Obstructions to global visibility of singularities in asymptotically flat spacetimes

Consider an $(N+1)$-dimensional asymptotically flat spacetime and a future-directed, affinely parametrized outgoing null generator $\gamma$ of an achronal boundary $\partial J^+(S_\varepsilon)$, where $\{S_\varepsilon\}$ is a nested family of smooth compact codimension $2$ surfaces approaching a singular boundary set $S$ in the past. In the twist-free case and under the null energy condition, the Raychaudhuri equation on the $m:=N-1$ dimensional screen bundle reads, $$\theta'=-\frac1m\theta^2-\|\sigma\|^2-\mathrm{Ric}(k,k),$$ where $k$ is the tangent to $\gamma$. This equation linearizes, via the rescaling $u:=A^{1/m}$ with $A := |\det D|$ the Jacobi-map $m$-volume, to the Sturm-type ODE $$u''+\frac1m f\,u=0,\qquad f:=\|\sigma\|^2+\mathrm{Ric}(k,k)\ge 0.$$ We develop two purely generator-wise criteria forcing a first zero of $u$: (i) an exact Volterra identity combined with concavity leads to a barrier-weighted integral inequality, and (ii) Sturm comparison and a Pr\"ufer-angle estimate yields failure of disconjugacy whenever $\int_c^d \sqrt{f/m}\,d\lambda>\pi$ on a subinterval. We prove that $u(\lambda_\ast)=0$ is equivalent to the existence of a focal (conjugate) point and implies $\theta= m u'/u\to-\infty$ at $\lambda_\ast$. Using the standard structure of achronal boundaries, this yields a geodesic-wise obstruction: if every generator that could reach $\mathscr I^+$ satisfies one of the above conditions in the regular spacetime region, then $J^+(S_\varepsilon)\cap \mathscr I^+=\emptyset$, and hence $S$ is not globally visible. As an application, we illustrate one of these criteria in the Einstein-massless scalar field collapse model of Christodoulou.