Extended Scalar-Tensor Theories of Gravity

TL;DR

This paper classifies and analyzes new scalar-tensor gravity theories that avoid instabilities, explores their relation to Horndeski theories, and discusses their implications for dark energy modeling and future cosmological tests.

Contribution

It provides a comprehensive classification of quadratic scalar-tensor theories with constraints to prevent extra modes and links them to effective field theories of dark energy.

Findings

Identified conditions for primary constraints in scalar-tensor theories.

Classified the most general quadratic theories consistent with these constraints.

Connected these theories to operators in the effective field theory of dark energy.

Abstract

We study new consistent scalar-tensor theories of gravity recently introduced by Langlois and Noui with potentially interesting cosmological applications. We derive the conditions for the existence of a primary constraint that prevents the propagation of an additional dangerous mode associated with higher order equations of motion. We then classify the most general, consistent scalar-tensor theories that are at most quadratic in the second derivatives of the scalar field. In addition, we investigate the possible connection between these theories and (beyond) Horndeski through conformal and disformal transformations. Finally, we point out that these theories can be associated with new operators in the effective field theory of dark energy, which might open up new possibilities to test dark energy models in future surveys.