Bulk Viscous Fluid Cosmological Models Research Articles

Behaviour of viscous fluid in string cosmological models in the framework of Lyra geometry

In this communication, we studied the aspects of bulk viscous fluid cosmological model with quadratic equation of state in the presence of strings loaded with particles in a higher dimensional (5- dimensional) Bianchi type-III geometry in Lyra’s Manifold (Lyra, 1951). Using physically plausible circumstances, an exact model of the universe is presented by obtaining the solutions of the Einstein’s field equations. Important geometrical and dynamical parameters of the model universes are premeditated and physical significance regarding their prospect in modern cosmology are discussed in details. Interestingly it is seen that both bulk viscosity and quadratic equation of state are acting crucial jobs throughout the evolution of the model which is expanding with acceleration so it represents dark energy model universe. Hence our model can be thought as a realistic universe.

A comparative study of cosmological models in alternative theory of gravity with LVDP & BVDP

In this communication we have presented a comparative study of Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological models in alternative theory of gravity with linearly varying deceleration parameter (LVDP) and bilinearly varying deceleration parameter (BVDP) as suggested by Mishra and Chand (Astrophys. Space Sci. 361:259, 2016c). The role of viscosity in cosmology have been studied by several researchers. Under the influence of such study we have also studied bulk viscous fluid cosmological models in alternative $f(R,T)$ theory of gravity along with comparison of results by taking LVDP and BVDP. The main conclusion of the paper is that BVDP law provides better results in comparative with Berman’s constant deceleration law (CDP) and LVDP law. Also the BVDP law provides an envelope for CDP law and LVDP law.

Kaluza–Klein Bulk Viscous Fluid Cosmological Models and the Validity of the Second Law of Thermodynamics in f(R, T) Gravity

Abstract: The authors considered the bulk viscous fluid in f(R, T) gravity within the framework of Kaluza–Klein space time. The bulk viscous coefficient (ξ) expressed as ξ = ξ 0 + ξ 1 a ˙ a + ξ 2 a ¨ a ˙ , $\xi = {\xi _0} + {\xi _1}{{\dot a} \over a} + {\xi _2}{{\ddot a} \over {\dot a}},$ where ξ 0, ξ 1, and ξ 2 are positive constants. We take p=(γ−1)ρ, where 0≤γ≤2 as an equation of state for perfect fluid. The exact solutions to the corresponding field equations are given by assuming a particular model of the form of f(R, T)=R+2f(T), where f(T)=λT, λ is constant. We studied the cosmological model in two stages, in first stage: we studied the model with no viscosity, and in second stage: we studied the model involve with viscosity. The cosmological model involve with viscosity is studied by five possible scenarios for bulk viscous fluid coefficient (ξ). The total bulk viscous coefficient seems to be negative, when the bulk viscous coefficient is proportional to ξ 2 a ¨ a ˙ , ${\xi _2}{{\ddot a} \over {\dot a}},$ hence, the second law of thermodynamics is not valid; however, it is valid with the generalised second law of thermodynamics. The total bulk viscous coefficient seems to be positive, when the bulk viscous coefficient is proportional to ξ = ξ 1 a ˙ a , $\xi = {\xi _1}{{\dot a} \over a},$ ξ = ξ 1 a ˙ a + ξ 2 a ¨ a ˙ $\xi = {\xi _1}{{\dot a} \over a} + {\xi _2}{{\ddot a} \over {\dot a}}$ and ξ = ξ 0 + ξ 1 a ˙ a + ξ 2 a ¨ a ˙ , $\xi = {\xi _0} + {\xi _1}{{\dot a} \over a} + {\xi _2}{{\ddot a} \over {\dot a}},$ so the second law of thermodynamics and the generalised second law of thermodynamics is satisfied throughout the evolution. We calculate statefinder parameters of the model and observed that it is different from the ∧CDM model. Finally, some physical and geometrical properties of the models are discussed.

Bulk viscous fluid cosmological models in f(R, T) gravity

In the literature, a number of modified theories have been discussed to explain early and late time expansion of the universe. In this context, f(R) gravity (R being the Ricci scalar) is the simplest and most popular modification of general relativity. Other modified theories like f(T) gravity, where T is the torsion has been proposed to overcome many problems in cosmology. Kaluza–Klein cosmological models in f(R, T) theory of gravity are investigated when the source of gravitation is the bulk viscous fluid. In this work, it is considered that the function f(R, T) as f(R,T)=R+2f(T) or f(R,T)=f1(R)+f2(T). Such a choice of the functional f(R, T) leads to an evolving effective cosmological constant Λ, which depends on the stress energy tensor. f1(R) and f2(T) are arbitrary functions of R and T, respectively and f(R, T) = R f(T) may be considered to solve the field equations. We however do not consider the general case, but restrict ourselves to the following form f(R, T) = R + 2f(T), where f(T) is an arbitrary function of the trace of energy-momentum tensor of matter. The term 2f(T) in the gravitational action modified the gravitational interaction between matter and curvature. It is observed that the first case of the model be reduced to effective stiff fluid model of the universe while the second case gives general bulk viscous model of the universe. The exact solutions of the field equations are obtained. Keeping an eye on the accelerating nature of the universe in the present epoch, the dynamics and physical behaviour of the models are discussed.

Polytropic Bulk Viscous Cosmological Model with Variable G and Λ

We consider a Bianchi type-I Polytropic bulk viscous fluid cosmological model with variable G and Λ. To get a deterministic model, it is assumed that and , , , where is the pressure, ρ is the energy density, η is the coefficient of bulk viscosity, α, k, γ and ηo are constants, H is Hubble constant, where k1 > 0, k2 > 0. The solution obtained lead to inflationary phase and the results obtained match with the observations [31] [32]. The case n = 1 for α = 1 is also discussed, relating the results with the observations.

Bianchi types I and V bulk viscous fluid cosmological models in f(R, T) gravity theory

AbstractIn this paper we present non-singular Bianchi types I and V cosmological models, in the presence of bulk viscous fluid and within the framework of f(R,T) gravity theory. Exact solutions to the field equations are obtained by choosing a particular form of the function f(R,T) and a special value for the average scale factor of the model, which corresponds to a time- dependent deceleration parameter. The cosmological models initially accelerate for a certain period of time and thereafter decelerate. The physical and kinematical properties of the models of the universe are discussed.

Dust Fluid Cosmological Model

Bianchi type I tilted bulk viscous fluid cosmological model filled with dust fluid is investigated. We assume that (constant), where is the coefficient of bulk viscosity and is the expansion in the model. It has been assumed that the expansion in the model is only in two directions; that is, one of the components of Hubble parameters is zero. The physical and geometrical aspects of the model in the presence and absence of bulk viscosity are also discussed. Also, we have discussed two special models and their physical properties. From this, we present a particular example based on dust fluid.

Bianchi Type V Magnetized String Dust Bulk Viscous Fluid Cosmological Model with Variable Magnetic Permeability

Bianchi Type V magnetized string dust bulk viscous fluid cosmological model with variable magnetic permeability, is investigated. The magnetic field is due to an electric current produced along x-axis. Thus the magnetic fields is in yz-plane and F23 is the only non-vanishing component of electromagnetic field tensor Fij. To get the deterministic model in terms of cosmic time t, we have also assumed the condition ζθ=constant where ζ the coefficient of bulk viscosity and θ the expansion in the model. The behaviour of the model in presence and absence of magnetic field and bulk viscosity and singularities in the model are also discussed.