f(R) Gravities à la Brans-Dicke

Abstract

We extend f(R) theories via the addition of a fundamental scalar field. The approach is reminiscent of the dilaton field of string theory and the Brans-Dicke model. f(R) theories attracted much attention recently in view of their potential to explain the acceleration of the universe. Extending f(R) models to theories with scalars can be motivated from the low energy effective action of string theory. There, a fundamental scalar (the dilaton), has a non-minimal coupling to the Ricci scalar. Furthermore beyond tree level actions will contain terms having higher (or lower) powers of R compared to the canonical Einstein-Hilbert term. Theories with f(R) will contain an extra scalar degree on top of the ad-hoc dilaton and mixing of these two modes around a stable solution is a concern. In this work we show that no mixing condition mandates the form $V_{1}(ϕ)f(R)+V_{2}(ϕ)R^{2}$ for the action.