Quasimorphisms and Poincar\'e duality in dimension 3

Abstract

We study $\mathrm{PD}^3$ groups which admit an unbounded quasimorphism to $\mathbb{R}$ with coarsely connected quasikernel. We show that such a group $G$ must either arise as the fundamental group of a torus or Klein-bottle bundle over $S^1$, or be quasiisometric to a Riemannian manifold $(\mathbb{R}^3,g)$ of bounded geometry, with the quasikernel being coarsely equivalent to $\mathbb{H}^2$. If $G$ is moreover hyperbolic, it admits a faithful action on $S^1$ by quasisymmetric homeomorphisms. Our approach features a coarse generalisation of Shapiro's lemma, and a new definition of homological isoperimetric inequalities for metric spaces; these tools make use of Margolis's framework for coarse homological algebra.