Generalized Brans-Dicke theories

TL;DR

This paper explores generalized scalar-tensor theories extending Brans-Dicke models, demonstrating conditions for stable de Sitter solutions and analyzing their cosmological dynamics without potential terms.

Contribution

It introduces a broad class of modified gravity theories with stable de Sitter solutions, extending Brans-Dicke models without relying on scalar field potentials.

Findings

Stable de Sitter solutions are achievable in generalized theories.

Parameter regions avoiding ghosts and instabilities are identified.

Distinct evolution of field propagation speed differentiates these models from LCDM.

Abstract

In Brans-Dicke theory a non-linear self interaction of a scalar field allows a possibility of realizing the late-time cosmic acceleration, while recovering the General Relativistic behavior at early cosmological epochs. We extend this to more general modified gravitational theories in which a de Sitter solution for dark energy exists without using a field potential. We derive a condition for the stability of the de Sitter point and study the background cosmological dynamics of such theories. We also restrict the allowed region of model parameters from the demand for the avoidance of ghosts and instabilities. A peculiar evolution of the field propagation speed allows us to distinguish those theories from the LCDM model.