9 Acknowledgements
We are grateful to John Baez, Chris Beetle, Alejandro Corichi,
Sergio Dain, Olaf Dreyer, Jonathan Engle, Stephen Fairhurst, Greg
Galloway, Sean Hayward, Jose Luis Jaramillo, Jerzy Lewandowski,
Kirill Krasnov, Tomasz Pawlowski, Erik Schnetter, Deirdre
Shoemaker, Daniel Sudarsky, Chris van den Broeck, and Jacek
Wisniewski for collaboration on several of the results summarized
in this review. We have also benefited from stimulating discussions
with numerous colleagues, especially among them Marcus Ansorg, Ivan
Booth, Bobby Beig, Piotr Bizoń, Piotr Chruściel, Peter Diener, Sam
Finn, Éanna Flanagan, Jim Hartle, Ian Hawke, Gary Horowitz, Gerhard
Huisken, Jim Isenberg, Pablo Laguna, Luis Lehner, Richard Matzner,
Jorge Pullin, Oscar Reula, Bernd Schmidt, Walter Simon, Ken Smith,
Josh Willis, and Jeff Winicour. This work was supported in part by
the National Science Foundation grants PHY-0090091, the National
Science Foundation Cooperative Agreement PHY-0114375, the
Albert-Einstein-Institut, the Erwin-Schrödinger-Institut, the Kavli
Institute of Theoretical Physics, the Alexander von Humboldt
Foundation, and the Eberly research funds of Penn State.