This is the metric on a Penrose diagram, illustrated. Unlike the original metric, this metric describes, a manifold with a boundary, given by the restrictions on R and T . There are two null surfaces on this boundary, corresponding to past and future null infinity. Specifically, future null infinity consists of all points where T = π π − − R and 0 < R < π π , and past null infinity consists of all points where T = R − − π π and 0 < R < π π .