The Early Universe Sonified

Light travels fast, but space is so vast that even light needs time to reach us from distant objects. For a distant galaxy, it can take millions of years. As a result, we never see these objects as they are today, but as they were in the past. The deeper you look into space, the further you travel back in time.

Astronomers have pushed this cosmic time machine almost as far as physics allows. In every direction around us, when we look as far as possible, we observe a faint glow known as the Cosmic Microwave Background (CMB). This radiation was released about 380,000 years after the Big Bang, when the Universe had cooled enough for light to travel freely through space. What reaches us today is therefore a snapshot of the Universe nearly 13.8 billion years ago.

This ancient radiation is not perfectly uniform. Tiny variations are frozen into it: ripples left behind by enormous pressure waves that traveled through the hot plasma filling the young Universe. These primordial vibrations are called acoustic waves, analogous to sound waves in air. But instead of traveling through a gas, they propagated through a dense plasma of matter and radiation, with wavelengths stretching across thousands of light-years. They are the origin of the structures that would later become galaxies and galaxy clusters.

The European Space Agency's Planck satellite measured these ripples with remarkable precision. The resulting spectrum reveals a series of resonant peaks, each corresponding to a different scale of vibration in the early cosmos.

The idea for this soundscape came from Tod Lauer, an astrophysicist at NOIRLab (USA) and a fan of myNoise, who challenged me to turn these measurements into sound.

I have always been interested in scientific sonification, but often disappointed by it. Too many examples render scientific data with sampled instruments: a graph becomes a violin section, a telescope image becomes a choir. Beautiful, but often misleading, even if audiences love it. For this project, I wanted the data itself to generate the sound, which is, I think, what sonification should actually mean. It won't sound like music: it sounds like noise, and that feels exactly right to me.

The original frequencies represented in this soundscape are unimaginably lower than any sound humans could hear. The primordial waves measured by Planck stretched roughly 380,000 light-years across. To bring them into the audible range, they had to be shifted upward by roughly 10^21 in frequency, or about 70 octaves! What you hear is therefore not the sound of the early Universe itself, but a transposition of its resonant structure into the human hearing range.

So, the resonant peaks measured by the Planck satellite were mapped directly into the audible domain, each one assigned to its own slider. But the data reveals eight visible peaks, and this generator has ten sliders! The first slider therefore represents the noise that exists below the very first resonance. And the last one is an extrapolation toward a ninth peak, one the data hints at but that lies buried beneath the noise floor and beyond the limits of the graph above.

The original Planck spectrum is plotted on a linear frequency scale, while our ears work logarithmically. To recreate its structure, I could not rely on standard filters with fixed dB/octave slopes. Instead, the filters become progressively narrower and steeper as frequency increases. And within a given filter, its dB/octave value itself grows from one octave to the next. This is unusual in audio processing, yet it is what was required to faithfully reconstruct the structure of the Planck data. It's a subtle detail, but one that a trained ear may notice.

The reconstruction presented here is not the only possible one. I chose one that was enjoyable to listen to, while remaining faithful to the structure of the Planck data. Arbitrarily, I set the first resonant peak to 69.3 Hz, the frequency of C#, the common root frequency of all tonal myNoise generators.

When all sliders are combined, they produce a broadband noise whose spectral shape follows the famous CMB measurements. Hidden inside that noise are the frozen echoes of vibrations that once traveled through the newborn Universe...

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