We show that dynamical dark energy theories can imprint O(1) modifications on the quasi-normal mode (QNM) spectrum characterising black hole ringdown. The time dependence of dynamical dark energy naturally gives rise to cosmological 'hair' around a black hole. Taking the cubic Galileon as a concrete example, which admits the only known stable solution of this kind, we parametrically connect the cosmological and black hole regimes, derive the induced QNM shifts and forecast the resulting dark energy constraints. We find that the dark energy field profile can be constrained with an accuracy of up to 10−2 for LVK and 10−4 for LISA.
@article{arxiv.2603.23634,
title = {Testing Dark Energy with Black Hole Ringdown},
author = {Laurens Smulders and Johannes Noller and Sergi Sirera},
journal= {arXiv preprint arXiv:2603.23634},
year = {2026}
}