The ${\rm PSL}(2,\mathbb{R})^2$-configuration space of four points in the torus $S^1\times S^1$

Abstract

The torus $\mathbb{T}=S^1\times S^1$ appears as the ideal boundary $\partial_\infty AdS^3$ of the three-dimensional anti-de Sitter space $AdS^3$, as well as the F\"urstenberg boundary $\mathbb{F}(X)$ of the rank-2 symmetric space $X={\rm SO}_0(2,2)/{\rm SO}(2)\times{\rm SO}(2)$. We introduce cross-ratios on the torus in order to parametrise the ${\rm PSL}(2,\mathbb{R})^2$ configuration space of quadruples of pairwise distinct points in $\mathbb{T}$ and define a natural M\"obius structure in $\mathbb{T}$ and therefore to $\mathbb{F}(X)$ and $\partial_\infty AdS^3$ as well.