 Global causal step in the Penrose theorem

Assume future null geodesic completeness. The compactness of the future horismos of a trapped surface makes $E^{+}(S)$ a nonempty compact achronal boundary without edge. Projection along a complete timelike vector field into the Cauchy hypersurface is injective, open by invariance of domain, and closed by compactness. Connectedness forces surjectivity, contradicting the noncompact Cauchy hypersurface. This global step completes the contradiction.^[ref]