Degenerate Higher-Order Scalar-Tensor (DHOST) theories

TL;DR

This paper reviews degenerate higher-order scalar-tensor theories, including Horndeski and Beyond Horndeski, highlighting their classification, cosmological applications, and neutron star solutions, while emphasizing their stability and phenomenological relevance.

Contribution

It provides a comprehensive review and classification of DHOST theories, and discusses their implications in cosmology and neutron star physics.

Findings

DHOST theories avoid Ostrodradsky instability despite higher derivatives.

These theories unify dark energy and modified gravity descriptions.

Neutron star solutions are explored within specific DHOST models.

Abstract

This contribution reviews scalar-tensor theories whose Lagrangian contains second-order derivatives of a scalar field but nevertheless propagate only one scalar mode (in addition to the usual two tensor modes), and are thus not plagued with the Ostrodradsky instability. These theories, which encompass the so-called Horndeski and Beyond Horndeski theories, have recently been fully classified up to cubic order in second-order derivatives. After introducing these theories, I present a few phenomenological aspects. In cosmology, these theories can be included in the unified effective description of dark energy and modified gravity. Finally, neutron star solutions in some specific models are discussed.