Equivalence of two different notions of tangent bundle on rectifiable metric measure spaces
Nicola Gigli, Enrico Pasqualetto
Abstract
We prove that for a suitable class of metric measure spaces, the abstract notion of tangent module as defined by the first author can be isometrically identified with the space of $L^2$-sections of the `Gromov-Hausdorff tangent bundle'. The class of spaces $({\rm X},{\sf d},{\mathfrak m})$ we consider are PI spaces that for every $\varepsilon>0$ admit a countable collection of Borel sets $(U_i)$ covering ${\mathfrak m}$-a.e. ${\rm X}$ and corresponding $(1+\varepsilon)$-biLipschitz maps $\varphi_i:U_i\to{\mathbb R}^{k_i}$ such that $(\varphi_i)_*{\mathfrak m}\lower3pt\hbox{$|_{U_i}$}\ll\mathcal L^{k_i}$. This class is known to contain ${\sf RCD}^*(K,N)$ spaces. Part of the work we carry out is that to give a meaning to notion of $L^2$-sections of the Gromov-Hausdorff tangent bundle, in particular explaining what it means to have a measurable map assigning to ${\mathfrak m}$-a.e. $x\in {\rm X}$ an element of the pointed-Gromov-Hausdorff limit of the blow-up of ${\rm X}$ at $x$.