Anisotropic Bianchi Type-II Cosmological Models in Self-Creation Cosmology

Totally anisotropic Bianchi type-II cosmological solutions are presented in Barber's second self-creation theory of gravitation both in vacuum and in the presence of stiff-matter. The corresponding cosmological models have no finite singularity. The stiff-matter model gives essentially an empty universe for large time. Some physical and kinematical properties of the models are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cosmological Creation of Matter in Conformal Cosmology

The effect of rapid cosmological creation of vector W and Z bosons is studied in the framework of conformal cosmology, which unifies the general theory of relativity and the standard model for strong and electroweak interactions.     

Brans-Dicke Cosmology with non-vanishing Cosmological Constant and Non-Zero Curvature

A model of the universe based on Brans-Dicke theory with non-vanishing cosmological constant and non-zero curvature is studied. Equations (13) and (16) have been obtained by the assumption f(t) = φ(t)a ³ (t),which give the values of the scale factor, a(t) and scalar field, φ(t) in terms of the observable parameters. Also, for a particular case of matter dominated universe, Equation (20) is obtained which gives the relation between various parameters. This revised version was published online in July 2006 with corrections to the Cover Date.     

Star Formation and Cosmological Simulations

The inclusion of a detailed modeling of the short-scale baryonic physics in a large-scale cosmological simulation is crucial for a better comparison between observations and predictions from cosmological models. From a set of 3D hydrodynamical simulations which include a chemical model to account for the complex physics of the ISM at a sub-grid scale, we have been able to obtain a statistically significant sample of galaxy-type halos with observational properties, like colors and luminosities for different cosmological scenarios. From this data base, we have studied a number of different things, like Tully-Fisher relations, luminosity functions and environmental effects. Despite the progress made during the last few years in the modeling of the physics of ISM and star formation, more work is clearly needed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Two-Fluid Bianchi Type II Cosmological Models

In this paper we present a class of solutions of Einstein's field equations describing two-fluid models of the universe in a locally rotationally symmetric Bianchi type II space-time. In these models one fluid is the radiation distribution which represents the cosmic microwave background and the other fluid is the perfect fluid representing the matter content of the universe. It is found that both the fluids are comoving in the locally rotationally symmetric Bianchi type II space-time. The behaviour of the radiation density, matter density, the ratio of the matter density to the radiation density and the pressure has been discussed. A subclass of solutions is found to describe models of a spatially homogeneous and partially isotropic universe evolving from a radiation dominated era to a pressure free matter dominated era. This revised version was published online in July 2006 with corrections to the Cover Date.     

Inhomogeneous Cylindrically-Symmetric Models with Varying Cosmological Term

We consider Einstein's equations with variable cosmological constant in the presence of a perfect fluid for the inhomogeneous cylindrically-symmetric space–time. We find two solutions under this conditions and analyze our solutions with respect to the constants.     

Classical Matter Fields in Homogeneous Cosmological Models

The behaviour of classical massive scalar and vector fields as sources of homogeneous world models is examined. The consequence of the violation of the strong energy condition by the massive scalar field for the evolution of an cosmological model is discussed.     

Some Cosmological Models in Scalar-Tensor Theory of Gravitation

Field equations in a scalar-tensor theory of gravitation proposed by Saezand Ballester (1985) are obtained with the aid of (i) Friedmann-type metric (ii) a non static plane symmetric metric and (iii) spatially homogeneous Bianchi type – III metric. Some cosmological models corresponding to perfect fluid and bulk viscous fluid are presented. Physical and kinematical properties of the models are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Λ Varying Cosmological Models with Viscous Fluid

Evolution of a homogeneous, isotropic Universe with flat geometry filled with a viscous fluid is investigated in presence of a variable cosmological Λ. A non-singular solution leading to a variable deceleration parameter is obtained which reduces to the solution of Murphy in the ‘no Λ limit’ and to the recent solution of Vishwakarma in the context of ‘a Machian model of dark energy’ in the ‘no viscosity’ limit.     

