They go chirp!
Or they would, if you could listen to gravitational waves. Black holes go through a long spiraling wind-down before they actually collide, and the distortions they make in space as they circle each other at high speeds winds up creating waves in space-time, very like what happens if you swirl your finger in a pond. The formation of these waves carries energy away from the two black holes, which helps the two of them merge together. Like sound waves, gravitational waves have a frequency and amplitude. These are at extremely long wavelengths, so it’s nothing the human ear could hear normally, but if you scale the frequencies up by a couple thousand, you can make a sound file of what it would sound like. And it is a chirp, or, more onomatopoetically, a vwooop!
A research group at MIT has come up with a great series of audio clips of what this would sound like, based on theoretical models. (These will come in handy, as we’d like to find direct evidence of this happening, so we need to know what to look for.)
This is what it sounds like when a black hole collides with another black hole 10000 time larger than itself, and here’s what it sounds like when it merges with one that’s only 3 times larger than itself. That little chirp at the end is the gravity-sound of the black holes colliding, and becoming one larger object.
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