Dynamics of Screening in Modified Gravity

TL;DR

This paper investigates the behavior and stability of screened solutions in scalar-tensor modified gravity theories, revealing potential issues with predictability due to diverging characteristic speeds during collapse.

Contribution

It provides the first detailed analysis of the dynamical evolution and stability of screened stars in a scalar-tensor theory with derivative self-interactions.

Findings

Screened solutions are stable under large perturbations unless collapse occurs.

Characteristic speeds diverge during gravitational collapse, challenging predictability.

Divergence occurs before black-hole or sound horizon formation.

Abstract

Gravitational theories differing from General Relativity may explain the accelerated expansion of the Universe without a cosmological constant. However, to pass local gravitational tests, a "screening mechanism" is needed to suppress, on small scales, the fifth force driving the cosmological acceleration. We consider the simplest of these theories, i.e. a scalar-tensor theory with first-order derivative self-interactions, and study isolated (static and spherically symmetric) non-relativistic and relativistic stars. We produce screened solutions and use them as initial data for non-linear numerical evolutions in spherical symmetry. We find that these solutions are stable under large initial perturbations, as long as they do not cause gravitational collapse. When gravitational collapse is triggered, the characteristic speeds of the scalar evolution equation diverge, even before apparent black-hole or sound horizons form. This casts doubts on whether the dynamical evolution of screened stars may be predicted in these effective field theories.