Analysis of near-extremal quasinormal modes beyond the eikonal limit in higher-derivative gravity

Develop a computational framework to analyze and compute the quasinormal modes of near-extremal Kerr black holes with higher-derivative corrections to general relativity outside the eikonal (large-l) approximation, enabling accurate treatment of finite-l modes and arbitrary rotation to quantify spectrum modifications such as phase-boundary shifts between damped and zero-damping modes.

Background

The paper establishes large higher-derivative effects on the quasinormal-mode spectrum of highly rotating black holes using an eikonal (large-l) analysis enabled by an isospectral quartic-curvature effective field theory. These results include amplified corrections near the phase boundary separating damped and zero-damping modes, and potential changes in the number of damped modes.

However, current methods used in the paper apply only in the eikonal regime. A comprehensive understanding relevant for observational ringdown tests requires extending the analysis to low-l modes and arbitrary rotation, which the authors identify as beyond their present capabilities.