Black-hole spectroscopy from a giant quantum vortex

Black-hole spectroscopy aims to infer physical properties of black holes by detecting the spectrum of quasinormal modes (QNMs) they emit while settling towards equilibrium. Unlike normal modes, which are resonances of energy-conserving systems, QNMs are damped oscillations arising when a system loses energy due to open boundaries or via dissipation. The detection of the full QNM spectrum of black holes is challenging due to rapidly decaying amplitudes of these resonances, limiting observations only to the longest-lived mode. Theoretical and numerical studies suggest that environmental confinement due to surrounding plasma or dark matter modify the QNM spectrum. Here, we employ black-hole spectroscopy to show how spatial confinement similarly affects the spectrum of nanometre-scale interface waves surrounding a giant quantum vortex in superfluid helium-4, an experimentally accessible quantum system that emulates dynamics in rotating curved spacetime. In the available parameter space, we observe regimes in which multiple QNMs emerge from the interface noise spectrum. In agreement with theoretical predictions, their real and imaginary frequencies are shifted with respect to those expected in the unbounded system. Our results demonstrate the critical role of spatial confinement in shaping the QNM spectrum, highlighting the importance of environmental effects on spectral stability of astrophysical compact objects.