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Listen to the Sound of the Big Bang

By Rachel Swaby

How a University of Washington physicist tapped data from cosmic radiation to recreate the sound of the first 760,000 years of the universe's life. You'll want to turn up your speakers for this.

Fourteen billion years ago, when one tiny, dense point became an unfathomable explosion creating all the matter in the universe, no one was around to witness the spectacle. We may not have first hand accounts of just how hot the blast or just how fast the matter traveled, but that also doesn’t mean that our knowledge of the universe’s early years are blank pages. There is a record of what happened, and from it, you can make music—the big bang’s original sound track, in fact.

In 2003, the mother of an 11-year old contacted John Cramer, a physicist at the University of Washington, with a question about the big bang. She was helping her son on a school project, and she wondered if anyone had been able to record what the explosion sounded like. The answer, of course, was no, but he kept returning to the question.

Image credit: Flickr user altemark via Creative Commons.

Cramer was a frequent contributor to the magazine, Analog Science Fiction & Fact, and just two years earlier he had written enthusiastically about how recent research projects looking at the cosmic wave background allowed scientists to hear “the sound of a Big Bang from a distance of 14 billion light years!” Cramer’s linguistic flourish actually meant that the data gathered could be used to understand what the big bang sounded like over a period of hundreds of thousands of years as the universe rapidly expanded. But scientists hadn’t actually heard the sound with their ears. Cramer had access to enough information. Why not recreate the sound?

Staging a revival of a very, very old explosion took Cramer just 16 lines to program, and an one hour on a Saturday morning. He constructed the sound in the software Mathematica, which gives users the option to render mathematical functions as sound. For all his interest in the subject, Cramer explains now ten years later, “I didn’t know what I was going to get.”

Photo credit: Seattle P-I file

The sound (embedded below), compressed to cover the first 760,000 years of the universe’s life, shoots up and then drops into a chest-vibrating hum that sounds like an airplane landing mixed with the static of the television. What came out of the speakers shocked more than just the physicist. Cramer’s two Shetland Sheepdogs came running into the room to inspect what in the world was going on. It was something bigger.

In order to create the sound, Cramer downloaded the frequency spectrum of cosmic background radiation measured by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), which launched in 2001 with the goal of better understanding the properties of the universe. Cosmic background radiation is a research target because the stuff has been around since the beginning. Fourteen billion years ago, when the universe was still brand new, the cosmic background radiation that we observe today used to be light. In the first few minutes or hours, the landscape was dense and, as Cramer explains in one of his articles, it “closed back on itself, so the hypersphere of the universe rang like a bell.” It spent its next 300,000 or so years in a state of extreme heat, which meant conditions were too uncomfortable for positively and negatively charged particles to pair up. In these conditions, light was constantly being swallowed and scattered.

As the universe cooled, its shape and properties changed. Protons were finally able to combine with electrons, and all of a sudden the medium through which light traveled became neutral. Light finally stuck around long enough to make a run for it.

Image credit: NASA

But as the light traveled outward, the universe expanded, stretching everything—including the light waves—within it. Over time, the waves got longer and longer, expanding by a factor of 1000, and today what was once light from the big bang is now cosmic background radiation, our oldest and most reliable record of the universe’s earliest history.

The key to understanding what it sounded like back then comes from the temperature profile encoded in the cosmic background radiation’s waves. If you peer at it at angle less than two degrees, scientists can see a slight temperature inflections caused by the compression and rarefaction of primordial sound waves. Reconstruct them, and voila! The sound of the big bang.

The actual big bang frequencies were far too low in frequency to be audible, so Cramer boosted them by a factor of 1026.

The only trouble, wrote Cramer, is that “the actual big bang frequencies were far too low in frequency to be audible to humans (even had any been around).” So Cramer boosted the frequencies by a factor of 1026. “When Alien came out, the tagline was ‘In space, no one can hear you scream,’” recalls Cramer. “In the early big bang, there was plenty of dense material. You’d be able to hear yourself scream, but the particular [big bang] sound waves were on the order of the scale of the universe so they had to be scaled way up.”

The sound was a revelation. Cramer is an accomplished physicist, with a career of published papers. But his soundtrack to the universe’s biggest show pulled in more public applause than anything else he’d done. In 2003 he wondered about an encore. What’s next? The sound of the galaxy? The sound of the sun? Instead, Cramer settled on improving his impressive first composition. This year, armed with new data thanks the Planck satellite mission’s powerful tools, Cramer was able to compose a more accurate rendition. His latest piece fills in some higher frequencies absent in his weekend work a decade ago, and the 100-seconds is audibly more intense. It’s not a first person account, but from the sound of it, this show was better to hear about second hand.