Random Hamiltonians I: Probability measures and random walks on the Hamiltonian diffeomorphism group

Abstract

We construct a family of probability measures on the group of Hamiltonian diffeomorphisms of a closed symplectic manifold $(M,\omega)$. We show that these measures are Borel measures with respect to the topology induced by the Hofer metric. Further, we show that these measures turn any Hofer-Lipschitz function into a random variable with finite expectation. These measures have (for suitable choices of parameters) several desirable properties, such as full support on $\text{Ham}(M,\omega)$, explicit estimates of the measure of Hofer-balls, and certain controls under the action of the group. We also define a family of probability measures on the space of autonomous Hamiltonian diffeomorphisms. These measures have similar properties and give rise to a random walk on the group $\text{Ham}(M,\omega)$. Finally, we show that under certain limits this construction gives rise to probability measures on the space of Hamiltonian homeomorphisms and on the metric completion of $\text{Ham}(M,\omega)$ with respect to the Hofer metric and the spectral metric.