Exact Bianchi type-I cosmological model with matter in Brans-Dicke theory

In this paper an exact solution of the Brans-Dicke field equations in the presence of stiff matter is obtained for the Bianchi type-I cosmological space-time. The new solution represents an anisotropic homogeneous cosmological model which tends to anisotropic expansion. The behaviour of the solution near the singularities and late stages of the expansion is discussed.     

Spherically symmetric cosmological perturbation in a universe with background field

In a previous paper the equations of small cosmological perturbations of a theory of gravitation in flat space-time are derived. They are applied to a homogeneous, isotropic, nonsingular cosmological model with radiation, matter and background field. At the beginning of the universe small spherically symmetric inhomogeneities on almost all scales can arise by instability. Later on the density contrast of dust grows exponentially during a short time epoch. The solution during this time period is given analytically.     

On the Hubble Law in a Nonexpanding Nonstationary Universe with Cosmological Constant

A nonsingular, homogeneous, isotropic cosmological model with cosmological constant in flat space-time theory of gravitation is studied. The second law of thermodynamics yields a nonexpanding (nonstationary) universe without entropy production. At the beginning of the universe radiation, matter and vacuum energy given by the cosmological constant are zero and then emerge from gravitational energy. In the course of time the energy of radiation and matter decrease whereas the vacuum energy increases forever. Light emitted from a distant galaxy loses energy on his way to the observer producing the observed redshift. The velocity of light in the past is greater than the present one. This may explain superluminal velocities but only for large redshifts. The sum of the density parameters of matter, radiation and vacuum energy is a little greater than one. All the matter can be baryonic. There is no age problem of the universe. This revised version was published online in July 2006 with corrections to the Cover Date.     

Relativistic static spheres filled with infinitely conducting charged fluids

A class of spherically-symmetric cosmological models for space-time filled with infinitely conducting relativistic perfect fluids with constant magnetic permeability is developed for a specified choice of matter density and fluid pressure. Some salient features of all these solutions are 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.      

Cosmological variations in the space-time distribution of absorption systems in quasar spectra

Based on the catalog of Junkkarinen et al. (1991), we analyze the space-time distribution of absorption systems in quasar spectra at cosmological redshifts z=0–3.7. The z distribution of absorbing matter is shown to have a pattern of alternating maxima (peaks) and minima (dips). Within statistical uncertainty, the positions of such peaks and dips do not depend on the direction of observation. We have found a periodicity in the distribution of absorption systems in the functions ln(1+z) and (1+z)^?1/2. We show that the derived sequence of maxima and minima in the space-time distribution of absorbing matter is not a manifestation of the spatial large-scale structure alone, but it is more likely temporal in nature. The most probable source of the putative structure could be an alternation (in the course of cosmological evolution) of pronounced and depressed epochs with a characteristic time interval of 520±160 Myr, depending on the cosmological model chosen.     

Relation between physical and metric field in general relativity and cosmology

By assuming that the whole matter of the Universe possesses a dilatational degree of freedom, we attempted to show the equivalence between the curvedmetric of space-time and the flat metric of dilated space-time. In the framework of this procedure we supposed that the metrical field and the physical space and time change their roles. The basic result suggests that gravitation and dilatation are interrelated phenomena. In addition we discuss the possibility of the Universe which is of a hybrid type: it possesses at the same time properties of the evolutionary and stationary Universe. Finally, we discuss the lenghtening of day in time as an example which can support our ideas. There was suggested that this phenomenon appears partly as cosmological manifestation.     

Non-relativistic matter and dark energy in a quantum conformal model

We consider a generalization of the Standard Model whose action displays conformal invariance in d dimensions. The model contains a strongly coupled dark matter sector which breaks conformal symmetry dynamically. The model evades conformal anomaly and leads to identically zero vacuum energy in flat space-time. Hence it does not suffer from the problem of fine tuning of the cosmological constant. We determine the contribution of non-relativistic matter to the energy-momentum tensor and determine a parameter regime in which it approximately reduces to the standard result. We show how dark energy and dark matter arises in this model. We discuss the parameter range for which the model reduces to the ΛCDM model and hence is consistent with observations.     

Semi-Classical Cosmological Models with Scalar, Dirac and Electromagnetic Fields

Exact solutions of the semi-classical Einstein equations for conformally invariant free quantum fields in an homogeneous and isotropic space-time, with cosmological constant and containing a classical scalar field, dust matter, an unquantised Dirac field and electromagnetic radiation are found. The initial behaviour of the semiclassical models is investigated. Some of the solutions found avoid the singularity and do not have particle horizons. This revised version was published online in July 2006 with corrections to the Cover Date.     

Bianchi type VI1 cosmological model with wet dark fluid in scale invariant theory of gravitation

In this paper, we have investigated Bianchi type VI _h , II and III cosmological model with wet dark fluid in scale invariant theory of gravity, where the matter field is in the form of perfect fluid and with a time dependent gauge function (Dirac gauge). A non-singular model for the universe filled with disorder radiation is constructed and some physical behaviors of the model are studied for the feasible VI _h (h=1) space-time.     

