Do black holes keep ringing after they stop growing?

When a black hole grows steadily, its gravitational ringdown doesn't just oscillate like a struck bell. Researchers found purely imaginary eigenvalues in the quasinormal mode spectrum that cause exponential decay without oscillation—a hidden layer of the signal. These modes dominate the late-time gravitational wave tail and connect smoothly to the familiar power-law decay seen in static black holes, confirmed through both mathematical analysis and simulations.