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

TL;DR

This paper characterizes the long-term behavior of solutions to the focusing cubic nonlinear Klein-Gordon equation in 3D near the ground state energy, revealing a detailed trichotomy of solution types including blowup, scattering, and ground state convergence.

Contribution

It extends the understanding of solution dynamics beyond the ground state energy, providing a complete classification near the ground state for the Klein-Gordon equation.

Findings

Solutions near the ground state exhibit a trichotomy: blowup, scattering to zero, or scattering to the ground state.

The initial data space is divided into nine disjoint sets with distinct asymptotic behaviors.

A 'one-pass' theorem is established, ruling out certain complex orbit connections between ground states.

Abstract

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 Schr\"odinger 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.