Thanks are due to Piotr Chruściel and Alan Rendall for comments on an earlier version of this review. The author acknowledges the support of the Göran Gustafsson Foundation and the Swedish Research Council. This review article was in part written during a stay by the author at the Max Planck Institute for Gravitational Physics as a recipient of a Friedrich Wilhelm Bessel Research Award granted by the Alexander von Humboldt Stiftung. The author is a Royal Swedish Academy of Sciences Research Fellow supported by a grant from the Knut and Alice Wallenberg Foundation.
This work is licensed under a Creative Commons License.