Remarks on (super-)accelerating cosmological models in general scalar-tensor gravity

TL;DR

This paper analyzes the phase space and acceleration conditions in general scalar-tensor gravity cosmological models with arbitrary coupling functions, focusing on flat models with dust matter and scalar fields.

Contribution

It provides a comprehensive phase space analysis and necessary conditions for acceleration in scalar-tensor cosmologies with arbitrary coupling functions.

Findings

Necessary condition on coupling function for acceleration

Phase space properties of scalar-tensor models

Example illustrating acceleration conditions

Abstract

We consider Friedmann-Lemaitre-Robertson-Walker cosmological models in the framework of general scalar-tensor theories of gravity (STG) with arbitrary coupling functions, set in the Jordan frame. First we describe the general properties of the phase space in the case of barotropic matter fluid and scalar field potential for any spatial curvature (flat, spherical, hyperbolic). Then we address the question under which conditions epochs of accelerated and super-accelerated expansion are possible in STG. For flat models filled with dust matter (and vanishing potential) we give a necessary condition on the coupling function of the scalar field which must be satisfied to allow acceleration and super-acceleration. This is illustrated by a specific example.