Bianchi VI0 models in N=2, D=5 supergravity

We investigate Bianchi VI₀ cosmological models containing two interacting scalar fields. These models are derived from a dimensional reduction of theN=2,D=5 supergravity theory. Exact solutions are found and the existence of singularities for them is considered.     

Bianchi Types V and VI<Subscript>0</Subscript> Cosmic Strings Coupled with Maxwell Fields in Bimetric Theory

Spatially homogeneous Bianchi types V and VI₀ cosmological models are studied with source cosmic cloud strings coupled with electromagnetic field in Rosen’s (Gen. Relativ. Gravit. 4:435, 1973) bimetric theory of relativity. It is observed that Bianchi type V space time is feasible whereas Bianchi type VI₀ is not feasible. In the feasible case different equations of state for cosmic strings with Maxwell fields do not survive in this theory and the space-time turns out to be flat.     

Bianchi Type VI1 Cosmological Model in Scale-Invariant Theory

An attempt has been taken to study the feasibility of scale-invariant theory in Bianchi type VI _h space-time with a time dependent gauge function (Dirac gauge) and a matter field in the form of a perfect fluid with isotropic matter pressure. It is found that Bianchi type VI _h (h = 1) space-time is feasible in this theory whereas Bianchi type VI _h (h = -1) and VI _h (h = 0) space-times are not feasible. In the feasible case a non-singular model for the universe filled with disorder radiation is constructed and some of its physical behaviors are studied.     

Bianchi VI0 electric type cosmological models in General Relativity with stiff fluid and heat conduction

The paper consists of some exact tilted solutions for a homogeneous Bianchi type VI₀ universe. The material distribution is taken to be a stiff fluid with heat conduction. The physical and kinematical parameters have also been calculated to discuss the models in detail.     

Gravitational Entropy in Cosmological Models

We discuss whether an appropriately defined dimensionless scalar function might be an acceptable candidate for the gravitational entropy, by explicitly considering Szekeres and Bianchi type VI _h models that admit an isotropic singularity. We also briefly discuss other possible gravitational entropy functions, including an appropriate measure of the velocity dependent Bel-Robinson tensor. PACS: 04.20.-q 95.30.Sf 98.80.Jk     

Bianchi V models in N=2, D=5 supergravity

We investigate Bianchi V cosmological models containing two interacting scalar fields. These models are derived from a dimensional reduction of theN=2,D=5 supergravity theory. Exact solutions are found.     

Nonscalar singularities in spatially homogeneous cosmologies

Singularities in vacuum spatially homogeneous cosmological models are investigated. It is shown that in general the curvature scalarR _* ^abcd R^*abcddiverge and that the only solutions which have curvature singularities at which this scalar does not diverge describe certain plane-wave space-times. It is argued that with matter present these nonscalar singularities are even less likely to occur. The exceptional case of Bianchi type VI_–1/9 is not considered.     

Some exact solutions in Bianchi VI0 string cosmology

Following the techniques used by Letelier and Stachel some new physically relevant explicit Bianchi VI₀ solutions of string cosmology with magnetic field are reported. They include two models describing distributions of Takabayashi strings and geometric strings respectively.     

Irrotational and conformally Ricci-flat perfect fluids

When a space-time, containing an irrotational perfect fluid withw + p 0, is conformally Ricci-flat, three possibilities arise: (a) When the gradient of the conformal scalar field is aligned with the fluid velocity, the solution is conformally flat; (b) when the gradient is orthogonal to the fluid velocity, solutions are either shearfree, nonexpanding and (pseudo-) spherically or plane-symmetric, or they are conformally related to a particular new vacuum solution admitting a three-dimensional group of motions of Bianchi type VI_o on a timelike hypersurface; (c) in the general case solutions are (pseudo) spherically or plane-symmetric and have nonvanishing expansion.     

From 2-dimensional surfaces to cosmological solutions

We construct perfect fluid metrics with two symmetries by means of a recently developed geometrical method [1]. The Einstein equations are reduced to a single equation for a conformal factor. Under additional assumptions we obtain new cosmological solutions of Bianchi type II, VI₀ and VII₀. The solutions depend on an arbitrary function of time, which can be specified in order to satisfy an equation of state.     

