CONFORMAL SCATTERING FOR A NONLINEAR WAVE EQUATION

Abstract

We establish a geometric scattering theory for a conformally invariant nonlinear wave equation on an asymptotically simple space-time. The scattering operator is defined via some trace operators at null infinity, and the proof is decomposed into three steps. A priori linear estimates are obtained via an adaptation of the Morawetz vector field to the Schwarzschild space-time and a method introduced by Hörmander for the Goursat problem. A well-posedness theorem for the characteristic Cauchy problem on a light cone at infinity is then obtained. Its proof requires a control of the nonlinearity that is uniform in time and follows from, both, an estimate of the Sobolev constant and a decay assumption on the nonlinearity of the equation. Finally, the trace operators on conformal infinity are introduced and allow us to define the conformal scattering operator of interest.