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Abstract

We present simulations of binary black holes mergers in which, after the
common outer horizon has formed, the marginally outer trapped surfaces (MOTSs)
corresponding to the individual black holes continue to approach and eventually
penetrate each other. This has very interesting consequences according to
recent results in the theory of MOTSs. Uniqueness and stability theorems imply
that two MOTSs which touch with a common outer normal must be identical. This
suggests a possible dramatic consequence of the collision between a small and
large black hole. If the penetration were to continue to completion then the
two MOTSs would have to coalesce, by some combination of the small one growing
and the big one shrinking. Here we explore the relationship between theory and
numerical simulations, in which a small black hole has halfway penetrated a
large one.