Random walks on almost connected locally compact groups: Boundary and convergence

We prove that, given an arbitrary spread out probability measure mu on an almost connected locally compact second countable group G, there exists a homogeneous space G/H, called the mu-boundary, such that the space of bounded mu-harmonic functions can be identified with L-infinity(G/H). The mu-boundary is an amenable contractive homogeneous space. We also establish that the canonical projection onto the mu-boundary of the right random walk of law mu always converges in probability and, when G is amenable, it converges almost surely. The mu-boundary can be characterised as the largest homogeneous space among those homogeneous spaces in which the canonical projection of the random walk converges in probability.