Cosmological Models with Bulk Viscosity in the Presence of Particle Creation

The effect of bulk viscosity, with a time varying bulk viscous coefficient, on the evolution of frw models is investigated in the context of open thermodynamic systems, which allows for particle creation. It is seen that, by choosing an appropriate function for particle creation, the models presented exhibit non-singular beginnings.     

Kantowski-Sacks Bulk Viscous String Cosmological Models in the Presence of Zero-Mass Scalar Fields

The Kantowski-Sachs cosmological solutions of massive strings have been studied in the presence of zero-mass scalar field coupled with bulk viscosity. It is assumed that the coefficient of bulk viscosity is a power function of energy density of massive strings. Further we have considered the cosmological parameter as a function of cosmic time. We obtained the general solution of the field equations in polynomial and exponential forms respectively. The behaviors of these models are also discussed in the presence as well as in the absence of bulk.     

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.     

Five Dimensional Axially Symmetric String Cosmological Models with Bulk Viscous Fluid

Bulk viscous fluid distribution with massive strings in LRS Bianchi type-1 space time is studied. The exact solutions of the field equations are obtained by using the equation of state ρ=−λ and ρ=λ. We observed that the bulk viscous fluid does not survive for ρ=−λ whereas it survives for ρ=λ. Some physical and geometrical properties of the models are discussed.     

Bulk Viscous Cosmological Model with Interacting Dark Fluids

We study a cosmological model for a spatially flat Universe whose constituents are a dark energy field and a matter field comprising baryons and dark matter. The constituents are assumed to interact with each other, and a non-equilibrium pressure is introduced to account for irreversible processes. We take the non-equilibrium pressure to be proportional to the Hubble parameter within the framework of a first-order thermodynamic theory. The dark energy and matter fields are coupled by their barotropic indexes, which depend on the ratio between their energy densities. We adjust the free parameters of the model to optimize the fits to the Hubble parameter data. We compare the viscous model with the non-viscous one, and show that the irreversible processes cause the dark-energy and matter-density parameters to become equal and the decelerated–accelerated transition to occur at earlier times. Furthermore, the density and deceleration parameters and the distance modulus have the correct behavior, consistent with a viable scenario of the present status of the Universe.     

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 F ₂₃ is the only non-vanishing component of electromagnetic field tensor F _ij . To get the deterministic model in terms of cosmic time t, we have also assumed the condition ζ θ=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.      

Bianchi Type-II Bulk Viscous String Cosmological Models in General Relativity

Some Bianchi type II bulk viscous string cosmological models with electromagnetic field are investigated. To get a determinate solution, we assume the shear (σ) is proportional to the expansion (θ), which leads to a supplementary condition B=lA ⁿ , between metric potentials, is used where A and B are function of time alone. A particular solution for n=0 is also discussed. The physical and geometrical implications of the models are discussed.     

Cosmological Models with Variable Cosmological and Gravitational “Constants” and Bulk Viscous Models

A viscous model with variable gravitational and cosmological constant has been considered. Several solutions are presented and some are shown to be equivalent to Berman, Kalligas et al. and Brans-Dicke solutions.     

Cosmological Particle Creation and Dynamical Casimir Effect

We compute particle creation for a real massive scalar field conformally coupled to a spatially closed Robertson–Walker space-time background, with time-dependent scale factor. This is a dynamical Casimir effect with moving boundaries.     

Interacting Viscous Modified Chaplygin Gas Cosmology in Presence of Cosmological Constant

Here, we consider interacting viscous modified Chaplygin gas in presence of cosmological constant. We assumed bulk viscosity as a function of density. We consider interaction between modified Chaplygin gas and baryonic matter. Then, the effects of viscosities on the cosmological parameters such as energy, density, Hubble expansion parameter, scale factor and deceleration parameter investigated. This model may be considered as a toy model of our universe.     

Anisotropic Viscous Fluid Cosmological Models from Deceleration to Acceleration in String Cosmology

In the present study, anisotropic locally rotationally symmetric (LRS) Bianchi type II models representing massive strings have been presented with variable mean deceleration parameter. The field equations in scalar-tensor theory with energy momentum tensor proposed by Letelier (Phys. Rev. D 28: 2414, 1983), have been solved for the bulk viscous fluid and perfect fluid under the following physically relevant assumptions: (a) scale factor varies with time as $a(t) = (\sinh(\alpha t))^{\frac{1}{n}}$ , (b) Expansion (θ) in the model is proportional to shear (σ), (c) p=γρ, where γ (0≤γ≤1) is a constant, (d) ξ(t)=ξ ₀ ρ ^η . It has been observed that the presented universe has a phase transition from the early decelerating phase to the accelerating phase at present epoch which is in good agreement with the recent astronomical observations. Moreover, some physical and geometric properties of the models have been discussed in detail.     

Five Dimensional Bulk Viscous String Cosmological Models in Saez and Ballester Theory of Gravitation

LRS Bianchi type-I bulk viscous string cosmological models are obtained in scalar tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. A 113:467, 1986). It is shown that cosmic string does not survive for ρ+λ=0 whereas it survives for the equations of state ρ=(1+ω)λ (Takabayasi string) and ρ=λ (Geometric string). Some physical and geometrical properties of the exhibited model are discussed.     

Bianchi Cosmologies with Particle Creation and Bulk Viscosity

In the context of open thermodynamic systems, we study in this paper the evolution of Bianchi cosmologies with time-dependent bulk viscosity and particle production. The possibility of an initial non-singularity occurs with a suitable choice of parameters as seen in the case of FRW models [Desikan, K. (1997). Gen. Rel. Grav. 29, 435.].     

Bianchi Type-I Cosmological Model Filled with Viscous Fluid in a Modified Brans-Dicke Cosmology

Adding the cosmological term, which is assumed to be variable in Brans-Dicke theory we have discussed about a Bianchi type-I cosmological model filled with viscous fluid with free gravitational field of Petrov type-D. The effect of viscousity on various kinematical parameters has been discussed. Finally, this model has been transformed to the original form (1961) of Brans-Dicke theory including a variable cosmological term.     

Future Stability of the FLRW Fluid Solutions in the Presence of a Positive Cosmological Constant

We introduce a new method for establishing the future non-linear stability of perturbations of FLRW solutions to the Einstein–Euler equations with a positive cosmological constant and a linear equation of state of the form p = Kρ. The method is based on a conformal transformation of the Einstein–Euler equations that compactifies the time domain and can handle the equation of state parameter values 0 < K ≤ 1/3 in a uniform manner. It also determines the asymptotic behavior of the perturbed solutions in the far future.     

Bulk Viscus Fluid Plane Symmetric String Dust Magnetised Cosmological Model in General Relativity

In this paper we have investigated bulk viscous fluid plane symmetric dust magnetized string cosmological model. To get a deterministic model, it is assumed that ε=λ, and a relation between metric potential B=RA ⁿ . The behaviour of the model in the presence and absence of magnetic field together with physical and geometrical aspects of the model are also discussed.