Have you ever wondered what the "Big Bang" actually sounded like? Surely, you may be thinking, this is a trick question -- didn't it just sound like, well, a really big BANG! Surprisingly, perhaps, the answer is "no, not really". As is often the case with Nature, things are not so simple, and a more accurate description would be something like this: a moment of silence followed by a rapidly descending scream which builds to a deep roar and ends in a deafening hiss. Try playing sound1.wav now -- it's an accurate rendering of the first million years of primordial sound, shifted up into the human range, and compressed into 10 seconds. Oh yes, you should crank the volume up to 110 decibels, about as loud as a rock concert! In fact, like many rock concerts, the cosmic concert also ends in a spectacular light show: after a hundred million year ear-splitting crescendo, the highest pitch sounds dissolve to become the first generation of brilliant exploding stars, while the deepest bass notes slowly began to weave the tapestry of galaxies which now fills all of space.

The birth of the Universe, it transpires, had its own primal scream.

Was there really sound?

Let's postpone a discussion of how it is possible to know about primordial sound, and start by answering some immediate questions and concerns. First, weren't we all taught in high school that sound cannot exist in the vacuum of space? Well yes, but space wasn't so empty when the Universe was young. Remember, the Universe is expanding so it was smaller in the past, and all the matter we now see in stars and galaxies was spread out uniformly to make a hot thin gas, a kind of cosmic "atmosphere". It is within this atmosphere that sound waves could form, grow and move [Figure 1].

The next obvious question is to ask whether we could actually hear the sounds, if we were alive back then? Regrettably, the answer is a very definite "no", for several reasons. Even ignoring the fact that we would instantly suffocate and roast in the searing heat of that early fireball, the sound pitch is way too low for us to hear. Compared to concert pitch A, for which 440 sound waves pass us each second, a typical cosmic sound wave takes more like 50,000 years to pass by, about 50 octaves below the human range! On reflection, this is not too surprising -- something as large as the Universe must surely have an exceedingly deep voice; and indeed it does. Using a musical instrument as metaphor, the cosmic concerto is played on an ultra-ultra-bass piano, the seventh in an ever deeper series extending below the range of a human piano -- a truly GRAND piano of cosmic proportions [Figure 2]. With this in mind, we might now ask: what was the piano made of; who played it; and what music was being played? Here are the answers: the pianist's name was Gravity; his instrument was made of dark matter; and he played a single powerful chord which contained hidden within it both sadness and joy. With these intriguing statements, let's now begin to look at how sound was created in the early Universe.

Creating Primordial Sound

The first shock is to learn that the "Big Bang" actually commenced in total and utter silence. The initial expansion was so pure and "radial" that no part was catching up any other part -- there were no compression waves, no sound -- just quiet brilliant live expansion. However, although the expansion was perfectly smooth, the distribution of matter wasn't -- dotted here and there were slightly denser regions (just why the Universe was born lumpy, we'll return to later). Now, these denser regions created a slight roughness to the (3 dimensional) gravitational landscape with gently rolling "hills" and "valleys" into which the hot cosmic gas soon began to fall. With pressure acting like a spring, the gas proceeded to bounce in and out of these valleys, creating sound waves [Figure 3]. As time passed, the gas had longer to fall into ever larger valleys, making bigger sound waves. In this way the sound gradually dropped in pitch and grew in strength, and by the time half a million years had passed the longest waves had grown to about 200,000 light years, and had pressure variation from peak to trough of about about 0.01\%. Now, although this pressure variation seems extremely slight, in fact it corresponds in human terms to about 110 decibels -- about as loud as a rock concert! This wonderful fact tells us that the primordial sound was neither pathetically quiet nor fatally cacophonous, as far as humans are concerned, it was simply powerfully loud.

Cosmic Chords?!

But what kind of sound was it; was it a single note, a roar, a hiss, or what? Having discussed pitch and volume, we must now consider sound's third property; its "quality". What a sound "sounds like" depends on the relative loudness of each frequency. For example, a hiss has weak low frequencies but strong high frequencies; a deep roar has the opposite; while a chord has just a few single frequencies. Perhaps the most astounding aspect of the primordial sound is that it contains a sequence of harmonic tones, just like the sound from a musical instrument. The Universe is actually playing a chord! This bizarre property is tricky to understand, but worth the effort. Most musical instruments generate a harmonic sequence of tones because they have a fixed size. Between their ends (either strings or columns of air) one can fit only a whole number of waves; one, two, three etc; and this sequence of waves generates the sequence of harmonics. Now, although the Universe has no ends or edges, it is bounded in time. At any given moment, there has been a finite time since the Big Bang, and so there are specific region sizes across which exactly one, two, three, etc, full sound waves have passed. The sound waves created from these regions are, in a sense, louder than sound waves from regions of intermediate size, and hence they dominate the sound and create the harmonics. It is even possible to analyze the harmonic sequence and figure out what the chord actually is. Choosing the lowest two harmonics (which are the loudest) we find a slow change across the first million years from a major third (4 semitones) to a minor third (3 semitones). Stated more poetically, the Universe's symphony opens, appropriately, with a positive majestic major chord, but as time passes the mood shifts to a more reflective sadder one as the minor chord builds.

