Vacuum Effects in a Conformally Flat Brane-Cosmology and Radion Stabilization

Vacuum quantum effects in a conformally coupled scalar field on the background of a conformally flat geometry of brane worlds are studied. Based on the corresponding results for a Minkowski space-time in the general case of mixed boundary conditions on the branes, formulas are derived for the vacuum averaged energy-momentum tensor and for the vacuum forces acting on the boundaries. The important special case of AdS background is examined and applications to the Randall-Sundrum model are discussed. The possible stabilization of the radion by vacuum forces is demonstrated.     

Cosmology of Vacuum

Shortly the vacuum component of the Universe from the geometry point of view and from the point of view of the standard model of physics of elementary particles is discussed. Some arguments are given to the calculated value of the cosmological constant (Zel’dovich’s approximation). A new component of space vacuum (the gravitational vacuum condensate) is involved the production of which has fixed time in our Universe. Also the phenomenon of vacuum selforganization must be included in physical consideration of the Universe evolution.     

LRS Bianchi-V cosmology with decaying vacuum density and heat flow

In this paper we consider a locally-rotationally-symmetric (LRS) Bianchi type-V perfect fluid model with variable cosmological ‘constant’ representing the energy density of vacuum. The field equations are solved with and without heat conduction by using a variation law for the mean Hubble parameter, which is related to the average scale factor of the metric and yields a constant value of the deceleration parameter. A constant value of deceleration parameter generates power-law form of average scale factor which is used to find the exact solutions with and without heat conduction with decaying vacuum density. The solutions presented here satisfy all the necessary conditions for the physically acceptability. The thermodynamical relations in decaying vacuum fluid model are also studied in detail.     

Exact-Brans-Dicke-Friedmann-Robertson-Walker-Bianchi-Kantowski-Sachs vacuum solutions

Using a new method of reduction of the Brans-Dicke vacuum field equations we derive exact vacuum solutions for the class of spatially homogeneous space-times in a unified manner. Exact solutions are given for the isotropic Friedmann-Robertson-Walker models as well as for the anisotropic Bianchi types I-IX and for the related Kantowski-Sachs model.     

Vacuum solution of a quadratic red-shift based correction in f(R) gravity

In the present paper, using action derivative with respect to Ricci scalar and its expansion relative to red-shift, to the second-order and consequently without needing dark energy, the history of cosmos expansion is reconstructed in vacuum. Then, employing supernova data, free parameters of the model which are the expansion coefficients, are calculated. It will be seen that if the free parameters of the model are zero, action of general relativity, which is a boundary mode of the gravity, is found. Also the cosmic age for this model in vacuum is calculated. Finally, it is attempted to reconstruct the reference action in terms of its Taylor expansion. Thus, it will be found that the reconstructed action and their actions definitely pass the Solar system tests.     

Dark matter as an effect of the quantum vacuum

The interaction between the quantum vacuum and a weak gravitational field is calculated for the vacuum fields of quantum electrodynamics. The result shows that the vacuum state is modified by the gravitational field, giving rise to a nonzero interaction energy. This suggests a model that fits in the main properties of the hypothetical dark matter in galactic haloes.     

Plane symmetric vacuum bianchi type I cosmological model in Brans-Dicke theory

The paper presents an exact solution of the vacuum Brans-Dicke field equations for the metric tensor of a spatially homogeneous and anisotropic cosmological model. The Kasner metric is shown as a special case. Some physical properties of the model are discussed.     

The isotropization of the vacuum Bianchi type — V solution

In a recent paper, the general vacuum Bianchi type-V solution in Brans-Dicke theory was presented and the isotropization of the model was discussed. It is pointed out that this solution was derived previously. Some remarks are made about the isotropization of the model.     

Einstein-Rosen Universe in the Scale-Covariant Theory of Gravitation

Field equations in the presence of a perfect fluid distributionc for Einstein-Rosen cylindrically symmetric metric are obtained in the scale-covariant theory of gravitation proposed by Canuto et al. (1977a). A static vacuum model and a non-static cosmological model corresponding to perfect fluid are presented. Physical and Kinematical properties of the models are also discussed.     

