How to smooth a crinkled map of spacetime: Uhlenbeck compactness for $L^{\mathrm{\infty }}$ connections and optimal regularity for general relativistic shock waves by the Reintjes-Temple-equations

We present authors’ new theory of the RT-equations, nonlinear elliptic partial differential equations which determine the coordinate transformations which smooth connections $\mathrm{\Gamma }$ to optimal regularity, one derivative smoother than the Riemann curvature tensor $\mathrm{R}\mathrm{i}\mathrm{e}\mathrm{m}(\mathrm{\Gamma })$. As one application we extend Uhlenbeck compactness from Riemannian to Lorentzian geometry; and as another application we establish that regularity singularities at GR shock waves can always be removed by coordinate transformation. This is based on establishing a general multi-dimensional existence theory for the RT-equations, by application of elliptic regularity theory in $L^p$ spaces. The theory and results announced in this paper apply to arbitrary $L^{\mathrm{\infty }}$ connections on the tangent bundle $T\mathcal{M}$ of arbitrary manifolds $\mathcal{M}$, including Lorentzian manifolds of General Relativity.