We present a novel approach to the analysis of regularity and decay
for solutions of wave equations in a neighborhood of null infinity in
asymptotically flat spacetimes of any dimension. The classes of metrics
and wave-type operators we consider near null infinity include those
arising in nonlinear stability problems for Einstein’s field equations in
dimensions. In a neighborhood of null infinity, in an appropriate compactification of
the spacetime to a manifold with corners, the wave operators are of edge type at
null infinity and totally characteristic at spacelike and future timelike infinity. On a
corresponding scale of Sobolev spaces, we demonstrate how microlocal regularity
propagates across or into null infinity via a sequence of radial sets. As an
application, inspired by work of the second author with Baskin and Wunsch, we
prove regularity and decay estimates for forward solutions of wave-type equations
on asymptotically flat spacetimes which are asymptotically homogeneous with
respect to scaling in the forward timelike cone and have an appropriate structure at
null infinity. These estimates are new even for the wave operator on Minkowski
space.
The results obtained here are also used as black boxes in a global theory of
wave-type equations on asymptotically flat and asymptotically stationary
spacetimes developed by the first author.
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