Real Spectrum Compactifications of Universal Geometric Spaces over Character Varieties

TL;DR

This paper develops a universal geometric framework over the real spectrum compactification of character varieties, linking boundary points to geometric structures and extending classical projections to this compactified setting.

Contribution

It introduces a new construction of universal geometric spaces over the real spectrum compactification of character varieties, providing a geometric interpretation of boundary points and extending classical projections.

Findings

Fibers are homeomorphic to the Archimedean spectrum of $Y( ext{field})$

The spectrum is homeomorphic to real analytification

For $Y=\mathbb{P}^1$, the image is a real subtree

Abstract

We construct universal geometric spaces over the real spectrum compactification $\Xi^{\mathrm{RSp}}$ of the character variety $\Xi$ of a finitely generated group $\Gamma$ in $\mathrm{SL}_n$, providing geometric interpretations of boundary points. For an algebraic set $Y(\mathbb{R})$ on which $\mathrm{SL}_n(\mathbb{R})$ acts by algebraic automorphisms (such as $\mathbb{P}^{n-1}(\mathbb{R})$ or an algebraic cover of the symmetric space of $\mathrm{SL}_n(\mathbb{R})$), the projection map $\Xi \times Y \rightarrow \Xi$ extends to a $\Gamma$-equivariant continuous surjection $(\Xi \times Y)^{\mathrm{RSp}} \rightarrow \Xi^{\mathrm{RSp}}$. The fibers of this extended map are homeomorphic to the Archimedean spectrum of $Y(\mathbb{F})$ for some real closed field $\mathbb{F}$, which is a locally compact subset of $Y^{\mathrm{RSp}}$. The Archimedean spectrum is naturally homeomorphic to the real analytification, and we use this identification to compute the image of the fibers in their Berkovich analytification. For $Y=\mathbb{P}^1$, the image is a real subtree.