Now, if you've already listened to the sound file [sound1.wav], you may be puzzled by its apparent lack of any obvious musical quality -- where are those major and minor chords? Well, it turns out that as musical instruments go, the Universe really isn't a very good one. Human musical instruments are designed to be exceedingly good resonators, which means their harmonic tones are almost pure single frequencies. The cosmic harmonics are very different and the resonances which generate them create a broad range of frequencies which yield a sound more like a roar than a chord. If that were not bad enough, during the first half-million years the overall pitch takes a dramatic downward dive across many octaves and this makes it very difficult to hear any chord [Figure 4].

At this point one can have a little fun and try to unmask the hidden chords by simply replacing each broad harmonic by its single central frequency, thereby "cleaning" the sound for human consumption [Figure 5]. The sound files sound2.wav and sound3.wav each span the first million years and include just the first eight harmonics as single tones. While chord recognition is still virtually impossible in sound2.wav, which keeps the downward dive in pitch, in sound3.wav this effect has been removed by anchoring the fundamental to a fixed pitch of A220Hz, giving a stable set of tones (these tones have been forced to the nearest note of our modern musical scale, and this gives small steps as the chord evolves). Finally, one can discern the changing chord. What do you think? I think it has a rather eeire quality, which has its own charm and interest. Certainly, though, it's not exactly Mozart, but then don't forget it wasn't written for our ears. Indeed, as we shall learn, in order to create a Universe filled with galaxies and stars and people, this sound had to be just the way it was. To your ear it may not seem very creative or imaginative, but hidden in that sound lies the blueprint for all that is to come.

What caused the scream and hiss?

Leaving the chord analysis behind, let's now return to study the nature of the true sound, as heard in sound1.wav. After the drop in pitch, which spans roughly the first 380,000 years, a growing hiss is heard which ultimately drowns out all the harmonics. Why does the sound have these two qualities; a descending pitch and growing hiss? The pitch drop has two causes. The first is simply the stretching of sound waves as the Universe expands -- imagine expanding a violin into a cello into a double bass. The second cause comes from a curious phenomenon associated with times shortly after creation: history is finite, and processes that require time to develop cannot occur until sufficient time has elapsed. For example, the gravitational creation of sound across a valley 1000 light years in size can only occur after the Universe is 1000 years old. Why? Because it takes at least this long for gravity's pull, travelling at light-speed, to cross the region, allowing it to "wake-up" and respond to its own gravity. As time passes, then, the sound pitch drops as longer waves are added to the mix, creating the descending scream.

What about the encroaching hiss, what's that all about? To understand this we need to look at what our musical instrument, the Universe, is actually made of. There are three principle components, each with its own unique character: light, atomic matter, and dark matter. Dark matter, you may know, is an enigmatic material that outweighs atomic matter five to one, and although its exact nature is still unknown, we do know it behaves like an invisible dense pressureless cold gas that permeates everything. Now, the components that oscillate as sound (like the strings on a piano, or the air in a flute) are atomic matter and light. The component that provides the framework for this motion (the body of the piano, or the tube of the flute) is dark matter -- it is this that moulds the gravitational landscape, providing the stage on which sound is built. When the hot glowing atmosphere bounces in and out of the gravitational valleys, these valleys are made principally from concentrations of dark matter. As you might imagine, then, the number and size of dark matter concentrations (the shape of the piano or flute) will determine in large part the nature of the sound -- and so it does. There is, however, one fascinating complication -- the dark matter piano only works for the first 380,000 years, at which time the "strings snap" and the piano begins to function completely differently.

