Recent Developments in Degenerate Higher Order Scalar Tensor Theories

TL;DR

This paper reviews recent developments in DHOST theories, a broad class of scalar-tensor models that extend Horndeski theories, highlighting their properties, cosmological and astrophysical applications, and observational constraints.

Contribution

It provides a comprehensive review of DHOST theories, including their classification, phenomenology, and potential for explaining cosmological and astrophysical phenomena.

Findings

DHOST theories generalize Horndeski and beyond-Horndeski models.

They can describe both early universe inflation and late-time cosmic acceleration.

DHOST models modify the Vainshtein screening mechanism and allow for non-stealth black hole solutions.

Abstract

Degenerate Higher Order Scalar Tensor (DHOST) theories are the most general scalar-tensor theories whose Lagrangian depends on the metric tensor and a single scalar field and its derivatives up to second order. They propagate only one scalar degree of freedom, without being plagued by Ostrogradsky instabilities. This is achieved through certain degeneracies of the functions forming their Lagrangian. They generalise the Horndeski and beyond-Horndeski theories. Originally proposed to describe the late-time acceleration of the expansion of the universe, generalising the cosmological constant, they can also be used to build models of the early universe, to describe inflation or alternatives to standard inflation. In the late universe, they modify the standard Vainstein screening mechanism from Horndeski theories (which can have observable consequences) and are suited to build black hole models, featuring non-stealth Kerr black hole solutions. In this work their phenomenology is reviewed, looking at their basic properties, their parameterisations and classifications, focusing on solutions in the early and the late universe and at cosmological and astrophysical constraints.