Kinetic Brownian motion on Riemannian manifolds

We consider in this work a one parameter family of hypoelliptic diffusion processes on the unit tangent bundle T-1 M of a Riemannian manifold (M,g), collectively called kinetic Brownian motions, that are random perturbations of the geodesic flow, with a parameter sigma quantifying the size of the noise. Projection on M of these processes provides random C-1 paths in M. We show, both qualitively and quantitatively, that the laws of these M-valued paths provide an interpolation between geodesic and Brownian motions. This qualitative description of kinetic Brownian motion as the parameter sigma varies is complemented by a thourough study of its long time asymptotic behaviour on rotationally invariant manifolds, when sigma is fixed, as we are able to give a complete description of its Poisson boundary in geometric terms.