Scalar-tensor theories with an external scalar

TL;DR

This paper explores scalar-tensor gravity theories with an external scalar field, demonstrating their relation to GR and ST theories, and proposing external fields as potential screening mechanisms.

Contribution

It introduces External Scalar-Tensor (EST) gravity, showing it encompasses GR and ST theories, and presents a cosmological example of solutions that combine GR and ST features.

Findings

EST gravity generalizes GR and ST theories.

Solutions can smoothly join GR and ST regimes.

External fields may act as screening mechanisms.

Abstract

Scalar-tensor (ST) gravity is considered in the case where the scalar is an external field. We show that General Relativity (GR) and usual ST gravity are particular cases of the External Scalar-Tensor (EST) gravity. It is shown with a particular cosmological example that it is possible to join a part of a GR solution to a part of a ST one such that the complete solution neither belongs to GR nor to ST, but fully satisfies the EST field equations. We argue that external fields may effectively work as a type of screening mechanism for ST theories.