Astrophysics & Particle Physics-APPC 2026

Black hole spectroscopy is a state-of-the-art method of astrophysics that observes the signals of gravitational wave "ringdown" that occur after two black holes merge. When two black holes crash, the resulting black hole rings out like a bell, releasing gravitational waves at specific frequencies called quasinormal modes (QNMs). A study of these modes enables scientists to quantify the black hole's mass, spin, and other basic features.The idea is similar to atomic spectroscopy, in which light from electrons provides information about atomic structure. Black hole spectroscopy employs the gravitational wave spectrum to study the extreme gravity regime that Einstein's general relativity predicts. Finding deviations from expected quasinormal modes may signal new physics beyond general relativity or exotic compact objects.Dezes of black hole mergers have already been detected by observatories such as LIGO, Virgo, and KAGRA, and detailed analysis of their ringdown parts gives a vast amount of information.

Next-generation gravitational wave detectors, such as Einstein Telescope and Cosmic Explorer, will offer more accurate black hole spectroscopy, with which scientists can test the fundamental laws of physics in the strong-field regime.Black hole spectroscopy will also have a crucial role in multi-messenger astronomy. With the addition of gravitational wave observations and possible electromagnetic counterparts, astrophysicists can decipher black hole formation, merger environment, and relationships to phenomena like gamma-ray bursts.Black hole spectroscopy is a revolutionary instrument in contemporary astrophysics. By interpreting the gravitational wave "ringing" of merged black holes, it offers an unprecedented window into the physics of extreme gravity, black hole properties, and cosmic evolution.