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

TL;DR

Detecting quasinormal modes with high spin parameter could confirm the strong gravity regime near black holes and test Einstein's general relativity in the ergoregion using gravitational wave observations.

Contribution

The paper proposes using WKB analysis of quasinormal modes at high spin to test the strong gravity regime and Einstein's theory near black hole horizons.

Findings

Detection of quasinormal modes at $a/M \\sim 0.95$ can confirm strong gravity predictions.

Potential to verify Einstein's general relativity up to $1.33M$ from the black hole center.

If deviations are observed, it could indicate new physics beyond general relativity.

Abstract

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.