Self-accelerating solutions of scalar-tensor gravity

TL;DR

Scalar-tensor gravity models can produce self-accelerating expansion solutions, including de Sitter-like behavior, without exotic matter, and deviations from general relativity can help distinguish these models from standard dark energy scenarios.

Contribution

The paper demonstrates that scalar-tensor gravity can naturally produce self-accelerating solutions and provides methods to differentiate these from other dark energy models through observational deviations.

Findings

Scalar-tensor models exhibit de Sitter expansion even with negative cosmological constant.

Any expansion history can be represented within scalar-tensor gravity frameworks.

Deviations in post-Newtonian parameters and gravitational coupling variation can distinguish scalar-tensor models from standard cosmology.

Abstract

Scalar-tensor gravity is the simplest and best understood modification of general relativity, consisting of a real scalar field coupled directly to the Ricci scalar curvature. Models of this type have self-accelerating solutions. In an example inspired by string dilaton couplings, scalar-tensor gravity coupled to ordinary matter exhibits a de Sitter type expansion, even in the presence of a {\it negative} cosmological constant whose magnitude exceeds that of the matter density. This unusual behavior does not require phantoms, ghosts or other exotic sources. More generally, we show that any expansion history can be interpreted as arising partly or entirely from scalar-tensor gravity. To distinguish any quintessence or inflation model from its scalar-tensor variants, we use the fact that scalar-tensor models imply deviations of the post-Newtonian parameters of general relativity, and time variation of the Newton's gravitational coupling $G$. We emphasize that next-generation probes of modified GR and the time variation of $G$ are an essential complement to dark energy probes based on luminosity-distance measurements.