The materials presented here originated from research work of BLH with Antonio Campos, Nicholas Phillips, Alpan Raval, and Sukanya Sinha, and of EV with Rosario Martin and Albert Roura. We thank them as well as Daniel Arteaga, Andrew Matacz, Tom Shiokawa, and Yuhong Zhang for fruitful collaboration and their cordial friendship since their Ph.D. days. We enjoy lively discussions with our friends and colleagues Esteban Calzetta, Diego Mazzitelli, and Juan Pablo Paz whose work in the early years contributed toward the establishment of this field. We acknowledge useful discussions with Paul Anderson, Larry Ford, Ted Jacobson, Renaud Parentani, and Raphael Sorkin. This work is supported in part by NSF grant PHY98-00967, the MICYT Research Project No. FPA-2001-3598, and the European project HPRN-CT-2000-00131.
© Max Planck Society and the author(s)