Paper detail

Global dynamics above the ground state energy for the focusing nonlinear Klein-Gordon equation

We study the focusing, cubic, nonlinear Klein-Gordon equation in 3D with large radial data in the energy space. This equation admits a unique positive stationary solution, called the ground state. In 1975, Payne and Sattinger showed that solutions with energy strictly below that of the ground state are divided into two classes, depending on a suitable functional: If it is negative, then one has finite time blowup, if it is nonnegative, global existence; moreover, these sets are invariant under the flow. Recently, Ibrahim, Masmoudi and the first author improved this result by establishing scattering to zero in the global existence case by means of a variant of the Kenig-Merle method. In this paper we go slightly beyond the ground state energy and give a complete description of the evolution. For example, in a small neighborhood of the ground states one encounters the following trichotomy: on one side of a center-stable manifold one has finite-time blowup, on the other side scattering to zero, and on the manifold itself one has scattering to the ground state, all for positive time. In total, the class of initial data is divided into nine disjoint nonempty sets, each displaying different asymptotic behavior, which includes solutions blowing up in one time direction and scattering to zero on the other, and also, the analogue of those found by Duyckaerts and Merle for the energy critical wave and Schrödinger equations, exactly with the ground state energy. The main technical ingredient is a "one-pass" theorem which excludes the existence of "almost homoclinic" orbits between the ground states.