Cosmic strings and viscous matter in a Kantowski-Sachs universe

We study the dynamics of a universe where the matter content is represented by a radially directed distribution of cosmic strings plus matter admitting both shear and bulk viscosity. We assume specific forms for the equation of state of both the strings and matter and discuss the cosmological solutions obtained.     

Bianchi Type-V Bulk Viscous Fluid String Dust Cosmological Model In General Relativity

Bianchi Type-V bulk viscous fluid string dust cosmological model in General Relativity is investigated. It has been shown that if coefficient of bulk viscosity (ζ) is inversely 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 of the model are also discussed.     

Bianchi type I cosmological perfect fluid model in scale invariant theory

The intention of this paper is to study the perfect fluid distribution in scale invariant theory of gravity when the space-time is described by Bianchi type I metric with a time dependent gauge function. The cosmological equations for this space-time with gauge function are solved and some physical properties of the model are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Global Monopole and C-Field

It is well known that a C-field, generated by a certain source equation leads to interesting changes in the cosmological solutions of Einstein's equations. It is argued that different types of topological objects may have been created by the vacuum phase transitions in the early universe. In the cosmological arena, the defects have been put forward as a possible mechanism for structure formation. A global monopole is a heavy object formed in the phase transition of a system composed of a self coupling scalar field triplet φ^a whose original global 0(3) symmetry is spontaneously broken to U(1). In this article, we find a special solution for the space-time of a global monopole in presence of C-field. It is shown that the nature of the solution remains the same as in general relativity case i.e. monopole exerts no gravitational force on the matter surrounding it but space around it has a deficit solid angle. Pacs Nos: 98.80cq, 04.20jb, 04.50     

Cosmological electrovac model in Bertotti-Robinson space-time

Cosmological electrovac field equations are studied in Bertotti-Robinson-type space-time, and a class of cosmological solutions is obtained. The nature of the electromagnetic fields and singularities of the solution is studied. A technique is established to generate these solutions from a known vacuum solution with a non-zero cosmological constant.     

Qualitative analysis of two-fluid FRW cosmological models

This paper focuses attention on a qualitative analysis of the evolution of two-fluid flat FRW cosmological models.In the first model one of the fluid represents matter content of the universe comoving with respect to the another fluid that is the cosmic microwave background radiation (CMBR), these two fluids are interacting.The first model is most relevant to describe the scenario before the recombination epoch when matter and radiation were in an interactive phase and the photons was bound to electron through Thomson scattering. The second model describe two noninteracting fluids where the matter is comoving to the space-time coordinates and the CMBR is moving axially, relative to the matter thus modeling the relative velocity between galaxies and the CMBR (Phys. Rev. Lett. 39:898–901, 1977). This model portray the cosmic evolution in the postrecombination epoch when the two-fluid are noninteracting.In this epoch the photons got themselves free to form the CMBR being observed presently.      

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.     

Non,thermal Cosmic Gravitational Radiation and Redshift Fluctuations

Intergalactic gravitational radiation fields with wavelengths in the range of 1 to 10³ Megaparsecs have remarkable observational effects if the energy density of the fields becomes comparable to the equivalent “critical” cosmological matter density χo = 3H₀². In this case the perturbation in the space-time metric caused by the radiation field is high enough to change the frequency and to shift the path of light rays of extragalactic objects in a random manner by observable amounts. The present paper discusses the wave spectrum as a function of cosmic time and gives a statistical treatment of the redshift fluctuations.     

Vaidya black hole in non-stationary de Sitter space: Hawking’s temperature

In this paper we present a class of non-stationary solutions of Einstein’s field equations describing embedded Vaidya-de Sitter black holes with a cosmological variable function Λ(u). The Vaidya-de Sitter black hole is interpreted as the radiating Vaidya black hole is embedded into the non-stationary de Sitter space with variable Λ(u). The energy-momentum tensor of the Vaidya-de Sitter black hole is expressed as the sum of the energy-momentum tensors of the Vaidya null fluid and that of the non-stationary de Sitter field, and satisfies the energy conservation law. We study the energy conditions (like weak, strong and dominant conditions) for the energy-momentum tensor. We find the violation of the strong energy condition due to the negative pressure and leading to a repulsive gravitational force of the matter field associated with Λ(u) in the space-time. We also find that the time-like vector field for an observer in the Vaidya-de Sitter space is expanding, accelerating, shearing and non-rotating. It is also found that the space-time geometry of non-stationary Vaidya-de Sitter solution with variable Λ(u) is Petrov type D in the classification of space-times. We also find the Vaidya-de Sitter black hole radiating with a thermal temperature proportional to the surface gravity and entropy also proportional to the area of the cosmological black hole horizon.     

A cosmological model with both deceleration and acceleration

In this paper, we investigate a variation law for Hubble’s parameter in the curved, expanding background of spatially homogeneous, anisotropic Bianchi type I space-time. By choosing a particular form of the generalized Hubble’s parameter, which gives an early deceleration and late time acceleration for the anisotropic Bianchi type I cosmological model, we show that the model approaches isotropy and tends to a de Sitter universe at late times. The cosmological term asymptotically tends to a genuine cosmological constant and the solution is consistent with recent observations.