The conjugacy problem for automorphism groups of countable homogeneous structures

Abstract

We show that a version of L\'opez-Escobar's theorem holds in the setting of logic for metric structures. More precisely, let $\mathbb{U}$ denote the Urysohn sphere and let $\mathrm{Mod}(\mathcal{L},\mathbb{U})$ be the space of metric $\mathcal{L}$-structures supported on $\mathbb{U}$. Then for any $\mathrm{Iso}(\mathbb{U})$-invariant Borel function $f\colon \mathrm{Mod}(\mathcal{L}, \mathbb{U})\rightarrow \lbrack 0,1]$, there exists a sentence $\phi $ of $\mathcal{L}_{\omega_{1}\omega}$ such that for all $M\in \mathrm{Mod}(\mathcal{L},\mathbb{U})$ we have $f(M)=\phi ^{M}$. At the same time we introduce a variant $\mathcal{L}_{\omega_1\omega}^\ast$ of $\mathcal{L}_{\omega_1\omega}$ in which the usual quantifiers are replaced with category quantifiers, and establish the analogous theorem for $\mathcal{L}_{\omega_1\omega}^\ast$. This answers a question of Ivanov and Majcher-Iwanow. We prove several consequences, for example every orbit equivalence relation of a Polish group action is Borel isomorphic to the isomorphism relation on the set of models of a given $\mathcal{L}_{\omega_{1}\omega}$-sentence that are supported on the Urysohn sphere. This in turn provides a model-theoretic reformulation of the topological Vaught conjecture.