Critical Collapse of Gravitational Waves

Critical phenomena in gravitational collapse refer to self-similarity, universality, and scaling close to the onset of black-hole formation. While these phenomena are quite well understood in the context of spherical symmetry, many of their aspects remain enigmatic for the collapse of vacuum gravitational waves. I will review recent simulations of this collapse and will suggest that, while the assumption of universality does not seem to apply in the absence of spherical symmetry, there do exist families of initial data for which the threshold solution displays an at least approximate discrete self-similarity.