Random Ideal Hyperbolic Quadrilaterals, the Cross Ratio Distribution and Punctured Tori

Abstract

Earlier work introduced a geometrically natural probability measure on the group of all M\"obius transformations of the hyperbolic plane so as to be able to study "random" groups of M\"obius transformations, and in particular random two-generator groups. Here we extend these results to consider random punctured tori. These Riemann surfaces have finite hyperbolic area $2\pi$ and fundamental group the free group of rank 2. They can be obtained by pairing (identifying) the opposite sides of an ideal hyperbolic quadrilateral. There is a natural distribution on ideal quadrilateral given by the cross ratio of their vertices. We identify this distribution and then calculate the distributions of various geometric quantities associated with random punctured tori such as the base of the geodesic length spectrum and the conformal modulus, along with more subtle things such as the distribution of the distance in Teichm\"uller space to the central "square" punctured torus.