# Wasserstein geometry and Ricci curvature bounds for Poisson spaces

## TL;DR

This paper explores the geometric structure of Poisson spaces using optimal transport and Ricci curvature bounds, establishing a Riemannian framework, a Wasserstein-like distance, and key geometric inequalities.

## Contribution

It introduces a formal Riemannian structure and a Wasserstein-type distance on Poisson space measures, extending geometric analysis tools to this infinite-dimensional setting.

## Key findings

- Poisson space has Ricci curvature bounded below by 1 in the entropic sense.
- The Ornstein-Uhlenbeck semi-group is the gradient flow of the relative entropy.
- The extended distance satisfies an HWI inequality.

## Abstract

Let $\varUpsilon$ be the configuration space over a complete and separable metric base space, endowed with the Poisson measure $\pi$. We study the geometry of $\varUpsilon$ from the point of view of optimal transport and Ricci-lower bounds. To do so, we define a formal Riemannian structure on $\mathscr{P}_{1}(\varUpsilon)$, the space of probability measures over $\varUpsilon$ with finite first moment, and we construct an extended distance $\mathcal{W}$ on $\mathscr{P}_{1}(\varUpsilon)$. The distance $\mathcal{W}$ corresponds, in our setting, to the Benamou--Brenier variational formulation of the Wasserstein distance. Our main technical tool is a non-local continuity equation defined via the difference operator on the Poisson space. We show that the closure of the domain of the relative entropy is a complete geodesic space, when endowed with $\mathcal{W}$. We establish non-local infinite-dimensional analogues of results regarding the geometry of the Wasserstein space over a metric measure space with synthetic Ricci curvature bounded below. In particular, we obtain that: (a) the Ornstein--Uhlenbeck semi-group is the gradient flow of the relative entropy; (b) the Poisson space has a Ricci curvature, in the entropic sense, bounded below by $1$; (c) the distance $\mathcal{W}$ satisfies an HWI inequality.

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Source: https://tomesphere.com/paper/2303.00398