Healthy theories beyond Horndeski

TL;DR

This paper introduces a new class of scalar-tensor theories extending Horndeski models, which maintain second-order equations of motion and exhibit unique phenomenology, such as influencing matter sound speed.

Contribution

It presents a novel class of theories that extend Horndeski models while avoiding Ostrogradski instabilities and demonstrates their unique phenomenological implications.

Findings

Theories possess higher-order derivatives but have second-order propagating degrees of freedom.

Covariant galileon Lagrangians are part of this new class.

Scalar field affects matter sound speed even with minimal coupling.

Abstract

We introduce a new class of scalar-tensor theories that extend Horndeski, or "generalized galileon", models. Despite possessing equations of motion of higher order in derivatives, we show that the true propagating degrees of freedom obey well-behaved second-order equations and are thus free from Ostrogradski instabilities, in contrast to the standard lore. Remarkably, the covariant versions of the original galileon Lagrangians-obtained by direct replacement of derivatives with covariant derivatives-belong to this class of theories. These extensions of Horndeski theories exhibit an uncommon, interesting phenomenology: the scalar degree of freedom affects the speed of sound of matter, even when the latter is minimally coupled to gravity.