Quasinormal modes of near-extremal Reissner-Nordstr\"om-de Sitter spacetimes

We study quasinormal modes (QNMs) for the Klein-Gordon equation on Reissner-Nordstr\"om-de Sitter black holes with near-extremal charge. We locate all QNMs of size $\mathcal{O}(\kappa_{\rm C})$ where $\kappa_{\rm C}$ is the surface gravity of the Cauchy horizon (which vanishes at extremality): they are well-approximated by $\kappa_{\rm C}$ times QNMs of the near-horizon geometry ${\rm AdS}^2\times{\mathbb S}^2$ of the extremal limit.