Consistent Regularization of Signature-Changing BTZ Black Holes

Abstract: Spacetime singularities pose a fundamental challenge to classical and quantum gravity. We develop a mathematically consistent framework for signature-changing black holes by revisiting the $(2+1)$-dimensional BTZ metric, which undergoes a Lorentzian-to-Euclidean transition at the horizon. We identify and resolve a critical inconsistency in previous regularization schemes concerning second-order distributional terms $\varepsilon''(r)$, introducing a \emph{modified Hadamard regularization} that is rigorously defined within distribution theory. This yields a vacuum solution without surface layers or impulsive waves. Geodesic analysis shows that infalling observers require infinite proper time to reach the horizon, and curvature remains finite everywhere. Furthermore, we establish the physical viability of the solution by proving its linear stability, demonstrating consistent quantum scalar field propagation across the horizon, and reinterpreting the $r=0$ singularity as a removable topological boundary. Our work places signature change and the associated \emph{atemporality} on a solid foundation as a robust mechanism for singularity resolution.