Hypercontractivity of the heat flow on ${\sf RCD}(0,N)$ spaces: sharpness and equality

Abstract: Inspired by Bakry [Lecture Notes in Math., 1994], we prove sharp hypercontractivity bounds of the heat flow $({\sf H}t){t\geq 0}$ on ${\sf RCD}(0,N)$ metric measure spaces. The best constant in this estimate involves the asymptotic volume ratio, and its optimality is obtained by means of the sharp $L^2$-logarithmic Sobolev inequality on ${\sf RCD}(0,N)$ spaces. If equality holds in this sharp estimate for a prescribed time $t_0>0$ and a non-zero extremizer $f$, it turns out that the ${\sf RCD}(0,N)$ space has an $N$-Euclidean cone structure and ${\sf H}_{t_0}f$ is a Gaussian whose dilation factor is reciprocal to $t_0$ up to a multiplicative constant. Our results are new even on complete Riemannian manifolds with non-negative Ricci curvature, where a full characterization is provided for the equality case in the sharp hypercontractivity estimate of the heat flow.