Abstract. Let M be a closed manifold whose based loop space Ω(M ) is "complicated". Examples are rationally hyperbolic manifolds and manifolds whose fundamental group has exponential growth. Consider a hypersurface Σ in T * M which is fiberwise starshaped with respect to the origin. Choose a function H : T * M → Ê such that Σ is a regular energy surface of H, and let ϕ t be the restriction to Σ of the Hamiltonian flow of H.
Theorem 1. The topological entropy of ϕ t is positive.This result has been known for fiberwise convex Σ by work of Dinaburg, Gromov, Paternain, and Paternain-Petean on geodesic flows. We use the geometric idea and the Floer homological technique from [19], but in addition apply the sandwiching method. Theorem 1 can be reformulated as follows.Theorem 1'. The topological entropy of any Reeb flow on the spherization SM of T * M is positive.The following corollary extends results of Morse and Gromov on the number of geodesics between two points. Corollary 1. Given q ∈ M , for almost every q ′ ∈ M the number of orbits of the flow ϕ t from Σ q to Σ q ′ grows exponentially in time.In the lowest dimension, Theorem 1 yields the existence of many closed orbits.Corollary 2. Let M be a closed surface different from S 2 , ÊP 2 , the torus and the Klein bottle. Then ϕ t carries a horseshoe. In particular, the number of geometrically distinct closed orbits grows exponentially in time.