(Non-)Linear waves on asymptotically flat spacetimes. II: trapping, bound states, nonlinear applications

We study wave-type equations on dynamical spacetimes that settle down to a subextremal Kerr black hole spacetime. We prove strong estimates for solutions of (tensorial) linear wave-type equations when the time-translation-invariant model satisfies a spectral assumption of mode stability type. We allow for this model to admit zero energy bound states; besides the scalar wave operator (which has no bound states), examples include the wave operator on 1-forms and the linearization of the Einstein field equations in generalized harmonic gauge. We demonstrate the utility of our estimates by proving the global existence of solutions to some quasilinear wave equations, including in the presence of zero energy bound states. The results proved here are, moreover, crucial ingredients in the author's proof of the nonlinear stability of subextremal Kerr black holes.
Our key novel linear estimate controls linear waves in weighted $L^2$-based spacetime Sobolev spaces that encode b-regularity, by which we mean regularity with respect to spacetime scaling, spatial scaling (in a hyperboloidal foliation of spacetime), and angular derivatives; this estimate is moreover tame in the b-regularity order, as needed for its applicability in a Nash-Moser iteration scheme. Its proof combines four main ingredients: microlocal propagation estimates in the edge-b-setting near null infinity (as introduced by the author with Vasy) and in the author's 3b-setting in the forward cone; estimates for the stationary model operator; energy estimates on edge-b-spaces on finite time intervals; and commutations with b-vector fields. For the nonlinear applications, we moreover develop a dictionary between decay rates in different spacetime regimes on the one hand and weighted low-energy resolvent estimates on the other hand.
This paper builds on Part I only a broad conceptual level, and is largely self-contained.