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.     

A Cosmological Model with a Negative Constant Deceleration Parameter in Scale-Covariant Theory of Gravitation

An axially symmetric Bianchi type-I space-time is considered in the presence of perfect fluid source in the scale-covariant theory of gravitation formulated by Canuto et al. [1977a, Phys. Rev. Lett. 39, 429]. With the help of special law of variation for Hubble’s parameter proposed by Bermann [1983, Nuovo Cimento 74B, 182] a cosmological model with a negative constant declaration parameter is obtained in this theory. Some physical properties of the model are also discussed.     

Axially Symmetric String Cosmological Model In Brans-Dicke Theory of Gravitation

By adopting the co-moving coordinate system, an exact axially symmetric cosmological model with string dust cloud source is obtained in the framework of Brans-Dicke [Phys. Rev. 124, 925, (1961) Scalar – tensor theory of gravitation. Some physical and kinematical properties of the model are also discussed.     

String cosmology with Brans-Dicke theory

In this paper string cosmology has been developed in the presence of Brans-Dicke scalar field coupled to Einstein gravity. Solutions are obtained for both geometric andp-string models and physical situations are discussed.     

Cosmological Evolution of String Effective Gravitation with a Dilaton Potential and Higher-Loop Corrections. I

Cosmological evolution is investigated within the framework of low-energy string gravitation with higher-loop corrections to the dilaton coupling functions in the presence of a dilaton potential and a nongravitational source. It is shown that for homogeneous and isotropic models with a flat space, the cosmological system of equations reduces to an autonomous, third-order, dynamical system. Subclasses of models with a constant dilaton, which provide the basis for various cosmological mechanisms of dilaton stabilization, are considered. A class of solutions is distinguished with asymptotic scaling behavior of the energy density of the dilaton field.     

A String cosmological model in Brans-Dicke theory of gravitation

A spatially homogeneous and anisotropic exact Bianchi type-I string cosmological model is investigated in Brans-Dicke (1961) scalar-tensor theory of gravitation. Some properties of the model are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

String theory, cosmology and varying constants

In string theory the coupling `constants' appearing in the low-energy effective Lagrangian are determined by the vacuum expectation values of some (a priori) mass less scalar fields (dilaton, moduli). This naturally leads one to expect a correlated variation of all the coupling constants, and an associated violation of the equivalence principle. We review some string-inspired theoretical models which incorporate such a space time variation of coupling constants while remaining naturally compatible both with phenomenological constraints coming from geochemical data (Oklo; Rhenium decay) and with present equivalence principle tests. Barring a very unnatural fine-tuning of parameters, a variation of the fine-structure constant as large as that recently `observed' by Webb et al. in quasar absorption spectra appears to be incompatible with these phenomenological constraints. Independently of any model, it is emphasized that the best experimental probe of varying constants are high-precision tests of the universality of free fall, such as MICROSCOPE and STEP. This revised version was published online in July 2006 with corrections to the Cover Date.     

String cosmological models in five dimensional bimetric theory of gravitation

Five-dimensional spherically symmetric space-time is considered in bimetric theory of gravitation formulated by Rosen (Gen. Rel. Grav. 4, 435, 1973) in the presence of cosmic string dust cloud. Exact cosmological models which represent geometric (Nambu) string, p-string (Takabayasi string) and Reddy string (Astrophys. Space Sci. 301, 2006) are obtained in the static and non-static cases. Some physical properties of the models are also discussed.     

Higher Dimensional FRW Cosmological Models in Self-Creation Theory

The field equations of Barber's (1982) second self-creation theory of gravitation are solved for 5D Friedmann-Robertson-Walker space time using perfect fluid energy momentum tensor. By assuming an equation of state p= ε ρ, (0 ≤ ε ≤ 1), the solutions of the field equations, in different scenarios, in Barber's second self-creation theory are presented and discussed. Some properties of these models are also discussed.     

