Universal non-CD of sub-Riemannian manifolds

TL;DR

This paper demonstrates that sub-Riemannian manifolds with full-support Radon measures are not $ ext{CD}(K,N)$ unless Riemannian, and introduces cone-Grushin spaces with sub-Riemannian-like features that lack a scalar product.

Contribution

It generalizes non-$ ext{CD}$ results to broader measures and constructs new $ ext{RCD}$ structures called cone-Grushin spaces with unique geometric properties.

Findings

Sub-Riemannian manifolds with full-support Radon measures are never $ ext{CD}(K,N)$ unless Riemannian.

Constructed cone-Grushin spaces exhibit sub-Riemannian features without being true sub-Riemannian.

Analysis of tangent cones and geodesics underpins the non-$ ext{CD}$ results.

Abstract

We prove that a sub-Riemannian manifold equipped with a full-support Radon measure is never $\mathrm{CD}(K,N)$ for any $K\in \mathbb{R}$ and $N\in (1,\infty)$ unless it is Riemannian. This generalizes previous non-CD results for sub-Riemannian manifolds, where a measure with smooth and positive density is considered. Our proof is based on the analysis of the tangent cones and the geodesics within. Secondly, we construct new $\mathrm{RCD}$ structures on $\mathbb{R}^n$, named cone-Grushin spaces, that fail to be sub-Riemannian due to the lack of a scalar product along a curve, yet exhibit characteristic features of sub-Riemannian geometry, such as horizontal directions, large Hausdorff dimension, and inhomogeneous metric dilations.