Bianchi Type-V universe with a viscous fluid and Λ-term

We have studied the evolution of a homogeneous, anisotropic universe given by a Bianchi Type-V cosmological model filled with viscous fluid, in the presence of cosmological constant Λ. The role of viscous fluid and Λ-term in the Bianchi Type-V universe has been studied.      

Cosmic acceleration and Brans-Dicke theory

We study the accelerated expansion of the universe by exploring the Brans-Dicke parameter in different eras. For this, we take the FRW universe model with a viscous fluid (without potential) and the Bianchi type-I universe model with a barotropic fluid (with and without a potential). We evaluate the deceleration parameter and the Brans-Dicke parameter to explore cosmic acceleration. It is concluded that accelerated expansion of the universe can also be achieved for higher values of the Brans-Dicke parameter in some cases.     

Phase space analysis for anisotropic universe with nonlinear bulk viscosity

In this paper, we discuss phase space analysis of locally rotationally symmetric Bianchi type I universe model by taking a noninteracting mixture of dust like and viscous radiation like fluid whose viscous pressure satisfies a nonlinear version of the Israel–Stewart transport equation. An autonomous system of equations is established by defining normalized dimensionless variables. In order to investigate stability of the system, we evaluate corresponding critical points for different values of the parameters. We also compute power-law scale factor whose behavior indicates different phases of the universe model. It is found that our analysis does not provide a complete immune from fine-tuning because the exponentially expanding solution occurs only for a particular range of parameters. We conclude that stable solutions exist in the presence of nonlinear model for bulk viscosity with different choices of the constant parameter m for anisotropic universe.     

Plane Symmetric Viscous Fluid Cosmological Models with Varying <Emphasis Type="Italic">Λ</Emphasis>-Term

Plane symmetric viscous fluid cosmological models of the universe with a variable cosmological term are investigated. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density whereas the coefficient of shear viscosity is to be proportional to rate of expansion in the model. We have also obtained a special model in which the shear viscosity is assumed to be zero. The cosmological constant Λ is found to be a decreasing function of time and a positive which is supported by results from recent supernovae Ia observations. Some physical and geometric properties of the models are also discussed.     

Bulk Viscosity and Decaying Vacuum Density in Friedmann Universe

We present an isotropic and homogeneous flat cosmological model for bulk viscous fluid distribution. We consider the vacuum density proportional to Hubble expansion parameter and time dependent bulk viscosity related to the velocity and acceleration of universe. The behaviour of resulting solutions are in accordance with recent astronomical observations. The model obtained evolves with a decelerating expansion followed by late time acceleration. Cosmological term Λ being very large at initial epoch relaxes to a genuine cosmological constant asymptotically. Presence of bulk viscosity prevents the matter density to vanish asymptotically and the matter density continues to be of the order of vacuum density after a finite time. Thus, we obtain a universe having the possibility of cosmic coincidence.     

Bulk Viscous Bianchi Type-III Cosmological Model with Time-Dependent <Emphasis Type="Italic">G</Emphasis> and Λ

This paper deals with Bianchi type-III anisotropic cosmological model of the universe filled with a bulk viscous fluid with time varying gravitational and cosmological constants. It is shown that the field equations are solvable for any arbitrary cosmic scale function. Exact solutions of Einstein’s field equations are obtained which represent an expanding, shearing, non-rotating and decelerating universe. The physical behaviour of the model has also been discussed.     

Anisotropic Bulk Viscous Fluid Cosmological Model with Zero-Rest-Mass Scalar Field and Time-Dependent Cosmological Term

In this paper, we have investigated Bianchi type-III cosmological model in the presence of a bulk viscous fluid together with zero-rest-mass scalar field and time-dependent cosmological term. We have shown that the field equations are solvable for any arbitrary cosmic scale function. Exact solutions of Einstein’s field equations are obtained which represent an expanding, shearing, non-rotating and decelerating model of the universe. Some physical and geometrical behaviours of the cosmological model are discussed.     

The weak interaction and the isotropy of the universe

We consider viscous dissipation of anisotropy and the corresponding entropy production during the lepton era of the universe, and investigate their sensitivity to the viscosity coefficients of the cosmological fluid.     

Cardy–Verlinde formula in FRW Universe with inhomogeneous generalized fluid and dynamical entropy bounds near the future singularity

We derive a formula for the entropy for a multicomponent coupled fluid, which under special conditions reduces to the Cardy–Verlinde form relating the entropy of a closed FRW universe to its energy together with its Casimir energy. The generalized fluid obeys an inhomogeneous equation of state. A viscous dark fluid is included, and also modified gravity is included in terms of its fluid representation. It is demonstrated how such an expression reduces to the standard Cardy–Verlinde formula corresponding to the 2d CFT entropy in some special cases. The dynamical entropy bound for a closed FRW universe with dark components is obtained. The universality of the dynamical entropy bound near a future singularity (of all known four types), as well as near the Big Bang singularity, is investigated. It is demonstrated that, except from some special cases of Type II and Type IV singularities, the dynamical entropy bound is violated near the singularity even if quantum effects are taken into account. The dynamical entropy bound seems to be universal for the case of a regular universe, including the asymptotic de Sitter universe.     

Stability of the accelerated expansion in nonlinear electrodynamics

This paper is devoted to the phase space analysis of an isotropic and homogeneous model of the universe by taking a noninteracting mixture of the electromagnetic and viscous radiating fluids whose viscous pressure satisfies a nonlinear version of the Israel–Stewart transport equation. We establish an autonomous system of equations by introducing normalized dimensionless variables. In order to analyze the stability of the system, we find corresponding critical points for different values of the parameters. We also evaluate the power-law scale factor whose behavior indicates different phases of the universe in this model. It is concluded that the bulk viscosity as well as electromagnetic field enhances the stability of the accelerated expansion of the isotropic and homogeneous model of the universe.     

