Initial Data for Numerical Relativity Gregory B. Cook Department of Physics, Wake Forest University Winston-Salem, North Carolina 27109-7507, U. S. A. cookgb@wfu.edu http://www.wfu.edu/˜cookgb Published on 14 November 2000 Last amended on 15 December 2001 www.livingreviews.org/Articles/Volume3/2000-5cook Living Reviews in Relativity Published by the Max Planck Institute for Gravitational Physics Albert Einstein Institute, Germany Abstract Initial data are the starting point for any numerical simulation. In the case of numerical relativity, Einstein’s equations constrain our choices of these initial data. We will examine several of the formalisms used for specifying Cauchy initial data in the 3 + 1 decomposition of Einstein’s equations. We will then explore how these formalisms have been used in constructing initial data for spacetimes containing black holes and neutron stars. In the topics discussed, emphasis is placed on those issues that are important for obtaining astrophysically realistic initial data for compact binary coalescence. c 2000 Max-Planck-Gesellschaft and the authors. Further information on copyright is given at http://www.livingreviews.org/Info/Copyright/. For permission to reproduce the article please contact livrev@aei-potsdam.mpg.de.