Some string cosmological models in Bianchi Type I space-time

We obtain some cosmological models that are exact solutions of Einstein's field equations. The metric utilized is Marder's metric which is Bianchi Type I and the curvature source is a cloud of strings which are one dimensional objects. Bianchi type cosmological models play an important role in the study of the universe on a scale which anisotropy is not ignored. In this paper we have investigated the effect of cosmic strings on the cosmic microwave background anisotropy. Various physical and geometrical properties of the model are also discussed. The solutions have reported that the cosmic microwave background anisotropy may due to the cosmic strings.     

Bianchi Type I string dust cosmological model with magnetic field in general relativity

Bianchi Type I string dust cosmological models in presence and absence of magnetic field following the techniques used by Letelier and Stachel, are investigated. To get the deterministic solution, we have assumed that σ ₁¹ is proportional to the expansion (θ) where σ ₁¹ is the eigen value of shear tensor (σ _i ^j ) and which leads to A=N(BC)⁻ⁿ , n>0 where A,B,C are metric potentials and , N and ℓ are constants. The behaviour of the models in presence and absence of magnetic field are discussed. The other physical and geometrical aspects of the model are also discussed.     

Tilted Bianchi Type I stiff fluid magnetized cosmological model in general relativity

Tilted Bianchi Type I cosmological model for perfect fluid distribution in presence of magnetic field, is investigated. To get a determinate solution, it has been assumed that the universe is filled with stiff perfect fluid distribution together with A=(BC) ⁿ where A,B,C are metric potentials and n is a constant. The behaviour of the model in presence and absence of magnetic field is discussed. The various physical and geometrical aspects of the model, is also discussed. It has been shown that tilted nature of the Bianchi Type I model is preserved due to magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bianchi Type III magnetized massive string cosmological model for perfect fluid distribution in general relativity

Bianchi Type III massive string cosmological model for perfect fluid distribution in the presence of magnetic field, is investigated. It is assumed that the universe is filled with barotropic perfect fluid. We have attempted to investigate Bianchi Type III string cosmological model incorporating perfect fluid with magnetic field. To get the deterministic model in terms of cosmic time, we have assumed that the expansion (θ) in the model is proportional to the shear. We have also assumed that F ₁₂ is the only non-vanishing component of electromagnetic field tensor F _ij . The behaviour of the model in presence and absence of magnetic field together with singularities in these models are also discussed.     

LRS Bianchi Type V bulk viscous fluid string dust cosmological model in general relativity

An LRS Bianchi Type V bulk viscous fluid dust distribution string cosmological model in General Relativity is investigated. It has been shown that if coefficient of bulk viscosity (ζ) is proportional to the expansion (θ) in the model then string cosmological model for Bianchi Type V space-time is possible. In absence of bulk viscosity(ζ) i.e. when ζ → 0 then there is no string cosmological model for Bianchi Type V space-time. The physical and geometrical aspects are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bianchi type VI0 magnetized bulk viscous massive string cosmological model in General Relativity

The paper consists of some exact solutions for a homogeneous Bianchi type VI₀ universe. The material distribution is taken to be a magnetized bulk viscous fluid in presence of massive cosmological string. We assume that current is flowing along x-direction. Therefore, the magnetic field is in yz-plane. For deterministic model of the universe, we assume that shear (σ) is proportional to the expansion (θ) and ζ θ=constant=ξ where ζ the coefficient of bulk viscosity and θ the expansion in the model. The physical and kinematical parameters of the models thus formed are discussed.     

Bianchi type-VIh string cloud cosmological models with bulk viscosity

String cloud cosmological models are studied using spatially homogeneous and anisotropic Bianchi type VI_h metric in the frame work of general relativity. The field equations are solved for massive string cloud in presence of bulk viscosity. A general linear equation of state of the cosmic string tension density with the proper energy density of the universe is considered. The physical and kinematical properties of the models have been discussed in detail and the limits of the anisotropic parameter responsible for different phases of the universe are explored.     

Multi-dimensional cosmological models in effective string gravitation. I

We consider multi-dimensional cosmological models in the low-energy field theory of strings with a boson gravitational sector containing a metric, dilaton field, and antisymmetric Kalb-Ramon field. We study the conformal properties of the action and show that in the general conformal representation the theory is equivalent to a generalized scalar-tensor theory with a Lagrangian of nongravitating matter dependent on the dilaton. We find exact solutions of the flat homogeneous anisotropic model with structure R×M¹×...×Mⁿ and with equation of state p_i=a_i in the space Mⁱ. We discuss the picture of cosmological evolution in different conformal representations.Translated fromAstrofizika, Vol. 38, No. 2, 1995.     

