Largest projections for random walks and shortest curves in random mapping tori

Abstract

We show that the largest subsurface projection distance between a marking and its image under the nth step of a random walk grows logarithmically in n, with probability approaching 1 as n tends to infinity. Our setup is general and also applies to (relatively) hyperbolic groups and to $\mathrm{Out}(F_n)$. We then use this result to prove Rivin's conjecture that for a random walk $(w_n)$ on the mapping class group, the shortest geodesic in the hyperbolic mapping torus $M_{w_n}$ has length on the order of $1/{\log }^2(n)$.

Version 2: Added application to systole of random mapping tori. 19 pages. To appear in Math. Res. Lett