Rigidity of braid group actions on $\mathbb{R}$ and of low-genus mapping class group actions on $S^1$

Abstract

Every nontrivial action of the braid group $B_n$ on $\mathbb{R}$ by orientation-preserving homeomorphisms yields, up to conjugation by a homeomorphism of $\mathbb{R}$, a representation $\rho : B_n \rightarrow \mathrm{H\widetilde{ome}o}_+(S^1)$ and therefore determines a translation number for every element of $B_n$. In this manuscript we offer a simple characterisation of which actions of $B_n$ on $\mathbb{R}$ produce translation numbers that agree with those arising from the standard Nielsen-Thurston action on $\mathbb{R}$. Our approach is to prove an analogous statement concerning left orderings of $B_n$ via a technique that uses the space of left orderings of $B_n$, the isolated points in this space, and the natural conjugacy action of $B_n$. We use this result to extend recent rigidity results of Mann and Wolff concerning mapping class group actions on $S^1$ to the case of low-genus surfaces with marked points.