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Reliable computation of quasinormal modes for very highly spinning black holes in modified gravity

Determine quasinormal mode frequencies for black holes with dimensionless spin chi greater than or approximately equal to 0.95 in modified gravity theories (including effective field theory extensions and scalar-tensor theories), developing reliable methods that remain accurate in this near-extremal regime where existing approaches fail.

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Background

The paper reviews substantial recent progress in computing quasinormal modes for rotating black holes in effective field theories and scalar-tensor theories, using approaches such as the modified Teukolsky formalism, metric reconstruction, and spectral methods.

Despite these advances, the near-extremal spin regime (chi ≳ 0.95) remains technically challenging: numerical and perturbative methods used so far suffer from reliability issues as the black hole approaches extremal rotation.

A robust treatment of this regime is essential for tests of gravity using high-spin remnants expected in current and next-generation gravitational-wave observations.

References

"An important open question is the case of very highly rotating BHs, χ≳0.95. None of the methods reviewed above has achieved reliable results in this regime yet, as this is technically challenging and may require alternative approaches."

— Black hole spectroscopy: from theory to experiment (2505.23895 - Berti et al., 29 May 2025) in Subsection "Future directions" in Section 4.1 (Theory-specific spectra)