Ordered Probability Spaces

Abstract: Let $C$ be an open cone in a Banach space equipped with the Thompson metric with closure a normal cone. The main result gives sufficient conditions for Borel probability measures $\mu,\nu$ on $C$ with finite first moment for which $\mu\leq \nu$ in the stochastic order induced by the cone to be order approximated by sequences ${\mu_n},{\nu_n}$ of uniform finitely supported measures in the sense that $\mu_n\leq \nu_n$ for each $n$ and $\mu_n\to \mu$, $\nu_n\to \nu$ in the Wasserstein metric. This result is the crucial tool in developing a pathway for extending various inequalities on operator and matrix means, which include the harmonic, geometric, and arithmetic operator means on the cone of positive elements of a $C^*$-algebra, to the space $\mathcal{P}^1(C)$ of Borel measures of finite first moment on $C$. As an illustrative particular application, we obtain the monotonicity of the Karcher geometric mean on $\mathcal{P}^1(\mathbb{A}^+)$ for the positive cone $\mathbb{A}^+$ of a $C^*$-algebra $\mathbb{A}$.