Early Viscous Fluid Cosmological Model with Zero-Rest-Mass Scalar Fields

Einstein field equations are considered for zero-curvature Robertson-Walker models in the case of a viscous fluid distribution interacting with zero-rest-mass scalar fields. Exact solutions are obtained for two different phases of the early universe viz. the inflationary phase and the radiation-dominated phase, by using the 'gamma-law' equation of state p = (-1). The index describing the material content varies continuously with cosmological time. The gravitational 'constant' and bulk viscosity are both allowed to depend on the cosmic time. Some physical properties of the cosmological models are also discussed.     

Cosmological magnetic fields: their generation,evolution and observation

We review the possible mechanisms for the generation of cosmological magnetic fields, discuss their evolution in an expanding Universe filled with the cosmic plasma and provide a critical review of the literature on the subject. We put special emphasis on the prospects for observational tests of the proposed cosmological magnetogenesis scenarios using radio and gamma-ray astronomy and ultra-high-energy cosmic rays. We argue that primordial magnetic fields are observationally testable. They lead to magnetic fields in the intergalactic medium with magnetic field strength and correlation length in a well defined range.We also state the unsolved questions in this fascinating open problem of cosmology and propose future observations to address them.     

Shear and vorticity in inflationary Brans–Dicke cosmology with lambda-term

We find a solution for exponential inflation in Brans–Dicke cosmology endowed with a cosmological term, which includes time-varying shear and vorticity. We find that the scalar field and the scale factor increase exponentialy while shear, vorticity, energy density, cosmic pressure and the cosmological term decay exponentialy for negative beta, where beta is defined in the text.     

Bianchi type-VI0 universe in the presence of zero-mass scalar fields

An exact Bianchi type-VI₀ cosmological model in the presence of zero-mass scalar fields is obtained when the source of the gravitational field is a perfect fluid with pressure equal to energy density. Some physical properties of the model are discussed.     

Thermodynamics and two-fluid cosmological models

Exact solutions of Einstein's field equations and the laws of thermodynamics are presented in which both a comoving radiative perfect fluid (modelling the cosmic microwave background) and a non-comoving imperfect fluid (modelling the observed material content of the Universe) act as the source of the gravitational field as represented by the flat FRW line element. The tilting velocity of the imperfect fluid is associated with the peculiar velocity of our local cluster of galaxies relative to the cosmic microwave background. In these relativistic two-fluid cosmological models the temperatures of the radiation and matter fields are equal until hydrogen recombines at 4000 K, after which time thermal contact between the two fluids is broken. The models presented are physically acceptable cosmologies that are shown to give rise to numerical predictions consistent with current observations.     

Bianchi type-I universe in the presence of zero-mass scalar fields

An exact Bianchi type-I cosmological model in the presence of zero-mass scalar fields is obtained when the source of the gravitational field is a perfect fluid with pressure equal to energy density. Some properties of the model are discussed.     

A plane-symmetric universe in the presence of zero-mass scalar fields

A non-static plane-symmetric cosmological model in the presence of zero-mass scalar fields is obtained when the source of the gravitational field is a perfect fluid with pressure equal to energy density. Some properties of the model are discussed.     

Bianchi type-III cosmological model in the presence of zero-mass scalar fields

An exact Bianchi type-III cosmological model in the presence of zero-mass scalar fields is obtained when the source of the gravitational field is a perfect fluid with pressure equal to energy density. Some properties of the model are also discussed.     

Conformal analogs of the Jordan-Brans-Dicke theory. II. (models including vacuum effects)

This paper is part of a series based on a modified Jordan tensor-scalar theory of gravitation. Given the current importance of research on vacuum phenomena in cosmic evolution, we examine several standard cosmological models with a scalar field and a physical vacuum, including models that have a dominant scalar field with the vacuum energy taken into account in various conformal representations of the Jordan theory, as well as models in which ordinary matter that obeys the conventional equations of state is present. Some noteworthy results are obtained which are, to a certain extent, consistent with currently available observational data.     

Relativistic Cosmology with Zero Deceleration Parameter

The possibility that the deceleration parameter q might be a null constant is discussed; such possibilitiy is interesting because solves the horizon problem and the flatness problem with no need of inflation. A simple way to get q = const = 0 is explored: the way assumes Einstein's field equations without cosmological term and introduces a massless scalar field V with negative energy density. Both in the early and in the present universe one finds the Whitrow-Randall relation Gut² = const 1 G gravitational coupling, u mass-energy density, t cosmic time). The interaction between the V-field and the ordinary matter is briefly discussed; as possible consequence of this interaction the true value of the Hubble parameter might be one half the observed value. This revised version was published online in July 2006 with corrections to the Cover Date.     

A Study of an Inhomogeneous Bianchi-I Model in Lyra Geometry

The inhomogeneous Bianchi-I model based on Lyra's geometry has been studied in the cosmological theory in presence of a massless inhomogeneous scalar field whose potential has a flat part. The field equations are solved using separation of variables and it is shown that one of the time part of the field equations are solvable for any arbitrary other cosmic scale function. Solutions for a particular form of cosmic scale (time part) is presented. This revised version was published online in July 2006 with corrections to the Cover Date.     

