The sound of DHOST

TL;DR

This paper investigates how higher-order scalar-tensor theories, which are free from Ostrogradsky instabilities, influence matter perturbation propagation and sound speed, providing constraints on these models based on physical stability and observational effects.

Contribution

It demonstrates the impact of such theories on matter perturbations and offers new constraints on viable models by analyzing their effects on sound speed and stability.

Findings

Scalar field modifies matter perturbation propagation.

Altered sound speed in Earth's atmosphere due to scalar-tensor effects.

Constraints established on higher-order scalar-tensor models.

Abstract

We show that, in generic higher-order scalar-tensor theories which avoid the Ostrogradsky instability, the presence of a scalar field significantly modifies the propagation of matter perturbations, even in weakly curved backgrounds. This affects notably the speed of sound in the atmosphere of the Earth. It can also generate instabilities in homogeneous media. We use this to constrain the viable higher-order scalar-tensor models.