How loud was the Big Bang? It turns out, not that loud at all.
In decibels, it’s a bit less than 120db. Motörhead concerts are louder than the Big Bang. OK, the sound had wavelengths of hundreds of thousands of light years, so the notes are much too low to be audible, but it’s still 120db.
But if you transpose the pitch up 50 or so octaves, what did it sound like? We’ll show you in a minute, but first we need to set the scene.
Imagine that when you were born, your excited parents sent a text message. Who could receive it? The fastest that the signal can go is the speed of light, so that tells you the furthest that the news could have travelled in your lifetime.
Now imagine it wasn’t your parents, it was their parents, so the news was sent out a maybe 2 or 3 decades earlier. More of the Universe could have received the news. Send the signal earlier and earlier, and the news reaches further and further by today.
But it turns out there’s a limit. Even if you send the message as soon after the Big Bang as you like, it turns out the news can only reach so far. We call this limiting size the Observable Universe. Anything outside our Observable Universe can’t have received any news from us yet, nor us from them.
Now imagine a very early time in the history of the Universe. The Universe is so much smaller and denser that it’s opaque. There’s no stars or planets back here, just gas and light, plus whatever subatomic particles that happen to be around. The sound waves of the Big Bang are still wobbling through the gas, and the speed of sound isn't that far off the speed of light. At this unfathomably early time, the whole observable universe is acting like a resonating cavity!
These primordial sound waves left their imprint on the Universe.
As the Universe expanded it became transparent. If you were there, it would have seemed like fog clearing around you. But light takes time to travel from place to place, so you would first see the fog clearing around you, then you would see the fog clear in more distant parts (it takes a while for the light from there to get to you), then in still more distant parts, and so on. The Universe would look like it has a receding bank of fog, as the light from more and more distant parts finally makes it to you.
We are still in such a Universe, and this receding bank of fog is called the Cosmic Microwave Background. And it’s on this background that we can still find the imprints of these primordial sound waves, from when the whole observable Universe was acting like a resonating cavity.
The fundamental note depends on the size of the observable Universe back then, but the harmonics, the overtones, depend on the nature of the fluid that was sloshing around at the time. It turns out that the only way you can explain the mix of overtones is if you have a lot of dark matter sloshing around back then. You can tell the Universe has lots of dark matter just from the sound of the Big Bang. (For a primer on this, you can read my introduction to dark matter, watch a video where I explain it using a pint of Guinness, or take a 60-second adventure.)
And here’s what it sounded like, transposed up many many octaves, as calculated by the astronomer Mark Whittle from Virginia University:
Listen out for the fundamental note dropping down. This happens because the observable Universe grows while the Universe expands.
This cosmic musical story has a coda. The primordial acoustic ripples in the gas later condensed into stars and galaxies, and the imprint of these giant primordial sound waves can still be seen in the distribution of galaxies. Astronomers are using these giant frozen-in patterns to map out the geometry of the expanding Universe, and from that it’s become possible to figure out how much Dark Energy is flinging the Universe apart. (Here's an introduction to dark energy, and another 60-second adventure you can go on.)
So these ancient sound waves don’t just tell us that there’s dark matter sloshing around in the Universe, they also tell us something about the Dark Energy that’s flinging the Universe apart.