“…[19]. Moreover, as it is the case for the Minkowski and Schwarzschild lightcone, see [24] and [17] respectively, we note that any spacelike cross section Σ of N , i.e., a 2-dimensional, spacelike submanifold of N that intersects any integral curve of L exactly once, can be uniquely identified with a conformally round metric. To see this, recall that L(ρ) = 1, so ρ restricts to an affine parameter along N , and we may thus uniquely identify Σ with a function ω : Proposition 1], we find that the induced metric γ = γ ω and null second fundamental form χ = χ ω (with respect to L) of Σ = Σ ω satisfy…”