A Musical Universe
At the heart of the Perseus Cluster, 250 million years distant, lies a supermassive black hole that has been singing the same song for 2.5 billion years. Supermassive black holes are monstrous objects commonly found in the centres of galaxies, up to tens of billions of times more massive than our sun, but the common idea that nothing can escape a black hole is wrong. Some matter can actually be violently expelled as “relativistic jets”, surging out at a black hole’s poles. Normally there’s no sound in space because there’s no medium for the pressure waves to travel through, but in some places, such as in the Perseus Cluster, there’s hot gas, allowing the jets to slam into it and propagate through like a galactic drum. We can’t hear the waves, but we can see the peaks and troughs—the Chandra X-Ray Observatory has observed concentric ripples of bright and dark gas, indicating sound waves. The supermassive black hole plays one note registering 57 octaves below middle C—a B-flat—and the waves have a frequency of 10 million years, so they’re one million billion times lower than the lowest sound audible to the human ear. What’s interesting is that these waves keep the gas in the cluster warmer than it usually would be, affecting the rate of star formation, and so these black hole “songs” could help us understand the evolution and growth of the biggest structures in the universe: galaxy clusters.