What's so important about 380,000 years? A remarkable event occurs which changes everything -- the pressure in the cosmic atmosphere suddenly plunges by about a factor of a billion. Before 380,000 years, the high pressure prevents the atmosphere from settling into the dark matter valleys; instead the gas simply bounces in and out as a sound wave. However, after 380,000 years, with almost no pressure to support it, the atmosphere can't "hold itself up" to resist the pull of dark matter, and it begins to settle into the valleys, ultimately moulding itself to the complex dark matter landscape. Strictly speaking, at this time the atmosphere ceases its acoustic oscillations and true sound dies away. In the current work, however, I have chosen (for pedagogical reasons) to keep the acoustic representation going, using the spatial variations in density to generate sound even though these variations no longer actually oscillate in time. With this one caveat in mind, we find that the growing hiss comes from the cosmic atmosphere falling into the smallest dark matter concentrations. In fact, with no pressure to stop the gas building up, the hiss gets louder and louder, ultimately drowning out all other aspects of the sound.

Why does the pressure drop so suddenly at 380,000 years? Because at that time, like the clearing of a morning mist, the Universe turned transparent. Early on, the Universe was foggy because its hot atmosphere was ionized -- it contained free protons and electrons. However, as the Universe expanded it also cooled and at 380,000 years its temperature dipped below 5000 F allowing the protons and electrons to combine as neutral atoms. With the free electrons gone, the gas turned transparent. But why should transparency affect gas pressure? Because in a fog light pushes on the gas and adds to its pressure. When the Universe turned transparent, light no longer contributed to the pressure. Was this a significant loss? You bet. Within the fog, each proton, electron and photon contributed equally to the pressure, and since light was enormously abundant (a billion photons for each proton or electron) light totally dominated the pressure. After fog clearing, then, the atmosphere lost essentially all its pressure, and from then on it couldn't bounce out of dark matter's valleys but instead fell ever more deeply into them.

This brings to a close the acoustic era; the overture ends with a cacophonous white noise. But the show is not over, indeed it is about to begin -- the stage has been prepared for the entry of stars, to which we will return, after a brief intermission...

The Microwave Background

Having described these remote and wild times, let's now come back to Earth and look at how it is possible to know, with any certainty, what creation sounded like. The framework for our understanding has been in place for almost a century now, starting with the discovery of cosmic expansion in the 1920s by Edwin Hubble. The expansion implied the Universe was born in an initial "explosive" event which was later called, slightly mockingly by Fred Hoyle in 1950, the "Big Bang" -- a name which, for good or ill, has stuck. For the present acoustic story, the two most important subsequent breakthroughs were the discovery of a faint microwave glow across the whole sky in 1963 by Arno Penzias and Robert Wilson, and then the further discovery in 1992 of slight patchiness in this glow by NASA's COBE satellite and science team. This patchiness is extremely slight, comparable to the height of a bacterium on a bowling ball, and its detection and measurement pose a very difficult experimental challenge. Because of its importance, however, during the 1990s a number of groups worked hard to make ever more detailed maps of the microwave sky, culminating most recently in 2003 with the all-sky microwave map produced by NASA's WMAP satellite and science team [Figure 6 and Figure 7].

It is this Cosmic Microwave Background (CMB), and in particular its patchiness, which holds the key to Big Bang acoustics. Let's briefly review what the microwave background actually is [Figure 8 and Figure 9]. Rather unbelievably, it comes from an ancient piece of the infant Universe which was only 380,000 years old. The fact that we can directly observe ancient history is an old astronomy trick: the light arriving from a distant object left it long ago, and so we see it as it was then, not as it is now. Look far enough away and you can see back almost to the Big Bang itself. But not quite. Remember that the early Universe contained a bright glowing fog. Hence, we can look out through the transparency of space as far as, but no further than, that glowing fog. Here's the spooky part. Because all directions look back in time, we see the fog in all directions -- the whole sky should be glowing with the light from the Big Bang. And it is! We just don't see it with our eyes. Cosmic expansion shifts the light to become microwaves -- as many microwave photons fall to Earth from the sky as do light photons from the full moon. If we had microwave sensitive eyes, even at night we could find our way and cast shadows by the light of creation! This extraordinary ability to witness, directly and in full panorama, the light from the Big Bang is one of Nature's most remarkable and generous gifts.

