Cosmological mass model in general relativity

Relativistic cosmological field equations are obtained for a non-static stationary Bertotti-Robinson-type space-time for interacting perfect fluid and electromagnetic field. The cosmological solution to the field equations are obtained and the nature of the electromagnetic field as well the perfect fluid are studied. The electromagnetic field generated here corresponds to a special generic case and the perfect fluid distribution degenerates into a barotropic perfect fluid with equation of statep+=0, >0. It is shown here that the interacting barotropic fluid can generate gravitation only when the cosmological constant being a function ofx in a dynamic field.     

Spherically symmetric electromagnetic mass models with cosmological parameter ⋀

An exact solution has been obtained for the Einstein-Maxwell field equation with corresponding to a spherically-symmetric charged perfect fluid distribution. Here the cosmological constant is assumed to be a scalar variable depending on the radial coordinater of the spherical system, viz., = (r). The solution set thus obtained presents an electromagnetic mass model.     

Bulk viscous cosmological model

Robertson—Walker cosmological models with bulkviscosity are investigated explicitly with equation of statep=(-1). In particular, the physical nature of the extreme cases, i.e., degenerate vacuum bulkviscous fluid model and bulkviscous stiff fluid model are studied in detail.     

Zatrikean pregeometry: simple cosmological scenarios

In this paper basic cosmological consequences of zatrikean pregeometry are studied. It is shown that many universal quantities can be calculated with simple applications of this theory. An equation directly linking the angular velocity of the universe, the Hubble constantH and the mean density of the universe is derived. The relation between the massM and the radiusR of the universe is examined. This relation leads to various cosmological scenarios, including variations in physical constants and/or violation of the mass conservation and/or variable geometrical properties.     

Exact solution to radiation-filled Brans-Dicke closed space cosmology

Under the assumption of a power law (k·R ⁿ=C,C=const.) between the gravitational constantk and the radius of curvatureR of the Universe and forP=1/3 the exact solution is sought for the cosmological equations of Brans and Dicke. The solution turns out to be valid for closed space and the parameter of the scalar-tensor theory is necessarily negative. The radius of curvature increases linearly with respect to the age of the Universe while the gravitational constant grows with the square of the radius of curvature. It has been shown (Lessner, 1974) that in this case (KR ²) the spatial component of the field equations is independent of the remaining equations. However, our solution satisfies this independent equation. This solution for the radiation-dominated era corresponds to the solution for the matter-dominated era found by Dehnen and one of the authors (Dehnen and Obregón, 1971). Our solution, as is the solution previously obtained for the matter-dominated era, is in contradiction to Dirac's hypothesis in which the gravitational constant should decrease with time in an expanding Universe.     

A class of cosmological solutions of Brans-Dicke theory with cosmological constant

We present a class of exact cosmological solutions of Brans-Dicke (B-D) equations with cosmological constant in flat Robertson-Walker metric. These solutions are based on the relation øR ⁿ= constant between the B-D field and the scale factor of the universe. This relation turns out to be consistent with the equation of statep =m for the cosmic matter, provided thatn andm are suitably related to each other. Several special cases and asymptotic solutions are derived and discussed.     

Exact solutions of a flat cosmological model in BSTT

We solve the cosmological equations for the bimetric scalar—tensor theory of gravitation (BSTT) for a flat model of the Friedmann type with the equation of state p = a. In the initial stage of expansion, the energy density of the scalar field dominates over the energy density of matter. As a result, the behavior of the solution in this limit does not depend on a. For later stages of expansion of the Universe, the solution obtained goes to a special solution having the form of a power law function of time. In this case, the relative change in the gravitational scalar is proportional to the Hubble parameter. In the limit of large values for the parameter of the theory, only a simple solution with zero value of the constant of integration goes to the corresponding Friedmann solution of general relativity theory.Translated from Astrofizika, Vol. 37, No. 2, pp. 351–362, April–June, 1994.I would like to thank L. Sh. Grigoryan for valuable discussions and support.     

A note on variable-G cosmologies

An example of a cosmological model with variable gravitational couplingG and a time-dependent cosmological term , has recently been presented. It has been shown that there is no creation of matter and that the rest mass of particles stays constant in this model. In this paper we will generalize the field equations to the case where bothG and depend both on time and position. It is shown that even in this case there may be no creation.     

On verification of the asymptotic model of the first kind

For the case of closed Friedmann models with the cosmological constant, an N - dependence is plotted, whereN is the conformal time of existence of the Universe, and is the ratio of the cosmological constant to its Einstein's value. The observational properties of the asymptotic model of the first kind (the so-called Al model) corresponding to the maximum of this dependence are analyzed. For the matter-dominated models, the maximum is achieved at the point = 1. Formulae for the calculation of the age of the Universe and for that of the photometric distance in the Al model are deduced. The observational properties of the A1 model are compared to the corresponding properties of the standard cosmological model ( = 0) which does not occupy any special distinguished position on the N - diagram. It is shown that from the standpoint of the modern observational cosmology these two models cannot be told one from another. However, the A1 model has better standing from the viewpoint of the strong wordings of the anthropic cosmological principle.     