The Decay of Massive Scalar Field in Non-Static Gödel Type Universe with Viscous Fluid and Heat Flow

In this study, we have investigated the dynamics of non-static Gödel type rotating universe with massive scalar field, viscous fluid and heat flow in the presence of cosmological constant. For various cosmic matter forms, the behavior of the cosmological constant (Λ), shear (η) and bulk (ξ) viscosity coefficients and other kinematic quantities have studied in the early universe. We have showed the decay of massive scalar field in the non-static rotating Gödel type universe and we have obtained constant scalar field with and without source density. Also, we have investigated the effects of massive scalar field on the matter density and pressure. From solutions of the field equations, we have a cosmological model with non-zero expansion, shear, heat flux and rotation. Also some physical and geometrical aspects of the model discussed.     

Radiating viscous universes coupled with zero-mass scalar field: Exact solutions

The dynamics of a radiating viscous fluid universe coupled with zero-mass scalar field is investigated in the Einstein formalism and two exact solutions are obtained. Both the solutions give expanding models. Their many physical and geometrical properties are studied. The model universe corresponding to the first solution turns out to be a big bang model. The second model shows an interesting feature of absorbing radiation rather than emitting it under certain conditions.     

Bulk viscous L.R.S. Bianchi type V tilted stiff fluid cosmological model in general relativity

Locally rotationally symmetric (L.R.S.) Bianchi type V bulk viscous tilted stiff fluid cosmological model is investigated. To get the deterministic model of the universe, we have also assumed a condition A=Bⁿ between metric potentials A, B where n is the constant. The behaviour of the model in presence and absence of bulk viscosity and singularities in the model are also discussed. In general, the models represent accelerating, shearing, tilted and non-rotating universe. The models have point type singularity in presence and absence of bulk viscosity both.     

Viscous Fluid Cosmology in Bianchi Type-I Space-Time

The paper presents a spatially homogeneous and anisotropic Bianchi type-I cosmological model consisting of a dissipative fluid. The field equations are solved explicitly by using a law of variation for mean Hubble parameter, which is related to average scale factor and yields a constant value for deceleration parameter. We find that the constant value of deceleration parameter describes the different phases of the evolution of universe. A barotropic equation of state (p=γ ρ) together with a linear relation between shear viscosity and expansion scalar, is assumed. It is found that the viscosity plays a key role in the process of the isotropization of the universe. The presence of viscous term does not change the fundamental nature of initial singularity. The thermodynamical properties of the solutions are studied and the entropy distribution is also given explicitly.     

The Hubble parameter in the early universe with viscous QCD matter and finite cosmological constant

The evolution of a flat, isotropic and homogeneous universe is studied. The background geometry in the early phases of the universe is conjectured to be filled with causal bulk viscous fluid and dark energy. The energy density relations obtained from the assumption of covariant conservation of energy‐momentum tensor of the background matter in the early universe are used to derive the basic equation for the Hubble parameter H. The viscous properties described by ultra‐relativistic equations of state and bulk viscosity taken from recent heavy‐ion collisions and lattice QCD calculations have been utilized to give an approximate solution of the field equations. The cosmological constant is conjectured to be related to the energy density of the vacuum. In this treatment, there is a clear evidence for singularity at vanishing cosmic time t indicating the dominant contribution from the dark energy. The time evolution of H seems to last for much longer time than the ideal case, where both cosmological constant and viscosity coefficient are entirely vanishing.     

Stability Analysis of the Viscous Polytropic Dark Energy Model in Einstein Cosmology

The viscous polytropic gas model as one model of dark energy is hot-spot and keystone to the modern cosmology.We study the evolution of the viscous polytropic dark energy model interacting with the dark matter in the Einstein cosmology.Setting the autonomous dynamical system for the interacting viscous polytropic dark energy with dark matter and using the phase space analysis method to investigate the dynamical evolution and its critical stability,we find that the viscosity property of the dark energy creates a benefit for the stable critical dynamical evolution of the interaction model between dark matter and dark energy in the flat Friedmann-Robertson-Walker universe and the viscosity of dark energy will soften the coincidence problem just like the interacting dark energy model.     

Two-Fluid Cosmological Models in Bianchi Type-V Space-Time

We have presented anisotropic, homogeneous two-fluid cosmological models using Bianchi type-V space-time. Here one fluid represents the matter content of the universe and another fluid is chosen to model the CMB radiation. The radiation and matter content of the universe are in interactive phase. Also we have discussed the behavior of fluid parameters and kinematical parameters.     

Higher Dimensional Plane Symmetric Cosmological Models with Two-Fluid Source in General Relativity

We consider anisotropic, homogeneous two-fluid plane symmetric cosmological models in higher dimensions. Here one fluid represents the matter content of the universe and another fluid is chosen to model the CMB (cosmic microwave background) radiation. The radiation and matter content of the universe are in interactive phase. Also we have discussed the behaviour of fluid parameters and kinematical parameters.     

Viscous Cosmological Model with Variable Gravitational and Cosmological Constants

We have considered some cosmological solutions with variable gravitational and cosmological constants with bulk viscosity. It is found that the solutions are singularity free and the deceleration parameter is in general not a constant unless we assume perfect fluid with equation of state in the standard cosmologies. Moreover, the deceleration parameter is a function of the scale factor and changes sign with evolution, so our solution is a generalization of those obtained by Arbab I. Arbab. The introduction of viscosity not only free from singularity but also give the deceleration parameter a freedom to vary with scale factor. Thus, a viscous cosmological fluid gives a more general situation in the early universe.     

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

In 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.