In this paper we propose a class of local definitions of weak lower scalar curvature bounds that is well defined for C 0 metrics. We show the following: that our definitions are stable under greater-than-second-order perturbation of the metric, that there exists a reasonable notion of a Ricci flow starting from C 0 initial data which is smooth for positive times, and that the weak lower scalar curvature bounds are preserved under evolution by the Ricci flow from C 0 initial data.Date: September 12, 2019.
Theorem 1.5 implies:Corollary 1.6. If (M, g) is a closed Riemannian manifold with C 0 metric g, and if there exists β ∈ (0, 1/2) such that, at every point in M , g has scalar curvature bounded below by κ in the β-weak sense, then there exists a sequence of C 2 metrics on M with scalar curvature bounded below by κ that converges uniformly to g.Theorem 1.5 also implies:Theorem 1.7. Let g be a C 0 metric on a closed manifold M which admits a uniform approximation by C 0 metrics g i such that, for some β ∈ (0, 1/2), g i has scalar curvature bounded below by κ i in the β-weak sense everywhere on M , where κ i is some sequence of numbers such that κ i − −− → i→∞ κ for some number κ. Then g has scalar curvature bounded below by κ in the β-weak sense. In particular, any regularizing Ricci flow (g(t)) t∈(0,T ] for g satisfies R(g(t)) ≥ κ for all t ∈ (0, T ], so g admits a uniform approximation by smooth metrics with scalar curvature bounded below by κ.As a corollary of Theorem 1.7, we may answer the following question, posed by Gromov in [8]:Question 1 ([8, Page 1119]). Let g be a continuous Riemannian metric on a closed manifold M which admits a C 0 -approximation by smooth Riemannian metrics g i with R(g i ) ≥ −ε i − −− → i→∞ 0. Does M admit a smooth metric of nonnegative scalar curvature?By setting κ i = −ε i in Theorem 1.7, we obtain the following response:Corollary 1.8. If (M, g) is as in Question 1, then any regularizing Ricci flow (g t ) t∈(0,T ] for g satisfies R(g t ) ≥ 0 for all t ∈ (0, T ]. In particular, M admits a smooth metric of nonnegative scalar curvature, and moreover, g admits a uniform approximation by smooth metrics with nonnegative scalar curvature.We use similar methods to show a torus rigidity result, motivated by the scalar torus rigidity theorem, which was first proven by Schoen and Yau [14] for dimensions ≤ 7, and later proven by Gromov and Lawson [7] for all dimensions, and which says that any Riemannian manifold with nonnegative scalar curvature that is diffeomorphic to the torus must be isometric to the flat torus. We show: Corollary 1.9. Suppose g is a C 0 metric on the torus T, and that there is some β ∈ (0, 1/2) such that g has nonnegative scalar curvature in the β-weak sense everywhere. Then (T, g) is isometric as a metric space to the standard flat metric on T.Remark 1.10. Corollary 1.9 is in fact the optimal result, i.e. it is not possible to show that there is a Riemannian isometry between g and the standard flat metric. In the case where g 1 and g 2 are smooth metrics, a metric space...