Cosmological Models with Variable G and Λ

Einstein's equations with variable gravitational and cosmological constants are considered in the presence of a perfect fluid for the anizotropic Bianchi I universe in a way which conserving the energy-momentum tensor. Two solutions are found, one of which the cosmological term varies inversely with power law of time. The other of which cosmological term is constant.     

Cosmological Dynamics of Brane Models and Vacuum Effects

The imposition of boundary conditions on the background fields of brane-world models leads to nonzero vacuum averages for the corresponding energy-momentum tensor. Methods from the qualitative theory of dynamic systems are used in this paper to study the cosmological evolution generated by this tensor. All the possible types of cosmological dynamics are examined for models with homogeneous and isotropic subspaces on the branes and the behavior of the corresponding solutions is studied in the early and late stages of evolution.     

Some Bianchi Type I Viscous Fluid Cosmological Models with a Variable Cosmological Constant

Some Bianchi type I viscous fluid cosmological models with a variable cosmological constant are investigated in which the expansion is considered only in two direction i.e. one of the Hubble parameter is zero. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density whereas the coefficient of shear viscosity is considered as constant in first case whereas in other case it is taken as proportional to scale of expansion in the model. The cosmological constant Λ is found to be positive and is a decreasing function of time which is supported by results from recent supernovae Ia observations. Some physical and geometric properties of the models are also discussed.     

Cylindrically-Symmetric and Inhomogeneous Cosmological Models with Viscous Fluid

In this paper, the Einstein field equations have been solved for cylindrically symmetric and inhomogeneous cosmological models with viscous fluid. Various physical and geometrical properties of the models are also discussed.     

Some Viscous Fluid Cosmological Models in General Relativity

The motivation of this paper is to investigate two viscous fluid cosmological models in General Relativity in which the expansion is only in two directions i.e. one of the Hubble parameters is zero. In the first model, coefficient of shear viscosity is assumed to be constant while in the second model, the coefficient of shear viscosity is proportional to the rate of expansion in the model. Here no additional condition is assumed except for coefficient of shear viscosity. These models are new and different from those models obtained by Bali and Jain (1987, 1988) in which free gravitational field was assumed to be Petrov Type D and non-degenerate for Marder (1958) metric and coefficient of shear viscosity is proportional to the rate of expansion in the model. The various physical and geometrical aspects of the models are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Vacuum Cosmological Models in Einstein and Barber Theories

An attempt has been made to solve the field equations with perfect fluid in an inhomogeneous space-time governed by the metric

Higher Dimensional FRW Cosmological Models in Self-Creation Theory

The field equations of Barber's (1982) second self-creation theory of gravitation are solved for 5D Friedmann-Robertson-Walker space time using perfect fluid energy momentum tensor. By assuming an equation of state p= ε ρ, (0 ≤ ε ≤ 1), the solutions of the field equations, in different scenarios, in Barber's second self-creation theory are presented and discussed. Some properties of these models are also discussed.     

Early Cosmological Models with Bulk Viscosity in Brans-Dicke Theory

The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously as the Universe expands, are obtained by using the power-law relation φ = bR ⁿ , which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation dominated era of the universe. The physical properties of cosmological solutions are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Testing Cosmological Models using the Kinematics of High Redshift Galaxies

The scaling of the apparent angular diameter of galaxies with redshift θ(z) is a powerful discriminator of cosmological models. In this paper we argue that the rotational velocity of distant galaxies, when interpreted as size indicator, may be used as an interesting tool to select high redshift standard rods. Upcoming deep redshift surveys will allow an implementation of this classical geometrical test to measure directly the amplitude of the cosmological constant Λ, or to constrain the cosmic equation of state parameter for a smooth dark energy component (w = p/ρ, —1 ≤ w < 0).