BASIL · Black Holes and Singularities in Low-Regularity

Einstein’s general relativity models gravity through the geometry of spacetime. While traditionally formulated within smooth differential geometry, the physics of general relativity has made it clear that such a smooth description is insufficient for key gravitational phenomena. This has motivated a surge in research in low-regularity spacetimes. The BASIL project explores the nature of black holes and singularities in such low-regularity settings, i.e., without assuming a smooth geometric structure. By combining new methods from Lorentzian metric geometry and optimal transport, it aims to advance our understanding of black holes and singularities beyond the classical smooth framework.The first part investigates whether cosmological spacetimes can be extended beyond their past ""big bang"" singularity, focusing on $C^0$-inextendibility and applying Sbierski’s methods to identify obstructions to any such extension.The second part examines whether $C^0$-inextendible spacetimes admit extensions as Lorentzian length spaces (the Lorentzian analogue of metric length space in metric geometry). It involves employing optimal transport techniques to estimate synthetic curvature bounds and link inextendibility to curvature blow-up.The third part consists of defining conformal completion in Lorentzian length spaces, extending our prior work on conformal transformations. It involves developing the notion of asymptotic flatness using the optimal transport formulation of Einstein’s vacuum equations (Mondino-Suhr), and deriving criteria for conformal boundaries to be null, which are then used to define black holes.The three research goals are thematically connected through their focus on low-regularity Lorentzian geometry, yet each addresses a distinct question using independent methods. Their success is not interdependent