The Hawking and Penrose singularity theorems for low regularity Finsler spacetimes

Abstract: We prove the Hawking and Penrose singularity theorems for Lorentz--Finsler structures of mixed horizontal/vertical regularity $C<sup>{(1,2)}$ in the $C<sup>1$ weighted case and $C<sup>{(1,3)}$ in the unweighted case. At this regularity, geodesics need not be uniquely determined by their initial conditions, and the (weighted) Ricci curvature must be interpreted distributionally. Our approach relies on a two-stage approximation procedure: first, a homogeneity-preserving convolution, which is of independent interest in the broader semi-Riemann--Finsler setting, and second, a causality-adapted approximation à la Chruściel--Grant. In the special case of $C<sup>1$ Lorentzian metrics, our results extend the $C<sup>1$ singularity theorems of Graf to the $C<sup>1$ weighted case.