String-driven inflationary and deflationary cosmological models

Abstract

A detailed study is made of inflationary universe driven by the quantum production of infinitely thin Witten strings on super-horizon scales in the very early universe. We show that these models are equivalent to the addition of an effective bulk viscosity to perfect fluid cosmological models with zero curvature. A large number of new exact solutions describing the effects of string creation upon isotropic universes are found. They exhibit either inflationary expansion to a de Sitter state from an initial Friedmann singularity or deflationary approach to the standard Friedmann universe from a de Sitter state at t= - ∞. We show that the deflationary models, which possess an initial string-dominated de Sitter phase with a maximum string temperature given by the Gibbons-Hawking temperature of the de Sitter event horizon, cease to exist in the presence of expansion anisotropies or the presence of further matter fields uncoupled from the string density. In both cases the initial string-dominated de Sitter era is replaced by an initial singularity at which the string density vanishes. We show that application of the generalized second law of black hole thermodynamics to the event horizons of the string-dominated models excludes some evolutionary behaviours. We give examples of globally static de Sitter universes which are not in local thermal equilibrium.