The detection of quasinormal mode with $a/M \sim 0.95$ would prove a sphere $99\%$ soaking in the ergoregion of the Kerr space-time

Understand

Recent numerical relativity simulations of mergers of binary black holes suggest that the maximum final value of $a/M$ is $\sim 0.95$ for the coalescence of two equal mass black holes with aligned spins of the same magnitude $a/M=0.994$ which is close to the upper limit $a/M=0.998$ of accretion spin-up shown by Thorne.

• Using the WKB method, we suggest that if quasinormal modes with $a/M \sim 0.95$ are detected by the second generation gravitational wave detectors, we could confirm the strong gravity space-time based on Einstein's general relativity up to $1.33M$ which is only $\sim 1.014$ times the event horizon radius and within the ergoregion.
• One more message about black hole geometry is expected here.
• If the quasinormal mode is different from that of general relativity, we need to find the true theory of gravity which deviates from general relativity only near the black hole horizon.