Logarithmic local energy decay for scalar waves on a general class of asymptotically flat spacetimes

TL;DR

This paper proves that solutions to the scalar wave equation on a broad class of asymptotically flat spacetimes exhibit at least logarithmic local energy decay, extending previous results to more general black hole spacetimes.

Contribution

It establishes sharp logarithmic decay rates for scalar waves on general stationary asymptotically flat spacetimes, including those with ergoregions and trapped sets.

Findings

Logarithmic decay rate is sharp for the class of spacetimes considered.

Includes black hole spacetimes with no restrictions on trapped sets.

Provides an asymptotic completeness result in the absence of ergoregions.

Abstract

This paper establishes that on the domain of outer communications of a general class of stationary and asymptotically flat Lorentzian manifolds of dimension $d+1$, $d\ge3$, the local energy of solutions to the scalar wave equation $\square_{g}\psi=0$ decays at least with an inverse logarithmic rate. This class of Lorentzian manifolds includes (non-extremal) black hole spacetimes with no restriction on the nature of the trapped set. Spacetimes in this class are moreover allowed to have a small ergoregion but are required to satisfy an energy boundedness statement. Without making further assumptions, this logarithmic decay rate is shown to be sharp. Our results can be viewed as a generalisation of a result of Burq, dealing with the case of the wave equation on flat space outside compact obstacles, and results of Rodnianski--Tao for asymptotically conic product Lorentzian manifolds. The proof will bridge ideas of Rodnianski--Tao with techniques developed in the black hole setting by Dafermos--Rodnianski. As a soft corollary of our results, we will infer an asymptotic completeness statement for the wave equation on the spacetimes considered, in the case where no ergoregion is present.