# Listening to the Big Bang

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.

Image credit: Banff Centre in Canada.

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!

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But if the Big Bang occurs in a forest and no one is around to hear it, does it still make a sound?

Natalia and Connie, your references bring up the excellent points that this sound gets lower pitched as the Universe expands, like so (from John Cramer's page):

http://faculty.washington.edu/jcramer/BigBang/BBSnd100.wav

However, once you get past a day in age or so, the sound mostly drops out of the audible range, and the medium density becomes too low to carry audible sounds. The oscillations that happen in the CMB are interesting to model, but they are in no way actually audible; the recreated data you've both linked to have had their wavelengths/frequencies shifted so that we could listen to them.

I am a 58 year old electrician who started out as a physics major. I haven't lost my desire to learn, but I haven't had this question answered no matter who I ask or what web site I log into. What happens to all the light photons that enter our eyes? Thank you for the web site

By Ron Martinkosky (not verified) on 02 Sep 2009 #permalink

Ethan, are the density fluctuations of the sound still theoretically detectable (given a microphone the size of the paper folding sheet) or does Boltzmann distribution of speed sufficiently randomize the interstellar gas to make all fluctuations gone by now?

@Ron
Light photons that enter the eye interact with a protein (actually multiple variants) called rhodopsin which is "photoisomerized" to a different conformation. In other words the energy from the photon is transfered into a change in shape of the molecule. This change in shape triggers the first impulse that is eventually sent down via a protein signal cascade ending with a nerve impulse at your optic nerve to your brain. Different rhodopsins respond to different wavelengths of light and thus allow for our color vision.

I love this "the universe is expanding" talk. Expanding into what, pray tell? It makes ZERO sense, no matter how you slice it. Don't even get me started on this Big Bang theory...

I really hope DaTruth is joking.

Ethan: mad props to you for attempting to put a decibel figure on the big bang. That's a really fun and bold calculation, but I'd suggest the following amendments:

First, if you're looking at the fluctuations in the CMB, those are fluctuations at the time that the universe became optically thin due to the "re"combination of protons and electrons to form neutral hydrogen (if I'm remembering my intro cosmology correctly). I forgot the snappy name for this occurrence, but it took place a good million years or so after t=0. Which is not to diminish your answer at all (you've gotta specify SOME time after t=0 to estimate of the sound level), but you should probably specify the time you're using in your estimate.

Second, my understanding of the dB scale is that it's defined in terms of pressure, rather than density fluctuations. So if you're spec'ing the sound level at t=+1e6 years, you should scale the fractional density fluctuations by the density and temperature at whatever time you're measuring the sound level. I have no idea what the density was at that time (I could ballpark the temperature at around 10^4 K), so unfortunately I can't do the conversion.

By Anonymous Coward (not verified) on 02 Sep 2009 #permalink

What does this imply for the effects of an atomic blast in space? If there are no air molecules to compress, there would be no compression blast wave. Does this mean that the only effects of an atomic bomb in space would be in the form of heat and light?

What a strange question, and how wild that you actually produced an answer.

Okay, then if 195 db are the level at which you go deaf, and a jackhammer is about 100 dbs then would standing betwen two jackhammers make you deaf?

By Questioner (not verified) on 02 Sep 2009 #permalink

Okay, then if 195 db are the level at which you go deaf, and a jackhammer is about 100 dbs then would standing betwen two jackhammers make you deaf?

No, because the decibel is a logarithmic unit. If one jackhammer is 100 dB, then two jackhammers are about 103 dB.

Two questions:

1. You said: "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."

Isn't the "snow" we see on our TVs the result of EM waves? What does sound wave production have to do with it?

2. Why does the frequency of the bang decrease as time increases?

"A significant portion of the "snow" you see on your TV set comes from the Cosmic Microwave Background. "

OK, that has me confused. The VHF band used for TV is nowhere near the band designated "microwave" - should that be the "cosmic radio background" (cosmic TV background) or am I missing something?

