The Euclidian-hyperboidal foliation method and the nonlinear stability of Minkowski spacetime

We introduce a new method for analyzing nonlinear wave-Klein-Gordon systems and establishing global-in-time existence results for the Cauchy problem when the initial data need not have compact support. This method, which we call the Euclidian-Hyperboidal Foliation Method (EHFM), relies on the construction of a spacetime foliation obtained by glueing together asymptotically Euclidian and asymptotically hyperboloidal hypersurfaces. Well-chosen frames of vector fields (null-semi-hyperboloidal, Euclidian-hyperboloidal) allow us to exhibit the structure of the equations under consideration and analyze the decay of solutions in timelike and in spacelike directions. New Sobolev inequalities for Euclidian-hyperboloidal foliations involving the Killing fields of Minkowski spacetime (but not the scaling field), as well as pointwise bounds for wave and Klein-Gordon equations on curved spacetimes are established. Our bootstrap argument involves a hierarchy of (almost sharp) energy and pointwise bounds and distinguishes between low- and high-order derivatives of the solutions. We apply this method to the Einstein equations when the matter model is a massive field and the methods by Christodolou and Klainerman and by Lindblad and Rodnianski do not apply.