We prove global existence and decay for small-data solutions to a class of quasilinear
wave equations on a wide variety of asymptotically flat spacetime backgrounds,
allowing in particular for the presence of horizons, ergoregions and trapped null
geodesics, and including as a special case the Schwarzschild and very slowly rotating
Kerr
family of black holes in general relativity. There are two distinguishing aspects
of our approach. The first aspect is its dyadically localised nature: The
nontrivial part of the analysis is reduced entirely to time-translation-invariant
-weighted
estimates, in the spirit of Dafermos and Rodnianski (2010b), to be applied on dyadic time-slabs
which for large
are outgoing. Global existence and decay then both immediately follow by
elementary iteration on consecutive such time-slabs, without further global
bootstrap. The second, and more fundamental, aspect is our direct use of a
“black box” linear inhomogeneous energy estimate on exactly stationary
metrics, together with a novel but elementary physical-space top-order
identity that need not capture the structure of trapping and is robust to
perturbation. In the specific example of Kerr black holes, the required linear
inhomogeneous estimate can then be quoted directly from the literature (Dafermos
et al. (2016)), while the additional top-order physical-space identity can be shown
easily in many cases (we include in the Appendix a proof for the Kerr case
,
which can in fact be understood in this context simply as a perturbation of
Schwarzschild). In particular, the approach circumvents the need either for
producing a purely physical-space identity capturing trapping or for a careful
analysis of the commutation properties of frequency projections with the wave
operator of time-dependent metrics.
Keywords
nonlinear wave equations, black holes, general relativity
nonlinear stability