The fully nonlinear Loewner-Nirenberg problem: Liouville theorems and counterexamples to local boundary estimates

In this paper we give a complete classification of positive viscosity solutions $w$ to conformally invariant equations of the form \begin{align}\label{ab}\tag{$*$} \begin{cases} f(\lambda(-A_w)) = \frac{1}{2}, \quad \lambda(-A_w)\in\Gamma & \text{in }\mathbb{R}_+^n \newline w = 0 & \text{on }\partial\mathbb{R}_+^n, \end{cases} \end{align} where $A_w$ is the Schouten tensor of the metric $g_w = w^{-2}|dx|^2$, $\Gamma\subset\mathbb{R}^n$ is a symmetric convex cone and $f$ is an associated defining function satisfying standard assumptions. Solutions to \eqref{ab} yield metrics $g_w$ of negative curvature-type which are locally complete near $\partial\mathbb{R}_+^n$. In particular, when $(f,\Gamma) = (\sigma_1,\Gamma_1^+)$, \eqref{ab} is the Loewner-Nirenberg problem in the upper half-space. More precisely, let $\mu_\Gamma^+$ denote the unique constant satisfying $(-\mu_\Gamma^+, 1,\dots,1)\in\partial\Gamma$. We show that when $\mu_\Gamma^+ >1$ (e.g. when $\Gamma = \Gamma_k^+$ for $k<\frac{n}{2}$), the hyperbolic solution $w^{(0)}(x) := x_n$ is the unique solution to \eqref{ab}. More surprisingly, we show that when $\mu_\Gamma^+ \leq 1$ (e.g. when $\Gamma = \Gamma_k^+$ for $k\geq \frac{n}{2}$), the solution set consists of a monotonically increasing one-parameter family $\{w^{(a)}(x_n)\}_{a\geq 0}$, of which the hyperbolic solution $w^{(0)}$ is the minimal solution. In either case, solutions of \eqref{ab} are functions of $x_n$. Our proof involves a novel application of the method of moving spheres for which we must establish new estimates and regularity near $\partial\mathbb{R}_+^n$, followed by a delicate ODE analysis. As an application, we give counterexamples to local boundary $C^0$ estimates on solutions to the fully nonlinear Loewner-Nirenberg problem when $\mu_\Gamma^+ \leq 1$.