Actually a TV receiver's front end, the part closest to the antenna, does not have a sufficiently low noise figure to allow the background radiation to be heard. This is engineer speak for "the snow/noise is created in the receiver itself" because a low-noise alternative would cost too much and entail compromises not worth making (in the area of coping with interference from strong signals).

Anonymous coward,

We can use the CMB to extrapolate back to "what were the fluctuations' sizes and scales at the beginning?" That's where I get the 0.002 or 0.003% figure; they only grow a little bit over the first 380,000 years of the Universe.

The dB scale is in terms of relative pressure. In other words, an oscillation that's 100% the amplitude of the medium -- whether it be air, water, or the plasma of the early Universe -- gets up to 195 dB. 140 dB is enough to deafen you completely; 195 is just insane.

Kris,

You still get a blast wave from giant explosions in space; supernovae give them off! The blast wave gets generated where there is a medium, however, and the front of it continues to fly through the vacuum of space.

Questioner,

Dunc has you covered.

Everything we see from the CMB -- despite the word "microwave" being used -- is actually electromagnetic radiation. The CMB is a huge spectrum. It's called "microwave" because it has its spectral peak in the microwave, but it actually extends into the infrared in the high frequency domain and deep into the radio in the low frequency domain. Your bunny ears pick it up, and turn it into a watchable, listenable electronic signal. The static sound/noise you hear should be pretty similar to what the big bang sounded like, although hopefully you don't crank your TV to 100 dB when you watch channel 3.

Can you answer this question? If a search light is made to spin fast enough so that light at say 200,000 miles out travels faster than "C". What would happen. as an example: if you had two sensors on the moon and sent a laser beam up from earth and moved the laser back and forth fast enough, the sensors should be able to record that the light traveling from one sensor to the other could do so faster than "C" or would the beam bend like a lawn sprinkler as it turns.

Thanks, Phil

By phil nigro (not verified) on 03 Sep 2009 #permalink

This is one answer to DaTruth's query about our expanding Universe. Consider a two dimensional representation such as a balloon where its surface becomes the model of OUR Universe. Moreover, let the surface be resplendent with ink dot galaxies with one representing ours, the Milky Way. As the balloon is blown up it expands, hence all dots on the surface of the balloon appear to recede from one other. This is exactly what we observe when we examine the sky. One might therefore infer that the balloon universe expands into the future. At the same time it then follows that inside the balloon represents the past. Also, the radius of our balloon, assuming a perfect sphere, becomes the arrow of time. This is not a perfect representation of the facts as we perceive them, but allows one to grasp the idea. A more factual answer, however, is the supposition that OUR 4-dimensional Universe expands into a multiverse of higher dimensions which perhaps includes an infinite variety of other such universes of widely diverse properties.

"Okay, then if 195 db are the level at which you go deaf, and a jackhammer is about 100 dbs then would standing betwen two jackhammers make you deaf?"

"No, because the decibel is a logarithmic unit. If one jackhammer is 100 dB, then two jackhammers are about 103 dB."

Ok, we know that the *frequency* of both would be the same. But are you saying that the *volume* of two jackhammers going would not be doubly loud as one?

By Questioner (not verified) on 03 Sep 2009 #permalink

Phil, re: your spinning light question-

I love this question, and agonized over it as a kid.

The simple answer is, yes, the pinpoint of laser light could go FTL. But think about it for a second: the light isn't actually traveling in that axis. It's still only traveling from the tip of the laser itself to the point on the moon where it impacts, and that speed is only C.

Think of it like this: you're looking through a telescope at an image of a faraway galaxy. You can zoom your eye's focus from one edge of the galaxy to the other in a split second, essentially making your vision travel faster than light. But the light itself that is entering your eye is still only going C, from its starting point in deep space, through the universe, and finally into your telescope and eye. Causality isn't being violated.

