Quasilinear wave equations on asymptotically flat spacetimes with applications to Kerr black holes
Abstract
We prove global existence and decay for smalldata 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 $\vert a \vert \ll M$ 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 timetranslation invariant $r^p$weighted estimates, in the spirit of [DR09], to be applied on dyadic timeslabs which for large $r$ are outgoing. Global existence and decay then both immediately follow by elementary iteration on consecutive such timeslabs, without further global bootstrap. The second, and more fundamental, aspect is our direct use of a "blackbox" 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 [DRSR16], 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 $\vert a \vert \ll M$, 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 timedependent metrics.
 Publication:

arXiv eprints
 Pub Date:
 December 2022
 DOI:
 10.48550/arXiv.2212.14093
 arXiv:
 arXiv:2212.14093
 Bibcode:
 2022arXiv221214093D
 Keywords:

 General Relativity and Quantum Cosmology;
 Mathematical Physics;
 Mathematics  Analysis of PDEs;
 Mathematics  Differential Geometry
 EPrint:
 98 pages, 2 figures