On the non-collapsed RCD spaces with local bounded covering geometry

TL;DR

This paper proves that RCD spaces with local bounded covering geometry are biHölder homeomorphic to infranil-manifolds under small diameter conditions, and establishes a regular fibration theorem for converging RCD spaces.

Contribution

It extends Gromov's almost flat manifold theorem to RCD spaces and confirms the conjecture in the RCD+CBA setting, also providing a regular fibration result for converging RCD spaces.

Findings

RCD spaces with small diameter are biHölder homeomorphic to infranil-manifolds.

The Gromov almost flat manifold theorem holds in the RCD+CBA setting.

Sequences of RCD spaces admit fibrations with infra-nilmanifold fibers over smooth manifolds.

Abstract

We consider a RCD$(-(N-1),N)$ space $(X,d,\mathcal{H}^N)$ with local bounded covering geometry. The first result is related to Gromov's almost flat manifold theorem. Specifically, if for every point $\tilde{p}$ in the universal cover $\widetilde{X}$, we have $\mathcal{H}^N(B_1(\tilde{p})) \ge v > 0$ and the diameter of $X$ is sufficiently small, then $X$ is biH\"{o}lder homeomorphic to an infranil-manifold. Moreover, if $X$ is a smooth Riemannian $N$-manifold with $\mathrm{Ric} \ge -(N-1)$, then $X$ is biH\"{o}lder diffeomorphic to an infranil-manifold. An application of our argument is to confirm the conjecture that Gromov's almost flat manifold theorem holds in the $\mathrm{RCD}+\mathrm{CBA}$ setting. The second result concerns a regular fibration theorem. Let $(X_i,d_i,\mathcal{H}^N)$ be a sequence of RCD$(-(N-1),N)$ spaces converging to a compact smooth $k$-dimensional manifold $K$ in the Gromov-Hausdorff sense. Assume that for any $p_i \in X_i$, the local universal cover is non-collapsing, i.e., for any pre-image point $\tilde{p}_i$ of $p_i$ in the universal cover of the ball $B_3(p_i)$, we have $\mathcal{H}^N(B_{1}(\tilde{p}_i)) \ge v$ for some fixed $v>0$. Then for sufficiently large $i$, there exists a fibration map $f_i:X_i \to K$, where the fiber is an infra-nilmanifold and the structure group is affine.