Bianchi Type I String Cosmological Models with Bulk Viscosity and Magnetic Field

Some locally rotationally symmetric (LRS) Bianchi type I cosmological models for a cloud string with bulk viscosity and magnetic field are presented. Where an equation of state ρ = kλ and a relation between metric potential R = AS ⁿ are considered. The solution describes a shearing and nonrotating model with a big bang start. In the absence of magnetic field it reduces to a string model with bulk viscosity, where the relation between the coefficient of bulk viscosity and energy density is ζ ∝ ρ^1/2. After choosing k = , it further reduces to a string model without viscosity and magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.     

A self-consistent world model

The field equations of the generalized field theory constructed by Mikhail and Wanas have been applied to a well-established geometrical structure given earlier by H. P. Robertson in connection with the cosmological problem. A unique solution, representing a specified expanding Universe (withq ₀=0, ₀=0.75,k=–1) has been obtained. The model obtained has been compared with cosmological observations and with FRW-models of relativistic cosmology. It has been shown that the suggested model is free of particle horizons. The existence of singularities has been discussed.The two cases, when the associated Riemannian-space has a definite or indefinite metric are considered. The case of indefinite metric with signature (+ – – –) is found to be characterized byk=–1, while the case of +ve definite metric is characterized byk=+1. Apart from that difference, the two cases give rise to the same cosmological parameters. It has been shown that energy conditions are satisfied by the material contents in both cases.     

Cosmological solution in wesson's 5D theory of gravity

Cosmological solutions that obey the perfect fluid equation of state in Wesson's gravitational theory are presented assuming that the fifth dimension subspace is also homogeneous and isotropic like the usual homogeneous isotropic cosmological model. It is shown that the scale factor scales as for the vanishing fifth component of the energy momentum tensor. The role of the fifth component as a cosmological constant is remarked, and an inflationary model is thus obtained.     

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.     

Exact cosmological solutions in the scalar-tensor theory with cosmological constant

Under the assumption of a power-law between the expansion factor of the Universe, and the scalar field (a ⁿ=c=const.) tensor theory with cosmological constant are reduced to quadrature. Several exact solutions are obtained, among them inflationary universes that have barotropic equation of state.     

From inflation to Friedmann regime inR 2 cosmology

A cosmological model describing the different stages of the universe, i.e.: inflation, radiation dominated period and matter dominated (Friedmann-like) period is shown. The model consists of the usual gravitational lagrangian with a R ² term, and, for the matter content, the lagrangian of a massive conformally coupled scalar field. The effect of backreaction is evaluated by means of an extremum condition on the entropy at each time. The differential equation, obtained when the lowest quantum order is considered, describes all the periods of evolution of the universe. For a range of values of, inflation is unstable and the universe can reach the following regime.     

A viable dark fluid model

We consider a cosmological model based on a generalization of the equation of state proposed by Nojiri and Odintsov (2004) and ?tefan?i? (2005, 2006). We argue that this model works as a dark fluid model which can interpolate between dust equation of state and the dark energy equation of state. We show how the asymptotic behavior of the equation of state constrained the parameters of the model. The causality condition for the model is also studied to constrain the parameters and the fixed points are tested to determine different solution classes. Observations of Hubble diagram of SNe Ia supernovae are used to further constrain the model. We present an exact solution of the model and calculate the luminosity distance and the energy density evolution. We also calculate the deceleration parameter to test the state of the universe expansion.     

Slowly rotating cosmological viscous fluid Universe

The perturbation by a spherical rotating shell is investigated in a homogeneous and isotropic cosmological model of viscous fluid distribution to first order in angular velocity (r, t) of matter and the metric rotation function (r, t) which is uniform and non-uniform the exact solutions for (r, t) are obtained for all cosmological models. The physical properties of these solutions are discussed.     

A flat cosmological model in the bimetric scalar-tensor theory: An approach based on the qualitative theory of dynamical systems

We apply the methods of the qualitative theory of dynamical systems to study a flat cosmological model of the bimetric scalar-tensor theory (BSTT) of gravitation with the equation of state p=a, -⊃a1. We single out the basic classes of solutions and study their behavior in different limiting cases. We show that the properties of the model depend critically on the value of the dimensionless parameter of the theory. We find the bifurcation lines of the corresponding dynamical system and study the behavior of the phase trajectories in a neighborhood of these lines.Translated fromAstrofizika, Vol. 37, No. 3, 1994.The author is grateful to the participants in the seminar of the Department of Theoretical Physics at Erevan State University for discussion of the results of this paper.     

A study on Bianchi-IX cosmological model

In this paper the inflationary solutions are studied for the Bianchi-IX space-time in presence of a massless scalar field with a flat potential. Also a class of cosmological solutions of massive strings are obtained following the techniques used by Letelier and Stachel. Some solutions are calculated for pure massive strings following the Takabayashi equation of state =(1+w).     

Once more on quasar periodicities

In an earlier paper Arp, Bi, Chu and Zhu investigated various quasar samples and recognized a periodicity of remarkable degree in them. Their conclusion was that the existence of such a periodicity is a counterevidence against the cosmological origin of quasar redshifts. Since this question is of great importance in a cosmological context, here we reanalyze some of the samples together with a galaxy sample of the pencil-beam survey of Broadhurstet al. Our result is that on the plane of cosmological parameters (₀, ₀) there is a non-negligible region where two quasar samples and the galaxy sample are simultaneously fairly periodic. Pure periodicity is still compatible with cosmological principles, at least on length scales much longer than the period length. So the regularities can fit into a cosmological context.