For whom the black hole rings – November 29, 2023

This diagram shows the frequencies (horizontal axis) and damping times (vertical axis) of the two tones observed in the decay of GW190521 as colored areas. The search found two tones consistent with the predictions of general relativity.
Observation of multiple ringdown modes in a black hole merger
An international team led by researchers from the Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) in Hannover has found strong observational evidence for multiple gravitational-wave frequencies in a binary black hole ringdown. The team discovered that the intermediate-mass black hole formed in the GW190521 event vibrated briefly at at least two frequencies after the merger. This ringdown is a fundamental prediction from general relativity. Its observation allows tests of the theory and of the black hole no-hair theorem. The scientists found no violations of the theorem or deviations from general relativity. It was widely assumed that this observation of multiple tones would be impossible before the next generation of gravitational-wave detectors. Nevertheless, the unexpected massive merger remnant of GW190521 together with exquisite data analysis methods make the detection possible. The results were published in Physical Review Letters.