EXTENDED INFLATION IN SCALAR TENSOR THEORIES OF GRAVITY

Extended inflation based on a pure Brans-Dicke theory of gravity with constant parameter ω (ω>30 in order to avoid unacceptable distortions of the microwave background radiation) is ruled out by light-deflection and time-delay experiments (ω>500). We propose a dynamical mechanism to solve this problem, based on a general scalar-tensor theory of gravity with a function ω(f{cyrillic}) having a singularity at f{cyrillic}=f{cyrillic}c. We analyze a particular class of models, with a Brans-Dicke-like behaviour during inflation, which converge continuously to general relativity during the radiation- and matter-dominated eras. Solving numerically their equations of motion we find a subclass of models for which ω<30 at the end of inflation and ω>500 today, thus in agreement with all post-newtonian experiments. © 1990.