Finite time blowup for a supercritical defocusing nonlinear wave system

TL;DR

This paper demonstrates finite time blowup for certain supercritical defocusing nonlinear wave systems, showing solutions can develop singularities despite the defocusing nature, which contrasts with known global regularity results in subcritical cases.

Contribution

It constructs explicit examples of finite time blowup solutions for supercritical defocusing nonlinear wave systems with large target dimension, extending understanding of singularity formation.

Findings

Finite time singularity can occur in supercritical defocusing NLW systems.

Constructed solutions are discretely self-similar in a backwards light cone.

Large target dimension (m=40) is used to facilitate the construction.

Abstract

We consider the global regularity problem for defocusing nonlinear wave systems $$ \Box u = (\nabla_{{\bf R}^m} F)(u) $$ on Minkowski spacetime ${\bf R}^{1+d}$ with d'Alambertian $\Box := -\partial_t^2 + \sum_{i=1}^d \partial_{x_i}^2$, the field $u: {\bf R}^{1+d} \to {\bf R}^m$ is vector-valued, and $F: {\bf R}^m \to {\bf R}$ is a smooth potential which is positive and homogeneous of order $p+1$ outside of the unit ball, for some $p >1$. This generalises the scalar defocusing nonlinear wave (NLW) equation, in which $m=1$ and $F(v) = \frac{1}{p+1} |v|^{p+1}$. It is well known that in the energy sub-critical and energy-critical cases when $d \leq 2$ or $d \geq 3$ and $p \leq 1+\frac{4}{d-2}$, one has global existence of smooth solutions (for dimensions $d \leq 7$ at least) from arbitrary smooth initial data $u(0), \partial_t u(0)$. In this paper we study the supercritical case where $d = 3$ and $p > 5$. We show that in this case, there exists smooth potential $F$ for some sufficiently large $m$ (in fact we can take $m=40$), positive and homogeneous of order $p+1$ outside of the unit ball, and a smooth choice of initial data $u(0), \partial_t u(0)$ for which the solution develops a finite time singularity. In fact the solution is discretely self-similar in a backwards light cone. The basic strategy is to first select the mass and energy densities of $u$, then $u$ itself, and then finally design the potential $F$ in order to solve the required equation. The Nash embedding theorem is used in the second step, explaining the need to take $m$ relatively large.