Do remnant black hole ringdown tones match Kerr at percent precision?
In plain words
A newly merged black hole rings like a bell at frequencies fixed by its mass and spin if it is a Kerr black hole. Measuring two or more tones in one event tests this directly.
Precise statement
For merger remnants, measure the complex frequencies of at least two independent quasinormal modes, for example $(l,m,n) = (2,2,0)$ together with $(2,2,1)$, $(3,3,0)$ or $(4,4,0)$, and constrain the fractional deviations $\delta f$ and $\delta \tau$ from Kerr values for a common mass and spin. The target is $\mid\delta f\mid, \mid\delta \tau\mid$ below 0.01. An answer is the measured deviations with uncertainties from a catalog of events.
What would settle it
Multi-mode ringdown measurements with percent-level frequency and damping-time constraints from next-generation ground detectors or LISA.
Status in the literature
Unverified note
GW250114 gave two modes at $4.1\,\sigma$ with the overtone frequency consistent with Kerr to plus or minus $30\,\text{percent}$ and the damping rate unconstrained (LIGO-Virgo-KAGRA 2025).