Showing papers on "Friedmann–Lemaître–Robertson–Walker metric published in 1983"

The stability of general relativistic cosmological theory

Abstract: The authors analyse the behaviour of homogeneous and isotropic solutions to a gravity theory that arises from the variation of an arbitrary analytic function of the space-time scalar curvature. Such a theory generalises Einstein's general relativity wherein this function is linear in the curvature. They prove conditions for the existence and stability of the general relativistic de Sitter and Friedmann solutions within the general theory, prove necessary and sufficient conditions for the existence of cosmological singularities and particle horizons and analyse the asymptotic behaviour of ever-expanding Universe models. The conditions under which Minkowski space-time and Schwarzschild space-time are stable is investigated and their instability, together with the pathological behaviour of certain cosmological models, traced back to the non-minimality of the stationary action giving rise to the field equations. The significance of these results for quantum theories of gravity and the 'inflationary' model of the early Universe is discussed.

Isotropization of arbitrary cosmological expansion given an effective cosmological constant

Abstract: It is shown that given an effective cosmological constant it is possible to construct the asymptotic structure of inhomogeneous cosmological expansion. This structure would contain the maximum possible number of arbitrary functions of three coordinates and would describe exponentially rapid local isotropization of the universe.

On the global geometry of the Stephani universe

Abstract: A preliminary investigation of global properties of the Stephani solution of the Einstein field equations is presented. This solution generalizes those of Friedman-Robertson-Walker (FRW) in such a way that the spatial curvature indexk (a constant in the FRW models) is a function of the time coordinate. The de Sitter solution, which is also a special case of the Stephani solution, is analyzed in the Stephani coordinates to gain insight into the global structure of the manifold and its foliation. The general metric is found to have several properties in common with this example. It has singularities which can be avoided either by matching the solution to an (as yet unknown) empty-space solution or confining the curvature index to be positive at all times.

Zero-Curvature Friedmann-Robertson-Walker Models as Exact Viscous Magnetohydrodynamic Cosmologies

Abstract: It is shown that FRW cosmologies, in particular the zero-curvature models,do not necessarily represent perfect fluid solutions but also can be exact solutions of the field equations for a viscous fluid, with or without an electromagnetic field, and that these solutions can be physically acceptable. The viscous, magnetohydrodynamic model with metric identical to that of the perfect fluid model with equation of state p-bar = ..gamma..rho-bar is investigated and particular attention is given to the Einstein-de Sitter model which corresponds to ..gamma.. = 0. The behavior of the expansion of the velocity congruence in the viscous interpretation is quite different from that of the standard model, which suggests that the usual treatment of the critical density problem may be questionable.

General solutions for a cosmological Robertsonwalker metric in the Brans-Dicke theory

Abstract: The general vacuum solutions of the Brans-Dicke theory in a cosmological Robertson-Walker-type metric are explicitly given. Several families of solutions have properties which essentially differ from the conventional Einstein theory. The geometry is not uniquely determined by the equations of motion, raising doubts about the “Machian” character of the Brans-Dicke theory. The role of the cosmological constant is emphasized, in agreement with modern ideas of fundamental particle interactions.

Some Comments on a Stability of the ˜S2 ⊗ S2-Type Space-Time

Abstract: In terms of the Abbott-Deser energy method, the stability for a small perturbation of N ariai's 5, ® s,. type solution of the vacuum Einstein equations with non-vanishing cosmological constant is studied. It is shown that the space-time is stable for some perturbations which arise only inside its event horizon. Recently, an effective cosmological term (A­ term) has been considered by many author·s in relation to the phase transition in an early stage of the big-bang universe. The de Sitter space­ time is famous as an exact solution of the vacuum Einstein equations with a positive cosmological constant A:

On the spectrum of particles created in a Robertson-Walker universe

Abstract: Created particle spectra are calculated in Robertson-Walker universes with scale factora∝tα(0 <α≦ 1) anda∝et, and discussed with a special emphasis on their dependence upon the initial and final times at which a WKB-like positive frequency condition should be imposed. It is shown that the obtained spectra are very sensitive to these times if the WKB approximation for the field equation is not valid in their neighborhood. It is also shown that the total number density of created particles remains finite if the final time is set to be finite.

Modified Robertson-Walker metric and matter-antimatter asymmetry

Abstract: A modified Robertson-Walker metric is used to obtain solutions for matter density in the time interval −∞