Modified gravity inside astrophysical bodies

TL;DR

This paper investigates how a new class of modified gravity theories, which partially break the Vainshtein screening inside astrophysical bodies, affects the density profiles of spherical matter distributions, revealing universal bounds on deviations from standard gravity.

Contribution

It introduces modifications to the Lane-Emden equations to account for new derivative interactions in beyond-Horndeski theories and identifies universal bounds on these deviations.

Findings

Partial breaking of Vainshtein screening inside matter

Modified Lane-Emden equations for polytropic spheres

Existence of a universal upper bound on gravity deviations

Abstract

Many theories of modified gravity, including the well studied Horndeski models, are characterized by a screening mechanism that ensures that standard gravity is recovered near astrophysical bodies. In a recently introduced class of gravitational theories that goes beyond Horndeski, it has been found that new derivative interactions lead to a partial breaking of the Vainshtein screening mechanism inside any gravitational source, although not outside. We study the impact of this new type of deviation from standard gravity on the density profile of a spherically symmetric matter distribution, in the nonrelativistic limit. For simplicity, we consider a polytropic equation of state and derive the modifications to the standard Lane-Emden equations. We also show the existence of a universal upper bound on the amplitude of this type of modified gravity, independently of the details of the equation of state.