Mode-Dependent Ringdown Inference in Binary Black Hole Mergers

This work investigates mode-dependent parameter inference in the ringdown phase of binary black hole mergers observed by the LIGO–Virgo–KAGRA collaboration. Independent estimates of the remnant mass and spin are obtained using different quasi-normal modes, with emphasis on comparisons between the dominant $(2,2)$ mode and higher-order harmonic modes such as $(3,3)$. Across multiple high signal-to-noise ratio events, a recurring and directional pattern is observed in which higher-order modes systematically favor slightly lower final spins than those inferred from the dominant mode. While all results remain statistically consistent with the Kerr black hole model within current uncertainties, the repeated nature of the offsets suggests the presence of a non-instantaneous post-merger relaxation toward the final stationary state. The results are discussed in a general theoretical context and shown to be naturally compatible with models in which the emergence of classical geometry occurs dynamically rather than instantaneously.