Cosmic sound

According to our understanding of the Big Bang, “cosmic sound” is older than neutral hydrogen. We can still spot its echoes today.

Lots of galaxies
ESA/Hubble, CC BY 4.0, via Wikimedia Commons 

The cosmologies of J. R. R. Tolkien and C. S. Lewis both begin with music. In The Silmarillion, the Ainur sing the universe into existence together (with Melkor providing the disharmony). In The Magician’s Nephew, Aslan sings the world into being. They were, as it turns out, not too far off the truth.

Sound cannot travel in the void of space. Sound waves need a medium to propagate: air, water, or some other matter in fairly close proximity. Particles in space just aren’t close enough for sound waves to spread out. (Shh, no-one tell George Lucas). But, crucially, this wasn’t always true.

Up until about 379,000 years after the Big Bang, space was not a vacuum. The whole universe was a soupy plasma of dark matter, photons, and baryons (free-floating protons and neutrons). Everything was packed so closely together that the universe was effectively opaque. And, crucially, everything was close enough together that sound-like waves could travel through it.

In this early universal plasma, some bits were slightly denser than their surroundings. These were the source and centre of waves that travelled out in every direction, behaving exactly like super-fast sound waves. The technical name is a “baryon acoustic oscillation” but it’s more commonly called the cosmic sound.

These waves kept on echoing out through the primordial plasma. At some point, though, the expanding universe spread out enough that they could go no further. The universe was no longer opaque, so the spheres of cosmic sound stopped in their tracks. And where they halted, they left behind matter.

How far did these waves travel? Well, factoring in the expansion of the universe since then, the cosmic sound travelled about 490 million light years. And this number is crucial, because the large scale structures of the universe follow the patterns set down by this matter. In effect, galaxies formed from those points where the cosmic sound stopped. We know this because, statistically speaking, more galaxies are 490 million light years apart than you would expect by chance. They are the remnants of a sound wave in the early universe: the clump of matter in the centre, and the clumps of matter at the edge of the waves.

(By the way, that 490 million light year measurement is a “standard ruler” – it helps us to figure out the rungs of the cosmic ladder.)

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