Gravitational Collapse in Einstein dilaton Gauss-Bonnet Gravity
  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.
