Null infinity is the ideal boundary reached by asymptotically escaping null geodesics in a suitable conformal completion. It has future and past components. A finite location on a Penrose diagram need not represent a finite affine parameter.
Past null infinity is the ideal boundary from which incoming null geodesics arrive in an asymptotically flat spacetime. In the Minkowski conformal compactification it is , with the retarded coordinate tending to and the advanced coordinate finite.
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Null infinity refers to a concept in the context of general relativity and asymptotic flatness, particularly in the study of asymptotic properties of spacetimes at "infinity." It is a way to describe the behavior of gravitational fields at very large distances from isolated systems, such as stars or black holes.