Einstein-Gleichungen mit Zusatztermen 4. Ordnung und Raum-Zeit-Modelle vom Bianchi-Typ I

In the vacuum case, Einstein's equations generalized by additive terms containing derivatives of the metric up to the 4th order are applied to cosmological Bianchi-type I model space-times.     

Non-existence of higher dimensional anisotropic cosmological model in bimetric theory of gravitation

A Five dimensional spherically symmetric space-time model in Rosen's (1973) bimetric theory of gravitation is considered. It is shown that, in this theory, the geometry of the five dimensional space-time does not admit perfect fluid source. Hence a vacuum model is presented and studied. It is observed that in the case of stiff fluid we again get the vacuum model. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stability of a compressible fluid cylinder ambient with a bounded conducting medium under general varying magnetic fields

The stability of a compressible fluid cylinder pervaded by a longitudinal uniform magnetic field-ambient with a bounded conducting medium of negligible inertia penetrated with general varying vacuum magnetic fields has been developed. The stability criterion describing the stability characteristics of that model is derived and discussed analytically in general terms. The axial fields have always stabilizing influences. The azimuthal vacuum field has a destabilizing effect, however, it becomes minimal if the perturbed and the unperturbed vacuum fields are not orthogonal. The magnetodynamic instability of the fluid jet is modified in the presence of the fluid compressibility. The stabilizing influence due to the latter may be realized more clearly on utilizing the numerical methods for investigating the eigenvalue relation.Several reported works can be recovered as limiting cases with appropriate simplifications.     

What is the most simple solution of Wheeler-DeWitt equation?

The Wheeler-DeWitt equation in Vilenkin model is solved via the ansatz approach when all terms related to vacuum, domain walls, strings, dust, relativistic matter, bosons and fermions and ultra stiff matter are present.     

Bianchi type-VI0 vacuum model in the general scalar-tensor theory of gravitation

The spatially-homogeneous and anisotropic Bianchi type-VI₀ cosmological model is obtained in the Nordtvedt-Barker general scalar-tensor theories of gravitation. Exact solutions of the Nordtvedt-Barker field equations in vacuum are presented and studied.     

Bianchi type-VI0 models in self-creation cosmology

Spatially-homogenous and anisotropic Bianchi type-VI₀ cosmological models are obtained, in Barber's second self-creation theory of gravitation, both in vacuum and in the presence of perfect fluid with pressure equal to energy density. Some properties of the model are discussed.     

Einstein–Rosen Universe in a Scalar-Tensor Theory of Gravitation

Field equations in the presence of a perfect fluid distribution are obtained in a scalar-tensor theory of gravitation proposed by Saez and Ballester (Phys. Lett. 113, 1985, 467) with the aid of Einstein–Rosen cylindrically symmetric metric. A static vacuum model and a non-static stiff fluid model are presented. The physical and geometrical properties of the stiff fluid model are studied.     

A static conformally-flat vacuum model in self-creation cosmology

Vacuum field equations for the conformally-flat spherically-symmetric static space-time are obtained in a self-creation theory of gravitation proposed by Barber. It is shown that the most general conformally-flat spherically-symmetric static vacuum model in this theory represents only purely flat space-time.     

Non-Existence of Five Dimensional Perfect Fluid Cosmological Model in Lyra Manifold

In this paper we have taken an attempt to construct a five dimensional perfect fluid cosmological model within the framework of Lyra manifold. It is found that neither perfect fluid nor dust distributions survive. Finally the exact solutions of the vacuum field equations are obtained.     

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.     

Bianchi Type-1 vacuum models in modified theory of general relativity

In this paper, the problem of spatially homogeneous and an isotropic Bianchi type-1 space time with perfect fluid distribution is considered in Barber's second theory of gravitation. To obtain determinate solutions, we have assumed the equation of statep= γρ, 0 ≤ γ ≤ 1. It is observed that the general fluid distribution degenerates isotropic vacuum model whenγ = 1 and Λ < 0. Further it is observed that the vacuum model obtained in case of γ = 0, ρ = 0 andΛ = 0, reduces to well known Kasner model in Einstein's theory. Some physical and geometrical aspects of the models together with singularities in the models are also discussed This revised version was published online in July 2006 with corrections to the Cover Date.