How typical is General Relativity in Brans-Dicke chaotic inflation?

Abstract

General Relativity is recovered from Brans-Dicke gravity in the limit of large $\omega$. In this article we investigate theories of Brans-Dicke gravity with chaotic inflation, allowing for either a constant or variable value of $\omega$, known as extended and hyperextended inflation respectively. The main focus of the paper is placed on the latter. The variation $\omega$ with respect to the Brans-Dicke field is based on higher-order corrections analogous to those of the dilaton field in string theory, following the simple principle that the Brans-Dicke and metric fields decouple asymptotically. The question addressed is whether a large value of $\omega$ is predominant in most regions of the universe, which would lead to the conclusion that a typical region is then governed by General Relativity. In these theories we find that it is possible to construct inflaton potentials that drive the evolution of the Brans-Dicke field to an appropriate range of values at the end of inflation, such that a large $\omega$ is indeed typical in an average region of the universe. However, in general this conclusion does not hold and it is shown that for a wide class of inflaton potentials General Relativity is not a priori a typical theory.