Gravity waves reveal colliding black holes, galaxies, the birth of a black hole in a supernova and the growth pains of our universe. Gravitational waves are unambiguous measured only at future null infinity.
Babiuc, M. C. and Winicour, J. (2012, April). Testing a model for the well-posedness of the Cauchy-characteristic problem. Slides presented at 15th Annual East Coast Gravity Meeting and Josh Goldberg Fest, Syracuse, NY.