Dynamical black holes in scalar-tensor theories

As the simplest generalization of Einstein's equations of gravitation, scalar-tensor theories have turned up a number of surprising results. Such results are the remarkable black hole properties discovered by Scheel, Shapiro and Teukolsky. In scalar-tensor theories there is a dynamical epoch where the surface area of the event horizon decreases over time and the apparent horizon which in general relativity always is confined to the interior of the black hole passes the event horizon and becomes timelike, No such processes occur in general relativity. We investigate these intriguing results further in a scalar-tensor theory with two interacting scalar fields. The scalar fields are massless and they are endowed in a space spanned by scalar fields which is described by a sigma metric. The scalar fields are coupled to collisionless matter with the problem having spherical symmetry and it is treated fully numerical. The spacetime location of the black hole horizons is calculated. The problem of gravitational collapse of matter to a black hole in a scalar-tensor theory with one scalar field and a two parameter scalar dependent coupling function (linear in the scalar field) is discussed. We show that this theory admits values for the coupling parameters where, in addition to the well-known violation of the apparent horizon theorem and the area theorem of black holes in general relativity, a further violation of these theorems occurs.