In your example, when the sensors on the moon detect the laser, the information isn't going back and forth between the sensors, it's coming from the earth, from where the laser is projecting. It would be just exactly like switching on a pair (or string) of lights, in a row. With a long enough string, and fast enough set of switches, you could make the bulb that is lighted travel from one end of the string to the other in sequence at faster than the speed of light, but nothing is actually breaking C; the light, the actual information, is still only traveling at C or slower.

Thank you for the quick and insightful answer to the spinning light question. I have asked that question many times but you have provided the only definitive answer.

Just one more thought. In the example of the laser from the earth to the moon; if you could see the laser beam itself (smoke, fog etc) would the beam stay straight or would it appear to bend looking from a point overlooking the earth moon. ( hence the rotating sprinkler analogy where the water droplets would be replaced with photons )

By phil nigro (not verified) on 03 Sep 2009 #permalink

Ignorant questions follows:
if our universe began with the Bang were all of todays electrons and protons created at that moment? If so would not the earliest universal density been very high? Like at least as high as a black hole? If so how did everything escape?

Phil, the answer you got here was correct.

Questioner, a whisper is about 30 dB, normal conversation is about 40 dB, and speaking in a clear, loud voice is about 60 dB. The 60 dB sound is not, however, twice as loud as the whisper. It's over 100 times as loud. So yes, if you were next to two jackhammers, the sound would be twice as loud. But twice as loud as 100 dB is not 200 dB, it is more like 103 dB. The decibel scale works the same as the Richter scale for earthquakes, if that helps.

Jim: no, yes, and no. We don't know when today's electrons and protons were created, but we know they were created after the big bang, but within the first second of the Universe. The earliest Universal density was extremely high, you are correct on that account. Denser than a black hole? No. A black hole has infinite density. But consider this: the density of a uranium nucleus is about 10^18 kg/m^3. A nucleus is -- in no way, shape, or form -- a black hole. So although the density of the early Universe was very high, it was not high enough to become a black hole, and that's why we're still here, because there was enough energy to prevent gravitational collapse of that dense stuff into a black hole.

Thanks Ethan. I hadn't known about the exponential increase. Then 195 decibles must be TRULY LOUD. Which makes me wonder if one would not be deaf way before then? Can you give an example of a noise that is 195 dbs?

By Questioner (not verified) on 03 Sep 2009 #permalink

Amazing! I love this blog!

I used to work with someone who in a previous job had done a lot with air powered ultrasonic whistles and had totally trashed his hearing. He said that you could hear 40 kHz, if it was loud enough. At the focal point of his ultrasonic whistles it was 185 dB, and if you put a tuft of cotton there it would catch on fire.

I think the 195 dB is the loudest sound that can happen in air at one atmosphere. The lowest pressure in the rarefaction part of the sound wave is zero, the maximum in air is one atmosphere. âSoundâ is considered to be fundamentally a linear process. When the media starts to respond non-linearly you donât have âsoundâ any more, you have shock waves.

In space essentially the only effects of an atomic explosion are the heat and light, the bomb products are so hot that the heat and light comes out as high energy gamma rays. Even for an atmospheric explosion, the initial expansion of the fireball is via radiation, x-rays from the bomb products have a short path length in air, they are stopped by the air and heat it up, that air then gets hot enough to radiate in x-rays, those x-rays have a short path length until they are absorbed. The fireball expands via radiation until it gets large enough that the surface temperature drops to where blackbody radiation from it is something that the atmosphere is transparent to, which is when the fireball becomes visible and a whole gigantic amount of energy gets transmitted by visible and infrared light (which ignites everything and starts the firestorm).

On some of the films of atomic bomb explosions you can see patterns in the fireball which reflect the distribution of energy release during the last few nanoseconds of fissioning. That is like the microwave background. The spots that were hotter due to greater fission energy release have cooled not by expansion but by radiation dilution with cool mass. Because the cooling is rapid and not due to movement or mixing of matter the initial patterns have remained and havenât been smeared out.

can you hear back ground radiation noise with just your ears like on old tv when station went off the air. I mean not from TV or radio just in every day life the sound is all around you.??? Don Mc