## Department Colloquium

### Friday, September 28th, 2018

**Time:** 2:30 p.m. **Place:** Jeffery Hall 234

**Speaker:** Jon Chaika (University of Utah)

**Title:** Horocycle orbits in strata of translation surfaces.

**Abstract: ** Ergodic theory is concerned with describing the long term behavior of orbits as time evolves. Ratner, Margulis, Dani and many others, showed that the horocycle flow have strong measure theoretic and topological rigidity properties that allow a good understanding of every such orbit. Eskin-Mirzakhani and Eskin-Mirzakhani-Mohammadi, showed that the action of $SL(2,\mathbb{R})$, and its upper triangular subgroup, on strata of translation surfaces have similar rigidity properties. We will describe how some of these results fail for the horocycle flow on strata of translation surfaces. In particular, 1) There exist horocycle orbit closures with fractional Hausdorff dimension; 2) There exist points which do not equidistribute under the horocycle flow with respect to any measure; 3) There exist points which equidistribute under the horocycle flow with respect to a measure, but they are not in the topological support of that measure. No familiarity with these objects will be assumed and the talk will begin with motivating the subject of dynamics and ergodic. This is joint work with John Smillie and Barak Weiss.

Jon Chaika works in the field of Dynamical systems. He did his undergraduate at the University of Iowa, obtained his Ph.D. from Rice, then went to the University of Chicago before coming to the University of Utah.