Spectral rigidity of manifolds with Ricci bounded below and maximal bottom spectrum

Abstract: We investigate the spectrum of the Laplacian on complete, non-compact manifolds $M^n$ whose Ricci curvature satisfies $\mathrm{Ric} \geq -(n-1)\mathrm{H}(r)$, for some continuous, non-increasing $\mathrm{H}$ with $\mathrm{H}-1 \in L^1(\infty)$. The class includes asymptotically hyperbolic manifolds with a mild curvature control at infinity. We prove that if the bottom spectrum attains the maximal value $\frac{(n-1)^2}{4}$ compatible with the curvature bound, then the spectrum of $M$ matches that of hyperbolic space $\mathbb{H}^n$, namely, $\sigma(M) = \left[ \frac{(n-1)^2}{4}, \infty \right)$. The result can be localized to an end $E$ with infinite volume.