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

Abstract

Recent numerical relativity simulations of mergers of binary black holes suggest that the maximum final value of a/Ma/M is ~0.95 for the coalescence of two equal-mass black holes with aligned spins of the same magnitude a/M=0.994a/M=0.994, which is close to the upper limit a/M=0.998a/M=0.998 of accretion spin-up shown by Thorne [Astrophys. J. 191, 507 (1974)]. Using the Wentzel–Kramers–Brillouin method, we suggest that, if quasinormal modes with a/M∼0.95a/M∼0.95 are detected by 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 ~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 that deviates from general relativity only near the black hole horizon.