THINKING ABOUT THE UNIVERSE

This article is largely a review and critical analysis of earlier pape rs by the author, and, within the broad background of the literature o n this subject, it discusses the problems in the early stages of evolu tion of the Universe close to the singularity point. Special attention is paid to analyzing the properties of the de Sitter and Friedmann un iverses, and discussing the physical conditions which could induce a t ransition from one type of universe to another. Various modifications of the usual Einstein equations are presented which could serve as the dynamical basis of such a transition. A version of gravity in which t he gravitational constant is replaced by a function of the matter (ene rgy) density approaching zero as the density increases to its maximum value is discussed in detail. The possible nontrivial role of black ho les during the inflationary stage of development of the Universe and a lso (for closed universes) during the final collapse stage is discusse d. The possibility that the physical laws change in regions of space o f size close to the Planck length, which could lead to the existence o f a special physics of the ultra-microscopic world, is especially emph asized. (C) 1998 American Institute of Physics.