In the dark heart of the Perseus galaxy cluster, 300 million light-years from Earth, a supermassive black hole has been singing the same note for 2.5 billion years. Its tone registers 57 octaves below middle C and, according to scientists at NASA's Chandra X-Ray Center, is a resounding B-flat. Yet, how is this possible in the vacuum of space?
Sound requires a medium, such as water or air, to travel. Here on Earth a sound wave moves from its origin by causing the surrounding air molecules to vibrate. The vibrations pass from one molecule to another; when they hit an ear, they are understood as noise. But because neither air nor water nor much of anything else exists in the majority of vast reaches of space, it is difficult for sound to travel there.
It takes a supermassive black hole—like a robust opera diva—to sing a resonant note in space. These monstrous celestial objects range from hundreds of thousands to tens of billions times our sun's mass and are commonly found in the center of active galaxies. For example, Sagittarius A*—a supermassive black hole—sits at the center of our own galaxy, the Milky Way.
Black holes are notorious for their gravitational might, which is so strong that nothing can escape, according to conventional wisdom. But this isn't quite correct—some matter does. A black hole's gravity pulls a mishmash of matter and energy into its surrounding accretion disk—a ringlike structure formed by gas and dust. But some of this matter is violently expelled from the black hole's poles as "relativistic jets." These jets surge into the scorching gas surrounding the hole and generate pockets in the otherwise uniform cloud.
"Sound waves are pressure waves. And black holes, or at least their relativistic jets, can generate enormous sound waves, which then propagate through surrounding galactic gas," explains astronomer Steven Allen, a professor of physics at Stanford University who studies the Perseus galaxy cluster. "When relativistic jets, which contain material moving at close to the speed of light, slam into the hot gas that pervades giant elliptical galaxies and clusters of galaxies, they beat a 'galactic drum,' as it were." The jet acts as the "stick," whereas the surface of the gas is the "drum."
Although people can't hear these waves (because sound can't travel through the vast vacuum separating this "drum" and us), we can "see" them using x-ray observations. As sound waves spread through the scorching gas in galaxies and galaxy clusters, regions of greater pressure (sound wave peaks) tend to appear brighter in x-rays; fainter regions (troughs) are dimmer.
Chandra x-ray telescope observations of the Perseus Cluster show roughly concentric ripples of brighter and fainter gas, which indicate sound waves. "We can't see the waves moving," Allen says. "The relevant timescales are too long, since the period of the waves is about 10 million years—but we have a clear 'snapshot' of them."
Perseus' black hole is not the universe's sole galactic vocalist. M87, a galaxy that holds one of the universe's most massive black holes, is also known to croon. Although its song isn't as steady as Perseus', it is more involved, with notes as deep as 59 octaves below middle C.
"There's no reason for black holes to sing the same note," says Peter Edmonds, an astrophysicist at the Chandra X-Ray Center. Galaxies that have more matter may provide a deeper sound, because this matter could lead to bigger, but less common eruptions from the black hole. There are bound to be other important factors contributing to a black hole's specific sound, such as the temperature of the gas and its location, but the details aren't well understood, says Edmonds.
Other interstellar objects and events produce sound waves as well, he adds. In fact, the echoes of the big bang have been humming and hissing since shortly after the universe's birth.