Inflationary Solutions for Higher Dimensional Anisotropic Cosmological Model in Scalar-Tensor Theory II

This is the second paper of the series where we have considered Brans-Dicke (B-D) theory as well as general scalar tensor theory of gravitation in higher dimensional space-time model in the false vacuum state. We have examined whether inflationary solutions are possible both for constant or variable coupling parameter ω. Also the nature of the scalar field and the coupling parameter are discussed in the asymptotic limit. This revised version was published online in July 2006 with corrections to the Cover Date.     

Some Cosmological Models in Scalar-Tensor Theory of Gravitation

Field equations in a scalar-tensor theory of gravitation proposed by Saezand Ballester (1985) are obtained with the aid of (i) Friedmann-type metric (ii) a non static plane symmetric metric and (iii) spatially homogeneous Bianchi type – III metric. Some cosmological models corresponding to perfect fluid and bulk viscous fluid are presented. Physical and kinematical properties of the models are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stationary axisymmetric fields in Generalized Theory of Gravitation

The problem of stationary axisymmetric gravitational fields is formulated within the framework of Generalized Theory of Gravitation. It is shown that solutions of the problem mentioned above may be found, if analogous solutions in General Relativity are obtained. As an illustration a Kerr-like solution is offered. A generation theorem for finding magnetostatic solution from stationary vacuum solutions is proposed.     

Cosmological Post-Newtonian Approximation in Flat Space-Time Theory of Gravitation

The 1-post-Newtonian approximation of perfect fluid in cosmological models of the theory of gravitation in flat space-time is studied. The equations of motion are given in evolution form. At high redshifts the terms of post-Newtonian approximation are important in studying the development of inhomogeneities on scales smaller than galaxies but at present time these terms only take effect on very large scales. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gravitation and inertia; a rearrangement of vacuum in gravity

We address gravitation and inertia in the framework of a general gauge principle (GGP) which accounts for the gravitation gauge group G _R generated by a hidden local internal symmetry implemented on the flat space. Following the method of phenomenological Lagrangians, we connect the group G _R to a non-linear realization of the Lie group of the distortion G _D of the local internal properties of six-dimensional flat space, M ₆, which is assumed as a toy model underlying four-dimensional Minkowski space. We study the geometrical structure of the space of parameters and derive the Maurer–Cartan’s structure equations. We treat distortion fields as Goldstone fields, to which the metric and connection are related, and we infer the group invariants and calculate the conserved currents. The agreement between the proposed gravitational theory and available observational verifications is satisfactory. Unlike the GR, this theory is free of fictitious forces, which prompts us to address separately the inertia from a novel view point. We construct a relativistic field theory of inertia, which treats inertia as a distortion of local internal properties of flat space M ₂ conducted under the distortion inertial fields. We derive the relativistic law of inertia (RLI) and calculate the inertial force acting on the photon in a gravitating system. In spite of the totally different and independent physical sources of gravitation and inertia, the RLI furnishes a justification for the introduction of the Principle of Equivalence. Particular attention is given to the realization of the group G _R by the hidden local internal symmetry of the abelian group U ^loc=U(1) _Y ×diag[SU(2)], implemented on the space M ₆. This group has two generators, the third component T ³ of isospin and the hypercharge Y, implying Q ^d =T ³+Y/2, where Q ^d is the distortion charge operator assigning the number −1 to particles, but +1 to anti-particles. This entails two neutral gauge bosons that coupled to T ³ and Y. We address the rearrangement of the vacuum state in gravity resulting from these ideas. The neutral complex Higgs scalar breaks the vacuum symmetry leaving the gravitation subgroup intact. The resulting massive distortion field component may cause an additional change of properties of the spacetime continuum at huge energies above the threshold value.     

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.     

String membranes in higher dimensions

We shall study homogeneous and isotropic clouds formed of String membranes. These membranes are considered in extra dimensions of a higher dimensional Bianchi-I space-time. Some solutions are found for the field equations of the system.