Asymptotically flat spacetime

Summary

An asymptotically flat spacetime is a spacetime whose geometry approaches that of flat Minkowski space far away from its sources. It is the model of an isolated system, such as a star, a black hole or a binary, in an otherwise empty universe. The notion is made precise in two ways. One uses fall-off conditions on the metric at spatial infinity; the other, due to Penrose, uses a conformal compactification with a null boundary at infinity. It is the setting in which total mass, momentum and outgoing gravitational radiation are well defined.