Margen
In 2003, NASA's Chandra X-ray Observatory detected pressure waves rippling through the hot gas of the Perseus galaxy cluster. In other words: sound. The source was the supermassive black hole at the cluster's centre, and astronomers calculated its pitch: a B flat.
Admittedly, a B flat 57 octaves below middle C. Each oscillation takes about 9.6 million years to complete, so no ear in the universe will ever hear it — it's the deepest note ever detected.
But how can anything sound in space, if a vacuum can't carry sound? Because a galaxy cluster isn't empty: it's filled with diffuse gas, and the black hole pushes waves through it as it swallows matter, like an impossibly slow loudspeaker. It's still out there now, holding the note it has sustained for millions of years. The universe, too, has its pedal note.