Future null infinity (${\mathcal{I}}^{+}$)

Future null infinity is the boundary reached by future-directed outgoing null geodesics in an asymptotically flat spacetime. In four dimensions it has topology $\mathbb{R}\times S^2$ under the standard asymptotic-flatness assumptions.

It is the future component of null infinity; the past component is ${\mathcal{I}}^{-}$.