Probing quadratic gravity with black-hole ringdown gravitational waves measured by LIGO-Virgo-KAGRA detectors

Quadratic gravity theories emerge as the low-energy limit of many grand-unified and quantum-gravity theories. We report the first gravitational-wave ringdown constraints on such theories obtained purely from the quasinormal-mode spectra of rapidly-spinning black-hole remnants. We find no quadratic-gravity signatures from ringdown signals measured by the LIGO-Virgo-KAGRA detectors, thereby constraining the coupling length scale of axi-dilaton gravity below 34 km, dynamical Chern-Simons gravity below 49 km, and scalar Gauss-Bonnet gravity below 47 km.