Spectral gap and origami expanders

Abstract

We construct the first measure-preserving affine actions with spectral gap on surfaces of arbitrary genus $g>1$. We achieve this by finding geometric representatives of multi-twists on origami surfaces. As a major application, we construct new expanders that are coarsely distinct from the classical expanders obtained via the Laplacian as Cayley graphs of finite quotients of a group. Our methods also show that the Margulis expander, and hence the Gabber--Galil expander, is coarsely distinct from the Selberg expander.

To appear in the Commentarii Mathematici Helvetici