Positive and negative results concerning the Gromov-Lawson-Rosenberg conjecture

Abstract

The Gromov-Lawson-Rosenberg-conjecture for a group G states that a closed spin manifold M^n (n>4) with fundamental group G admits a metric with positive scalar curvature if and only if its C^*-index A(M) in KO_n(C^*_r(G)) vanishes. We prove this for groups G with low-dimensional classifying space, provided the assembly map for G is injective. On the other hand, we construct a spin manifold with no metric with scal>0 but so that already its KO-orientation in KO_*(B pi_1(M)) vanishes. Therefore a corresponding weakened version or the GLR-conjecture is wrong. Last we address non-orientable manifolds. We give a reformulation of the minimal surface method of Schoen and Yau (extended to dimension 8) and introduce a non-orientable version of it. We then construct a manifold M whose orientation cover admits a metric of positive scalar curvature but where M itself does not. M is also a counterexample to a twisted analog of the GLR-conjecture because its twisted index vanishes.