Homogeneous Bianchi type VI0 perfect fluid space-times

An algorithm is presented for generating new exact solutions of the Einstein equations for spatially homogeneous cosmological models of Bianchi type VI₀. The energy-momentum tensor is of perfect fluid type. Starting from Dunn and Tupper's dust-filled universe, new classes of solutions are obtained. The solutions represent anisotropic universes filled with perfect fluid not satisfying the equation of state. Some of their physical properties are studied.     

Exact anisotropic viscous fluid solutions of Einstein's equations

A method for obtaining anisotropic, rotationless viscous fluid matter solutions of Bianchi type I and Segré type [1, 111] with the barotropic equation of state is presented. Solutions for which the anisotropy decreases exponentially or with a power law as well as solutions with average Hubble parameterH t ^–1 are discussed. Also, a class of solutions with constant anisotropy and Bianchi type VI_h is found. The dominant energy condition holds and the transport coefficients show the right sign.     

General Behaviour of Bianchi VI0 Solutions with an Exponential-Potential Scalar Field

The solutions to the Einstein-Klein-Gordon equations without a cosmological constant are investigated for an exponential potential in a Bianchi VI₀ metric. There exists a two-parameter family of solutions which have a power-law inflationary behaviour when the exponent of the potential, k, satisfies k² < 2. In addition, there exists a two-parameter family of singular solutions for all k² values. A simple anisotropic exact solution is found to be stable when 2 < k².     

Bianchi type-I,V and VIo models in modified generalized scalar-tensor theory

In modified generalized scalar-tensor (GST) theory, the cosmological term Λ is a function of the scalar field ϕ and its derivatives . We obtain exact solutions of the field equations in Bianchi Type-I, V and VIo space-times. The evolution of the scale factor, the scalar field and the cosmological term has been discussed. The Bianchi Type-I model has been discussed in detail. Further, Bianchi Type-V and VIo models can be studied on the lines similar to Bianchi Type-I model.      

Viscous Bianchi universes with a magnetic field. I. Effect of magnetoviscosity on the metric

It is shown that viscosity induced by a magnetic field excludes certain Bianchi models. In fact, it is proved that a homogeneous, nonrotating universe of Bianchi class A, containing a viscous fluid and a large-scale magnetic field, can be only of Bianchi type I or VI₀.     

Letter: On Tilted Perfect Fluid Bianchi Type VI0 Self-Similar Models

We show that the tilted perfect fluid Bianchi VI₀ family of self-similar models found by Rosquist and Jantzen [K. Rosquist and R. T. Jantzen, Exact power law solutions of the Einstein equations, 1985 Phys. Lett. 107A 29–32] is the most general class of tilted self-similar models but the state parameter lies in the interval (6/5, 3/2). The model has a four dimensional stable manifold indicating the possibility that it may be future attractor, at least for the subclass of tilted Bianchi VI₀ models satisfying n =0 in which it belongs. In addition the angle of tilt is asymptotically significant at late times suggesting that for the above subclasses of models the tilt is asymptotically extreme.     

Bianchi Type VI0 Cosmological Models with Perfect Fluid and Dark Energy

We consider a self-consistent system of Bianchi Type VI₀ cosmology and binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be one obeying the usual equation of state p=??? with ????[0,1]. The dark energy is considered to be either the quintessence or Chaplygin gas. Exact solutions to the corresponding Einstein??s field equations are obtained as a quadrature. Models with power-law and exponential expansion have discussed in detail.     

Anisotropic supergravity cosmologies

Anisotropic cosmologies of Bianchi types I, II, VI₀, VII₀, VIII and IX are studied in the bosonic sector of d = 11 supergravity. Exact solutions are given for all types in s = 4 dimensions coupled to the “round” (d ? s = 7) seven-sphere solutions due to Freund, Rubin, Duff and Pope, and Englert. The generalized Kasner, the Taub-NUT-de Sitter and some “mixmaster” solutions with a cosmological constant are derived. In addition, we wish to point out that the isotropic solution given recently by Myung and Kim does not exist. Another inconsistent solution was also given by Gleiser et al.     

Bianchi VI cosmological models representing perfect fluid and radiation with electric-type free gravitational fields

A homogeneous Bianchi type VI_h cosmological model filled with perfect fluid, null electromagnetic field and streaming neutrinos is obtained for which the free gravitational field is of the electric type. The barotropic equation of statep = (–1) is imposed in the particular case of Bianchi VI₀ string models. Various physical and kinematical properties of the models are discussed.