical quantities that are important in cosmology are proper volume , average scale factor , expansion scalar , shear scalar , and Hubble parameter .For the metric (1), they have the following form: An important observational quantity in cosmology is the deceleration parameter  which is defined as The sign of  indicates whether the model inflates or not.The positive sign of  corresponds to a standard decelerating model, whereas negative sign indicates inflation.

Exact Solutions
We now solve the field equations ( 5)-( 11) by using the method developed by Mazumdar [27] and further used by Verma and Ram [28].From (9), we obtain where  is an integration constant.Without loss of generality, we take  = 1.Using ( 17), ( 5)-( 11) reduce to From ( 18) and ( 19), we get This is an equation involving two unknown functions  and  which will admit solution if one of them is a known function of .To get a physically realistic model, Bali and Chandanani [25] and V. K. Yadav and L. Yadav [26] have assumed a supplementary condition  =   between the metric potentials  and .This condition is based on the physical assumption that the shear scalar  is proportional to the expansion scalar .In order to obtain a simple but physically realistic solution, we make the mathematical assumption that where  is positive real number.For this, we first show that ( 23) is solvable for an arbitrary choice of .Multiplying ( 23) by  2 , we get This can be written in the following form: The first integral of ( 26) is where  1 is a constant of integration.We can write (27) in the following form: where The general solution of ( 28) is given by where  2 is a constant of integration.Note that in this case the solution of Einstein's field equations reduces to the integration of (30) if  is explicitly known function of .Making use of ( 24) in ( 29) and (30), we obtain the general solution  2 of the form For further discussion of the solutions, we take  1 =  2 = 0.Then, Hence, the metric of our solutions can be written in the form where 0 <  < 1.

Some Physical and Kinematical Properties of the Model
The model (33) represents an anisotropic Bianchi type-III cosmological universe filled with a bulk viscous fluid in the framework of Lyra geometry.From ( 22), (24), and (32), the gauge function  has the expression given by where  1 is an integration constant.Using ( 24), (32), and (34) into ( 18) and ( 20), we obtain It is clear that, given (), we can determine isotropic pressure .In most of the investigations involving bulk viscosity, () is assumed to be simple power function of energy density (Weinberg,[29]): where  0 and  > 0 are constants.The case  = 1 has been considered by Murphy [30] which corresponds to a radiating fluid.We take  = 1 so that From ( 4), ( 13), (36), (37), and (38), we obtain Clearly, the viscosity contributes significantly to the isotropic pressure of the fluid.The physical and kinematical parameters of the model (33) have the following expressions: The deceleration parameter is positive which shows the decelerating behavior of the cosmological model.It is worthwhile to mention the work of Vishwakarma [31], where he has shown that the decelerating model is also consistent with recent CMB observations model by WNAP, as well as with the high-redshift supernovae Ia data including 1997ff at  = 1.755.We observe that the spatial volume  is zero at  = 0, and it increases with the cosmic time.This means that the model starts expanding with a big bang at  = 0.All the physical and kinematical parameters , , , and  diverge at this initial singularity.The physical and kinematical parameters are well defined and are decreasing functions for 0 <  < ∞, and ultimately tend to zero for large time.The gauge function () and bulk viscosity coefficient () are infinite at the beginning and gradually decrease as time increases and ultimately tend to zero at late times.Since / = (1 − )/ √ 3( + 2) = const, the anisotropy in the universe is maintained throughout the passage of time.

Conclusions
We have presented an anisotropic Bianchi type-III cosmological model in the presence of a bulk viscous fluid within the framework of Lyra's geometry with time-dependent displacement vector.The model describes an expanding, shearing, and decelerating universe with a big-bang singularity at  = 0.All the physical and kinematical parameters start off with extremely large values, which continue to decrease with the expansion of the universe and ultimately tend to zero for large time.As  tends to zero as  tends to infinity, the model would essentially give an empty space-time for large time.