Testing the Kerr hypothesis with orbital frequencies: lessons from spacelike singularities and black bounces
Abstract
The inner horizon of stationary black holes is subject to classical and semiclassical instabilities, suggesting that the Kerr interior should be modified. Quantum backreaction may drive the curvature to the Planck scale, producing a spacelike singularity, or halt its growth at lower curvatures, leading to a regular black bounce. We investigate the phenomenology of these two possibilities using, respectively, a Gürses--Gürsey geometry with a spacelike singularity and the rotating Simpson--Visser black bounce. Both geometries reproduce the light ring and ISCO frequencies of a Kerr black hole with the same ADM mass and angular momentum, by calibration in the first case and identically in the second. We find that the null and timelike convergence conditions are violated outside the horizon, up to a finite radius beyond the light ring in the Gürses--Gürsey geometry and everywhere in the rotating Simpson--Visser spacetime. Moreover, their fundamental scalar quasinormal modes deviate from those of Kerr, most markedly in the damping, which is enhanced in the Gürses--Gürsey geometry and reduced in the rotating Simpson--Visser one. Thus, agreement with Kerr in a limited set of observables is not sufficient to establish the Kerr nature of a compact object, motivating complementary observational tests.
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BibTeX
@misc{neshat2026testing,
title = {{Testing the Kerr hypothesis with orbital frequencies: lessons from spacelike singularities and black bounces}},
author = {Hooman Neshat and Stefano Liberati},
howpublished = {arXiv},
year = {2026},
month = nov,
doi = {10.48550/arxiv.2611.07024},
eprint = {2611.07024},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2611.07024},
}
RIS
TY - UNPB
TI - Testing the Kerr hypothesis with orbital frequencies: lessons from spacelike singularities and black bounces
AU - Hooman Neshat
AU - Stefano Liberati
PY - 2026
DA - 2026/11/04
PB - arXiv
DO - 10.48550/arxiv.2611.07024
UR - https://arxiv.org/abs/2611.07024
AB - The inner horizon of stationary black holes is subject to classical and semiclassical instabilities, suggesting that the Kerr interior should be modified. Quantum backreaction may drive the curvature to the Planck scale, producing a spacelike singularity, or halt its growth at lower curvatures, leading to a regular black bounce. We investigate the phenomenology of these two possibilities using, respectively, a Gürses--Gürsey geometry with a spacelike singularity and the rotating Simpson--Visser black bounce. Both geometries reproduce the light ring and ISCO frequencies of a Kerr black hole with the same ADM mass and angular momentum, by calibration in the first case and identically in the second. We find that the null and timelike convergence conditions are violated outside the horizon, up to a finite radius beyond the light ring in the Gürses--Gürsey geometry and everywhere in the rotating Simpson--Visser spacetime. Moreover, their fundamental scalar quasinormal modes deviate from those of Kerr, most markedly in the damping, which is enhanced in the Gürses--Gürsey geometry and reduced in the rotating Simpson--Visser one. Thus, agreement with Kerr in a limited set of observables is not sufficient to establish the Kerr nature of a compact object, motivating complementary observational tests.
ER -