A Lorentzian Lasry-Lions regularization theorem

Abstract: The main goal of this paper is to establish a general Lorentzian Lasry-Lions regularization theorem: let $u$ be a function defined on a globally hyperbolic spacetime. Assume that its forward Lax--Oleinik evolution $Tu$ is locally semiconcave in a neighbourhood of $(t_0,y_0)$ and has future-directed timelike superdifferentials there. Then, for $t$ close to $t_0$ and sufficiently small $s>0$, the function $\hat T_s\circ T_tu$ is of class $C_{\mathrm{loc}}^{1,1}$ in a neighbourhood of $y_0$. We give sufficient conditions ensuring the assumptions of the theorem and present an application to optimal transport: under quite general assumptions, for any two intermediate measures along a displacement interpolation, there exists a $C^{1,1}_{loc}$-regular maximizing pair in the dual formulation.