M87 is one noisy galaxy. Scientists detected sonic booms coming from a black hole in the galaxy . . . and it's not a scream for help as the event horizon approaches. (Photo courtesy NASA/CXC/CfA/W. Forman et al.)

M87 is one noisy galaxy. Scientists detected sonic booms coming from a black hole in the galaxy . . . and it's not a scream for help as the event horizon approaches. (Photo courtesy NASA/CXC/CfA/W. Forman et al.)


The Sound of Silence

By Eric Bland

They say love is blind, and I loved Star Wars as a kid (ok, still do). And love will make you believe in pretty crazy ideas. Like the idea that you can hear sound in space. But those twanging lasers and the eerie howl of TIE fighters chasing rebel X-wings around the Death Star were all lies (even though they sounded pretty cool). But space is a vacuum, and sound, unlike light, needs some kind of medium, be that solid, liquid, gas, or plasma, for it to travel through. Since all available mediums are absent in the vastness of outer space, sound can’t exist there.

Or can it?

As it turns out, sound can, and does, exist in space. Sound is just a matter of differences in pressure, and those exist aplenty in space. Space sounds might actually hold the keys to some of the more important questions in the universe, such as what happens to matter as it is sucked into a black hole and how the entire universe settled into its present configuration. And the Big Bang? Despite its acoustically prejudiced name, it didn’t generate a whisper. 

Several weeks ago, researchers from the Chandra X-ray Observatory reported detecting sonic booms coming out of a super-massive black hole in the Virgo constellation.  The galaxy it’s located in is known as M87. The booms were the result of matter rubbing together violently just before disappearing forever into the black hole. The friction generates waves of pressure that appear as loops and rings in the gas surrounding the black hole. Since sound is just alternating waves of compression and rarefaction, like the pressure waves surrounding M87, sound can propagate. The friction in the gas is so great that it also generates X-rays, which are what the Chandra Observatory actually detected.  It’s not like there’s an oversized microphone orbiting the Earth.

Using the data, Chandra’s scientists actually calculated that the sound of M87 would be about 56 octaves below middle C. But a deep cavity in the center of the gas cloud suggests that even deeper notes, about 58 or 59 octaves below middle C, could exist. These pressure waves, which are generated about every six million years, shake things up and keep the cloud from settling out and becoming new stars and planets.

This follows up on research Chandra released in 2003 when they first calculated the sound of a black hole in the Perseus cluster to be about 57 octaves below middle C.

One of the more interesting sides of space sound is found in the cosmic microwave background present no matter where you look in the sky. These microwaves are the left-over light from the Big Bang (which we’ll get to in a moment). Over the last decade and a half, more advanced telescopes, like NASA’s COBE and WMAP satellites,  have picked up subtle difference in the microwave background. The differences are so slight that it would be like measuring the height of a bacteria sitting on a bowling ball versus a clean ball. By measuring these minor differences, scientists, like Mark Whittle from the University of Virginia, made simulated recordings of what the cooling universe would have sounded like. In one sense, it’s the sound of light.

A few hundred thousand years (or, to put it in a human perspective, about the first 12 hours after conception) after the Big Bang, the universe’s sound begins with a descending scream, builds into a deep, rasping roar, and ends in a deafening hiss. Each part corresponds to a different age of the universe: the highest pitches gave birth to the first generation of stars, with the bass notes turning into the various galaxies that fill outer space. The volume of this cacophony would have been about 110 decibels, or about the same volume as a rock concert, but the pitch would have been to low for human ears to detect. At this point, the universe was still small enough and dense enough that sound waves could travel through it.

And the Big Bang itself? That early the universe was too hot and too uniform for any pressure differences to form that could have produced sound. It was only about 380,000 years later that matter clumped together enough for areas of high and low pressure to form and create the hissing, roaring, and screaming universe.