Some magnetized bulk viscous string cosmological models in?General Relativity

Some Bianchi type-I viscous fluid string cosmological models with magnetic field are investigated. The viscosity coefficient of bulk viscous fluid is assumed to be a power function of mass density ξ(t)=ξ ₀ ρ ^m , where ξ ₀ and m are constants. To get a determinate model, we assume conditions ρ=(1+ω)λ, where ρ is rest energy density, ω a positive constant and λ the string tension density and expansion θ is proportional to eigen value σ ₁¹ of the shear tensor σ _j ⁱ . The behaviour of the models from physical and geometrical aspects in presence and absence of magnetic field is discussed.      

Two-fluid cosmological models in a Bianchi type I space-times

In this paper we present anisotropic, homogeneous two-fluid cosmological models in a Bianchi I space-time. These classes of cosmological models picture two different scenarios of cosmic history; viz., when the radiation and matter content of the universe are in interactive phase and another when the two are non-interacting. The universe is highly anisotropic in the initial stages, however, anisotropy tapers out to insignificance in due course of cosmic evolution. In every model the anisotropy of the space-time is determined by the density parameter Ω₀ at the present epoch. For Ω₀=1, the anisotropy is washed out before long. An interesting class of models, having an inflationary epoch in finite future, is discovered.      

Bianchi type – IX string cosmological models with bulk viscous fluid in general relativity

Some Bianchi Type IX string cosmological models with bulk viscous fluid for massive string is investigated. To get a determinate solution, a supplementary condition a=b ⁿ, between metric potentials, is used wherea and b are function of time alone. A particular solution for n=0 is also discussed. The behaviour of the model in presence and absence of bulk viscosity, is discussed. The physical and implications of the model is also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

String cosmological models of Bianchi type-III

A technique to generate new exact Bianchi type-III cosmological solutions of massive strings in the presence of magnetic field is presented. Starting from Tikekar and Patel's strings models in the absence and presence of the magnetic field, new solutions are obtained. Some of their physical features are discussed.     

Bianchi type-V bulk viscous string cosmological model in f(R,T) gravity

In this paper, we investigate a spatially homogeneous and anisotropic Bianchi type-V cosmological model in a scalar-tensor theory of gravitation proposed by Harko et al. (Phys. Rev. D 84:024020, 2011) when the source for energy momentum tensor is a bulk viscous fluid containing one dimensional cosmic strings. To obtain a determinate solution, a special law of variation proposed by Berman (Nuovo Cimento B 74:182, 1983) is used. We have also used the barotropic equation of state for the pressure and density and bulk viscous pressure is assumed to be proportional to energy density. It is interesting to note that the strings in this model do not survive. Also the model does not remain anisotropic throughout the evolution of the universe. Some physical and kinematical properties of the model are also discussed.     

Bianchi type-II, VIII & IX perfect fluid magnetized cosmological models in Brans-Dicke theory of gravitation

Spatially homogeneous Bianchi type-II, VIII & IX charged perfect fluid cosmological models in Brans-Dicke theory of gravitation are obtained and presented. Various physical and geometrical features of the models are also discussed.     

LRS Bianchi type-II massive string cosmological model in General Relativity

The present study deals with locally rotationally symmetric (LRS) Bianchi type II cosmological model representing massive string. The energy-momentum tensor for such string as formulated by Letelier (Phys. Rev. D 28:2414, 1983) is used to construct massive string cosmological model for which we assume that the expansion (θ) in the model is proportional to the shear (σ). This condition leads to A=B ^m , where A and B are the metric coefficients and m is proportionality constant. For suitable choice of constant m, it is observed that in early stage of the evolution of the universe string dominates over the particle whereas the universe is dominated by massive string at the late time. Our model is in accelerating phase which is consistent to the recent observations of type Is supernovae. Some physical and geometric behavior of the model is also discussed.     

Anisotropic Bianchi type-I models in string cosmology

The present study deals with a spatially homogeneous and anisotropic Bianchi-I cosmological models representing massive strings. The energy-momentum tensor, as formulated by Letelier (1983), has been used to construct massive string cosmological models for which we assume the expansion scalar in the models is proportional to one of the components of shear tensor. The Einstein’s field equations have been solved by applying a variation law for generalized Hubble’s parameter in Bianchi-I space-time. We have analysed a comparative study of accelerating and decelerating models in the presence of string scenario. The study reveals that massive strings dominate in the decelerating universe whereas strings dominate in the accelerating universe. The strings eventually disappear from the universe for sufficiently large times, which is in agreement with current astronomical observations.     

Magnetic Bianchi type II string cosmological model in loop quantum cosmology

The loop quantum cosmology of the Bianchi type II string cosmological model in the presence of a homogeneous magnetic field is studied. We present the effective equations which provide modifications to the classical equations of motion due to quantum effects. The numerical simulations confirm that the big bang singularity is resolved by quantum gravity effects.