# Unscreening scalarons with a black hole

## TL;DR

This paper challenges the common belief that scalar degrees of freedom are always suppressed near black holes, showing that low accretion rate black holes can unscreen scalar fields, with new techniques developed for their analysis.

## Contribution

The authors develop a novel, accurate method to study scalar field dynamics around black holes, including static profiles and stability, applicable to various modified gravity models.

## Key findings

- Scalar fields can remain unscreened around low accretion rate black holes.
- New computational tools for analyzing scalar field stability near black holes.
- Application to multiple modified gravity models, including f(R) and symmetron.

## Abstract

It is typically believed that the additional degrees of freedom in any modification of gravity are completely suppressed by the large energy densities coexisting with an astrophysical black hole. In this paper, we find that this might not always be the case. This belief holds for black holes formed via gravitational collapse in very dense environments, whereas the black holes with sufficiently low accretion rates that have low matter densities inside innermost stable circular orbit will generally unscreen chameleons. We develop a novel technique to study the dynamics of accretion of a scalar field onto a Schwarzschild-like black hole which is accurate on both short and long time scales. In particular, we study the behavior of the extra scalar degree of freedom in the Starobinsky and Hu-Sawicki $f(R)$ theories, for the symmetron model, and for the Ratra-Peebles model. Aside from calculating non-trivial static field profiles outside the black hole, we provide the tools to study the (in)stability and evolution towards the equilibrium solution for any generic well behaved set of parameters and initial conditions. Our code is made publicly available for further research and modifications to study other models.