A rapidly spinning black hole, where spacetime is curved to the limit, resembles a bell that can hum for centuries. Its special oscillations barely decay, losing energy incredibly slowly.
A black hole, like a spinning wheel, twists a cloud of light particles, axions, around itself. These particles are the leading candidates for dark matter. The cloud follows the laws of quantum mechanics, resembling a giant atom: there are levels, and there are transitions between them.
The LIGO detectors caught the signal GW250114—a gravitational wave from the merger of two black holes. After the main burst, a decaying 'tail' remained — direct waves from the newborn hole's horizon. It was expected that their damping would reveal the gravitational strength at the horizon's surface. But the measured value came out smaller.
Imagine a quiet room where you need to pick out a single melody among many barely audible rustles. That's roughly what it's like to search for signals from extreme systems of two black holes — one huge, the other millions of times smaller.