On the growth rate of Reeb orbit on star-shaped hypersurfaces

Abstract

In this article, we study the growth rate of Reeb orbits on fiberwise star-shaped hypersurfaces in the cotangent bundle of a closed manifold. We prove that under a suitable topological condition on the base manifold the Reeb flow on any such hypersurface carries infinitely many simple closed orbits. Moreover, the number of simple Reeb orbits with period at most T grows at least like the prime numbers, that is, like T/log(T). The topological condition we assume is the existence of a non-nilpotent class in the homology of the free loop space of the manifold, with respect to the Chas-Sullivan product, lying in a connected component associated to a non-torsion class in the first homology of the manifold. In particular, for any Riemannian metric on a manifold satisfying such a topological condition, the number of geometrically distinct closed geodesics with length at most l grows at least like l/log(l). We also prove, using symplectic homology, that if a Liouville domain of dimension at least 4 with vanishing first Chern class admits a Reeb symplectically degenerate maximum representing a non-torsion first homology class of the domain, then the number of simple Reeb orbits with period at most T grows at least like T/log(T).