Conducting the cosmos through the Chandra X-ray Observatory

If a black hole could sing to us, what melody would it hum? At the centre of the Perseus galaxy cluster, its cosmic drones sound like a pod of humpback whales thousands of metres below the surface of the sea. But these ghostly moans aren’t cetaceans that have been recorded off the coast of Alaska. They’re the sound of pressure waves being sent out by a black hole, causing ripples in the cluster’s hot gas, 250 million light years away from Earth. Perseus has been “singing” to us like this for a couple of billion years.

In 2003 a team of Cambridge astronomers, studying data from NASA’s Chandra X-ray Observatory, found something extraordinary – pressure waves that could be translated into sound. It was the deepest note in the universe. A mellow B-flat, undetectable to the human ear, 57 octaves below middle C. Decades later, these pressure waves have been remixed into a pitch we can hear, 58 octaves above its original note; serenading us, you could say, across the abyss – and into our headphones.  

“There’s this idea that black holes are these monsters zipping through the universe, sucking things up like a cosmic rhombus, but there’s another side to them,” suggests Kimberly Arcand, visualisation scientist and emerging tech lead for NASA’s Chandra X-ray Observatory, as we talk about her recent project to turn spatial data from some of NASA’s greatest telescopes into music. They’re providers, she says. “They’re responsible for the care and feeding of their galaxies and there is a beautiful elegance in the way that they behave. We’re trying to capture some of that with the sonification.” 

It may seem counterintuitive to propose that any black hole – a dense object with a gravity field so strong that neither light nor sound can escape – can communicate with us this way.

“There is this very common misconception that there is no sound in space – because at least here, in our corner of the galaxy, there is no medium for a soundwave to travel to us,” Arcand tells me. “But in clusters of galaxies, for example, there can be immense amounts of superheated gas, and that gas can act as a medium that sound waves can travel through. So if there is a supermassive black hole that is belching in that cluster, it can expel matter via jets that push out into the clouds around it, causing the pressure waves or sound waves to propagate out. That’s what we’re listening to now.” 

Arcand joined the Chandra X-ray Observatory in 1998, a year before it was launched. It is the sister telescope to Hubble, she tells me, designed to detect X-ray emissions from the hottest regions of our universe; torrid and tempestuous objects and events such as colliding galaxies and exploding stars.

“Chandra offers a completely different piece of the puzzle,” she says, offering up the metaphor of a piano. “There are all these different kinds of light in the universe and if we only used human vision, it would be like only having middle C and a couple of keys either side.”

The Chandra telescope gives us so many more strings and hammers, enabling us to hear “this beautiful symphony”, as Arcand puts it, through a “multi-wavelength approach”.

Like most of us, Arcand was forced to rethink things during the Covid-19 pandemic. When her lab closed, and her students went home, she reached out to Matt Russo, an astrophysicist, musician, and sonification specialist who teaches physics at the University of Toronto. The move was largely inspired by the sonification work of her colleague and friend, Dr Wanda Díaz-Merced, an astronomer who lost her sight in her early twenties and pioneered the use of audible sound to study astrophysical data. 

Arcand thought the project would interest the low-vision community, but she wasn’t expecting the emotional response she received from others. Maybe it goes back to the pandemic, she speculates, and our collective need for escape. Or perhaps it’s more about reassurance. In a world where the only constant seems to be chaos, it doesn’t seem wild to suggest that these musical tracks are acting as a kind of salve, translating the most cryptic pockets of our universe into a language we can finally recognise and understand.  

When Arcand heard the complete version of their first sonification she was blown away by the musicality of what she was hearing: the playful violin plucks that represented the gaseous discs surrounding young stars. “You can really hear why we need different kinds of light to understand the universe,” she says.  

Head to Chandra X-ray Observatory’s YouTube page and you can hear the twinkling, effervescent melody for yourself; an interplay of three different kinds of light from the centre of our Milky Way (roughly 400 light years across in size) that can either be listened to as “solos” or together as
a group ensemble. The result is a panoramic and pulsating performance of sound: from the soft piano “detailing the cool bits of dust and gas” to the glockenspiel that animates “those high-pitched beeps and boops at the centre of that supermassive black hole”. 

Every day, for Arcand, is a symphony waiting to unfurl. “These are thousands of supermassive black holes dating back to earlier parts of the universe, you know? And that, in sound, is so much more interesting to me than just looking at a black rectangle with multicoloured dots.”

Kat Lister is a writer and editor
@Madame_George

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