# Gravitational Collapse in Einstein dilaton Gauss-Bonnet Gravity

## TL;DR

This paper numerically investigates gravitational collapse in Einstein dilaton Gauss-Bonnet gravity, revealing how modifications to general relativity influence black hole formation and scalar field behavior, especially in strong coupling regimes.

## Contribution

It provides the first detailed numerical analysis of spherical collapse in EdGB gravity, highlighting the transition from hyperbolic to elliptic PDE behavior and the emergence of scalar hair near black holes.

## Key findings

- Weak EdGB coupling solutions approach GR results.
- Strong coupling leads to elliptic regions where PDE character changes.
- Negative energy densities and violation of null convergence condition occur near elliptic regions.

## Abstract

We present results from a numerical study of spherical gravitational collapse in shift symmetric Einstein dilaton Gauss-Bonnet (EdGB) gravity. This modified gravity theory has a single coupling parameter that when zero reduces to general relativity (GR) minimally coupled to a massless scalar field. We first show results from the weak EdGB coupling limit, where we obtain solutions that smoothly approach those of the Einstein-Klein-Gordon system of GR. Here, in the strong field case, though our code does not utilize horizon penetrating coordinates, we nevertheless find tentative evidence that approaching black hole formation the EdGB modifications cause the growth of scalar field "hair", consistent with known static black hole solutions in EdGB gravity. For the strong EdGB coupling regime, in a companion paper we first showed results that even in the weak field (i.e. far from black hole formation), the EdGB equations are of mixed type: evolution of the initially hyperbolic system of partial differential equations lead to formation of a region where their character changes to elliptic. Here, we present more details about this regime. In particular, we show that an effective energy density based on the Misner-Sharp mass is negative near these elliptic regions, and similarly the null convergence condition is violated then.