Black hole ringdown spectroscopy

'Ringdown' comes from the idea where, when you hit a bell, it rings for a short period, vibrates at a certain frequency, and over time it quiets down. A black hole has a similar characteristic, except instead of sound waves, a black hole produces gravitational waves and ripples in spacetime itself. By studying the frequencies and decay rates of these waves, scientists can find the final masses of black holes, their spins, and how they behave the way general relativity predicts. Ringdown radiation has significant scientific potential because predicted quasi-normal mode frequencies and amplitudes can be compared with gravitational-wave data obtained from black-hole collisions.

The method uses 'spectroscopy' because it works similarly to spectroscopy in astronomy, where scientists study light from stars or gases to understand what they are made of. In black hole ringdown spectroscopy, scientists study spectrums of gravitational-wave vibrations from black holes. The frequencies from the black holes act as a mark of the final product of the black hole.

The ringdown phase

Binary black hole mergers are divided into 4 main stages: inspiral, plunge, merger, and ringdown. During the inspiral phase, the 2 black holes will orbit each other, and as they slowly move inward, they lose energy through gravitational waves. In the plunge phase, the 2 black holes will break out of their stable orbits and rapidly fall into each other, leading to the merger phase that combines the 2 black holes into a single distorted object in space, where the ringdown phase then settles as the product of the 2 black holes, leaving the final black hole in place.

According to general relativity, the final product from the black hole merger should eventually become a stationary and compact object with observable parameters such as mass, angular momentum, and charge. Specifically for charges, the charge for black holes is negligible, so mass and spin are only accounted for.