Scalar-tensor cosmologies without screening

TL;DR

This paper identifies specific scalar-tensor theories within Horndeski models that do not require screening mechanisms to satisfy observational constraints, and explores their cosmological implications.

Contribution

It characterizes subsets of Horndeski theories that naturally avoid the need for screening mechanisms and examines their compatibility with observational bounds.

Findings

Certain Horndeski theories do not need screening mechanisms.

Restrictions from gravitational wave speed and solar system tests limit these theories.

Some theories recover standard $ ext{ΛCDM}$ predictions in the quasi-static limit.

Abstract

Scalar-tensor theories are frequently only consistent with fifth force constraints in the presence of a screening mechanism, namely in order to suppress an otherwise unacceptably large coupling between the scalar and ordinary matter. Here we investigate precisely which subsets of Horndeski theories do not give rise to and/or require such a screening mechanism. We investigate these subsets in detail, deriving their form and discussing how they are restricted upon imposing additional bounds from the speed of gravitational waves, solar system tests and cosmological observables. Finally, we also identify what subsets of scalar-tensor theories precisely recover the predictions of standard (linearised) $\Lambda\text{CDM}$ cosmologies in the quasi-static limit.