Cosmic structures and gravitational waves in ghost-free scalar-tensor theories of gravity

TL;DR

This paper investigates cosmic structures within the quadratic Degenerate Higher Order Scalar Tensor (qDHOST) model, analyzing gravitational wave constraints and the Vainshtein mechanism to understand its viability as an alternative to General Relativity.

Contribution

It identifies parameter conditions in the qDHOST model that satisfy gravitational wave speed constraints and preserve the Vainshtein screening inside cosmic structures.

Findings

Vainshtein mechanism is broken inside structures but valid outside.

GW170817 constraints restrict qDHOST parameters.

Quadratic second-order derivative Lagrangians are ruled out for shift symmetric qDHOST.

Abstract

We study cosmic structures in the quadratic Degenerate Higher Order Scalar Tensor (qDHOST) model, which has been proposed as the most general scalar-tensor theory (up to quadratic dependence on the covariant derivatives of the scalar field), which is not plagued by the presence of ghost instabilities. We then study a static, spherically symmetric object embedded in de Sitter space-time for the qDHOST model. This model exhibits breaking of the Vainshtein mechanism inside the cosmic structure and Schwarzschild-de Sitter space-time outside, where General Relativity (GR) can be recovered within the Vainshtein radius. We then look for the conditions on the parameters on the considered qDHOST scenario which ensure the validity of the Vainshtein screening mechanism inside the object and the fulfilment of the recent GW170817/GRB170817A constraint on the speed of propagation of gravitational waves. We find that these two constraints rule out the same set of parameters, corresponding to the Lagrangians that are quadratic in second-order derivatives of the scalar field, for the shift symmetric qDHOST.