Non-minimal scalar-tensor theories

TL;DR

This paper analyzes non-minimal scalar-tensor theories, revealing their spectrum includes two massive scalar modes and a massless graviton, and discusses their dynamics and interactions in the Einstein frame.

Contribution

It provides a detailed spectrum analysis of scalar-tensor theories with general couplings, including the identification of degrees of freedom and their behavior in the Einstein frame.

Findings

Spectrum includes two massive scalar modes and one massless graviton.

The Einstein frame allows clear separation of graviton dynamics.

Interaction structure can be anticipated by geometrical considerations.

Abstract

We compute the spectrum of scalar models with a general coupling to the scalar curvature. We find that the perturbative states of these theories are given by two massive spin-0 modes in addition to one massless spin-2 state. This latter mode can be identified with the standard graviton field. Indeed, we are able to define an Einstein frame, where the dynamics of the massless spin-2 graviton is the one associated with the Einstein-Hilbert action. We also explore the interactions of all these degrees of freedom in the mentioned frame, since part of the coupling structure can be anticipated by geometrical arguments.