Accelerating universe with time variation of G and Λ

We study a gravitational model in which scale transformations play the key role in obtaining dynamical G and Λ. We take a non-scale invariant gravitational action with a cosmological constant and a gravitational coupling constant. Then, by a scale transformation, through a dilaton field, we obtain a new action containing cosmological and gravitational coupling terms which are dynamically dependent on the dilaton field with Higgs type potential. The vacuum expectation value of this dilaton field, through spontaneous symmetry breaking on the basis of anthropic principle, determines the time variations of G and Λ. The relevance of these time variations to the current acceleration of the universe, coincidence problem, Mach’s cosmological coincidence and those problems of standard cosmology addressed by inflationary models, are discussed. The current acceleration of the universe is shown to be a result of phase transition from radiation toward matter dominated eras. No real coincidence problem between matter and vacuum energy densities exists in this model and this apparent coincidence together with Mach’s cosmological coincidence are shown to be simple consequences of a new kind of scale factor dependence of the energy momentum density as ρ∼a ⁻⁴. This model also provides the possibility for a super fast expansion of the scale factor at very early universe by introducing exotic type matter like cosmic strings.     

Scalar field instability in multi-dimensional cosmology

The scalar field theory on the background of cosmological models with n(n ≥ 1) spaces of constant curvature is considered. We take the integrable case of Ricci flat internal spaces. The coupling between the scalar and the gravitational fields includes the minimal coupling as well as the conformal case. In the ground state of the scalar field we find the conditions for vacuum instability realized for most of the possible solutions to Einstein's equations if the coupling parameter takes appropriate values. For the excited states of the scalar field we show the induction of massive modes and discuss their properties.     

The influence of gravitational binding energy on cosmic expansion dynamics: new perspectives for cosmology

Confronted with microwave background observations by WMAP and with consternating supernova locations in the magnitude–redshift diagram modern cosmology feels enforced to call for cosmic vacuum energy as a necessary cosmological ingredient. Most often this vacuum energy is associated with Einstein’s cosmological constant Λ or with so-called “dark energy”. A positive value of Λ describes an inflationary action on cosmic dynamics which in view of recent cosmological data appears as an absolute need. In this article, however, we question the hypothesis of a constant vacuum energy density since not justifiable on physical grounds. Instead we show that gravitational binding energy of cosmic matter, connected with ongoing structure formation during cosmic expansion, acts similar to vacuum energy, since it reduces the effective gravitating proper mass density. Thus one may be encouraged to believe that actions of cosmic vacuum energy and gravitational binding energy concerning their cosmological effects are closely related to each other, perhaps in some respects even have identical phenomenologies.     

(An)Isotropic models in scalar and scalar-tensor cosmologies

We study how the constants G and Λ may vary in different theoretical models (general relativity with a perfect fluid, scalar cosmological models (“quintessence”) with and without interacting scalar and matter fields and a scalar-tensor model with a dynamical Λ) in order to explain some observational results. We apply the program outlined in section II to study three different geometries which generalize the FRW ones, which are Bianchi V, VII₀ and IX, under the self-similarity hypothesis. We put special emphasis on calculating exact power-law solutions which allow us to compare the different models. In all the studied cases we arrive at the conclusion that the solutions are isotropic and noninflationary while the cosmological constant behaves as a positive decreasing time function (in agreement with the current observations) and the gravitational constant behaves as a growing time function.     

Time dependent viscous string cloud cosmological models

Bianchi type-I string cosmological models are studied in Saez-Ballester theory of gravitation when the source for the energy momentum tensor is a viscous string cloud coupled to gravitational field. The bulk viscosity is assumed to vary with time and is related to the scalar expansion. The relationship between the proper energy density ρ and string tension density λ are investigated from two different cosmological models.     

Interacting holographic generalized cosmic Chaplygin gas model

In this paper we consider a correspondence between the holographic dark energy density and interacting generalized cosmic Chaplygin gas energy density in flat FRW universe. Then, we reconstruct the potential of the scalar field which describe the generalized cosmic Chaplygin cosmology. In the special case we obtain time-dependent energy density and study cosmological parameters. We find stability condition of this model which is depend on cosmic parameter.     

Inhomogeneous cosmological models in the presence of massless scalar fields

Non-static inhomogeneous cosmological models are obtained in general relativity for the case of a plane symmetric massless scalar field with cosmological constant A,when the source of the gravitational field is a viscous fluid.Some physical and geometrical behaviors of the solutions are also discussed.     

Bianchi type universe and superpotential reconstruction in scalar-tensor cosmology

The nature of scalar field potentials plays a dominant role in the cosmological dynamics of scalar-tensor gravity. The superpotential reconstruction technique is an interesting way to determine the nature of scalar field potentials by taking the Hubble parameter as a function of scalar field. The present study is an application of this technique in the gravitational framework of scalar-tensor gravity using LRS Bianchi type I universe. We explore the nature of scalar field potentials for some particular cases. It is found that the potentials in all cases turn out to be of polynomial nature and the anisotropy parameter m classifies its degree. The graphical behavior of the directional Hubble parameter shows monotonic behavior which is in contrast to the FRW case.     

Modified equations in the theory of induced gravity. Solution to the cosmological constant problem

This research is an extension of the author’s works, in which conformally invariant generalization of string theory was suggested to higher-dimensional objects. Special cases of the proposed theory are Einstein’s theory of gravity and string theory. This work is devoted to the formation of self-consistent equations of the theory of induced gravity in the presence of matter in the form of a perfect fluid that interacts with scalar fields. The study is done to solve these equations for the case of the cosmological model. In this model time-evolving gravitational and cosmological “constants” take place which are determined by the square of scalar fields. The values of which can be matched with the observational data. The equations that describe the theory have solutions that can both match with the solutions of the standard theory of gravity as well as it can differ from it. This is due to the fact that the fundamental “constants” of the theory, such as gravitational and cosmological, can evolve over time and also depend of the coordinates. Thus, in a rather general case the theory describes the two systems (stages): Einstein and “evolving”. This process is similar to the phenomenon of phase transition, where the different phases (Einstein gravity system, but with different constants) transit into each other.