So, I have a small confession for you, my readers. Since I first started writing this blog, I have always solicited questions, and promised to answer the best ones. Recently, this has come back to bite me, as I’ve started getting more questions than I possibly have time to answer. However, the ones that meet the following criteria:
- are of interest to a very broad, general audience,
- are clear, well-posed, and straightforward, and
- I can actually provide an answer to them,
will, more often than not, get answered right here. This one comes from reader Dan Noyes, who writes:
I have a question for you from my 10 year old son who loves fireworks,
and any kind of bang in general. He said the other day that he would
have loved to have heard the the Big One (the Big Bang) – “That must
have sounded awesome!” But did it make a sound? And where could you
have heard it?
What an interesting question! Because, on one hand, we all know this:
Sound doesn’t travel through space. But the reason sound doesn’t travel through space is that sound needs a medium to travel through. Sound waves come from whatever medium you’re in (like air) undergoing compression and rarefaction like so:
The change in pressure in this medium is what causes sound. Additionally, for a human ear, the compressions need to happen at the right frequency: between 20 and 20,000 Hz for a human to hear it. Most of the sounds we hear come through the Earth’s atmosphere, which is dense enough (about 1 kg of air per cubic meter) to allow sound to propagate. The average density of space, however, is about one proton per cubic meter, or about 10-27 kg per cubic meter. So, you might reason, there’s no sound in space.
And that’s true, for space today. But since the Universe is expanding, it was smaller and denser in the past.
Now, we know we’d be able to hear sound in an atmosphere as sparse as Mars’ (which is about 10 grams per cubic meter), and we know we can hear sound through more dense media as well, such as water and rock. If we extrapolate the Universe back until it was as least as dense as Mars’ atmosphere, this means that for about the first day after the big bang, the Universe is dense enough that sound audible to humans can travel through it.
But there’s a big question here: is there anything to hear? The answer is a resounding yes, and you can understand it from this famous picture:
The picture above is not, as most people think, a picture of the leftover glow from the Big Bang, known as the Cosmic Microwave Background. It is a picture of the fluctuations in the leftover glow. These fluctuations happen because there are tiny differences in density throughout the Universe, at the level of about 0.002% or 0.003%. Well, if you have a fluid with differences in density, what happens? The simple answer is that you get waves of a certain frequency, depending on the size of the density fluctuation.
The very early Universe has density fluctuations at all scales, meaning that it will produce sound waves at all frequencies. In fact, you can still see this “white noise” just by grabbing an old television with rabbit ears and turning to channel 3. A significant portion of the “snow” you see on your TV set comes from the Cosmic Microwave Background.
So now that you know that there is something to hear, what would you actually hear (assuming you could survive the radiation and the near-billion degree temperatures)? The human ear is most sensitive to lower frequency sounds, so that’s what you’d preferentially hear: the bass notes.
But how loud would it be? Would it be maximally deafening, which is about 195 dB? Would it be muted by all the ambient matter and energy? It turns out that the decibel level of the sound that you hear is simply related to the magnitude of the amplitude of the waves relative to the overall density of the medium. It’s easier for sound waves to travel through water than air, but water is denser than air, so the sounds are muffled, and have lower relative amplitudes. I already told you that the amplitude of the fluctuations in the early Universe are about 0.002 or 0.003% of the ambient density, which corresponds to a sound of about 100 decibels, or about as loud as standing near a running jackhammer.
This is loud, but not even as loud as an airplane passing a few hundred feet overhead. And this was the Big Bang! But it was muffled by all the matter and energy in the Universe, and so it comes in as a loud, but unremarkable noise in all directions. And that’s what the Big Bang sounds like for the first few dozen hours of the Universe, until the density becomes low enough that audible sounds can’t affect your ears anymore.
Thanks for the great question!