Complete 3-manifolds of positive scalar curvature with quadratic deca

Abstract

We prove that if an orientable 3-manifold M admits a complete Riemannian mètric whose scalar curvature is positive and has a subquadratic decay at infinity, then it decomposes as a (possibly infinite) connected sum of spherical manifolds and S2×S1 summands. This generalises a theorem of Gromov andWang by using a different,more topological, approach. As a result, the manifold M carries a complete Riemannian metric of uniformly positive scalar curvature, which partially answers a conjecture of Gromov. More generally, the topological decomposition holds without any scalar curvature assumption under a weaker condition on the filling discs of closed curves in the universal cover based on the notion of fill radius. Moreover, the decay rate of the scalar curvature is optimal in this decomposition theorem. Indeed, the manifold R2×S1 supports a complete metric of positive scalar curvature with exactly quadràtic decay, but does not admit a decomposition as a connected sum.