The authors are greatly indebted to Bernard F. Schutz for his helpful comments and valuable discussions while they visited the Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Germany. When the authors wrote this article, Y.I. stayed at Tokyo University, for which he particularly thanks Masaru Shibata for his hospitality. The authors would like to thank Takashi Fukumoto for fruitful discussion.
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 2.0 Germany License.