So far so good, but why have studies of the microwave sky spurred the development of Big Bang acoustics? Because most of the finest scale patchiness visible in the microwave maps shows, more or less directly, the peaks and troughs of sound waves moving through the hot gas of the young Universe [Figure 10]. One can actually see the primordial sound waves, not moving of course, but frozen in place as they crossed the wall of fog, caught just as the Universe turned transparent. The situation is not unlike looking down over the ocean and taking a photograph: a whole collection of water waves is visible, little ones on top of bigger ones on top of even bigger ones, all superposed. Analysing the complex pattern of patches, using a computer, can yield the relative number and strength of waves of different sizes -- in other words the relative loudness of high and low pitch notes. A graph of this is called the "sound spectrum" and is a precise way to characterize the collection of waves, and hence the quality and loudness of the sound. Our knowledge of the CMB sound spectrum has slowly improved over the last decade, and now spans about 10 octaves, of which the highest 5 correspond to acoustic waves (the lower octaves are a whole other story, and give a picture of the Universe less than a nanosecond after it was born). Remarkably, the upper sound spectrum shows many of the features of a musical instrument: a strong fundamental at a wavelength of about 220,000 light years, and a sequence of higher harmonic peaks with shorter wavelength. It is this sound spectrum which provides the starting point for the recreation of the primordial sound for human ears.

The Cosmic Genome Project

Before discussing that, however, lets clarify why the CMB sound spectrum has been a holy grail for cosmologists. It certainly is not just to find out what creation sounded like, though that is an added bonus. There are two nice metaphors which illustrate why it is so important. First, if you strike a wineglass and a teacup you instantly know that these two objects differ in many ways. The sound an object makes is like a fingerprint: it is unique to the object, and reveals much about its nature. The same is true for the Universe: its primordial sound carries a great deal of information about its structure and properties. Indeed, the CMB sound spectrum has played a crucial role, when combined with several other astronomical datasets, in establishing more than a dozen basic properties of the Universe. There is another, perhaps more appealing, human metaphor which also illustrates the importance of the microwave background [Figure 11]. Just 380,000 years into the life of the Universe is equivalent to just 12 hours into the life of a human. Now, 12 hours after conception, a human is tiny and formless, and all that is present is its DNA. Yet within that DNA, hidden and encoded, is information which determines much of what the developing child and adult will become. So too with the microwave background. It depicts a compact Universe which is virtually formless, and yet hidden within its delicate patchiness is encoded a huge amount of information, much of which determines how the Universe will subsequently evolve and grow. In a sense, then, studying the microwave background is to astronomy what the human genome project is to the life sciences. It is astronomy's "Cosmic Genome Project". Both genome projects, human and cosmic, present huge scientific challenges, but also promise huge scientific rewards.

The "Sound" of the Microwave Background

With all this background now in place, let's return to the more playful nature of this subject -- reproducing the primordial sounds suitable for human ears. The obvious first step is to use the observed CMB sound spectrum to create a sound, remembering to shift the pitch up by about 50 octaves. The actual shift is arbitrary, but a good choice is to place the fundamental peak at about 220 Hz, corresponding to the A below concert A (this in fact matches the acoustic frequency in Hz to the angular harmonic number "L" used by cosmologists to characterize the CMB sound spectrum). You can hear this "observed" sound by playing sound4.wav -- it gives a deep powerful roar which is really quite impressive, especially when played at the correct 110 decibels volume. Because the microwave background is a static image, this roar doesn't change in time -- it is an acoustic "snapshot".

Now, as far as using real data is concerned, that is about all one can do -- there is, after all, only one measured CMB sound spectrum. Fortunately, however, it is possible to go much further by using computer models of the early Universe. These are highly sophisticated programs which have been developed by a number of workers over the past decade, mainly to help interpret the observed CMB sound spectra and extract cosmic properties. These computer programs are publicly available and relatively easy to use, and it is these programs which really allow one to access a host of aspects of the primordial sounds. For example, it is possible to generate CMB sound spectra for different kinds of Universe, and then turn these into sounds. Sound5.wav, for example, compares three Universes of different density -- an overdense Universe with closed geometry has a deeper voice than an underdense Universe with an open geometry.

Computer access to the Big Bang

The computer programs offer two other extremely important options. The first is to correct for several non-acoustic distortions which significantly affect the patchiness seen on the microwave sky. In this sense, the Universe is not a perfect concert hall, and the primordial sounds suffer a number of distortions, both in situ and en route to us. For example, the foggy wall is not a perfect surface, it has significant depth, and so any sound waves shorter than this depth get blurred out and become invisible. This has the effect of suppressing the highest pitches, and is analogous to a thick carpet or drapes in a concert hall which absorb high frequencies and deaden the sound. Other effects have no simple analog. For example, some patchiness arises due to Doppler shifts from moving gas, or to gravitational shifts due to light crossing density variations. Fortunately, the computer simulations work principally with the undistorted sounds, and only at the end of the calculation do they fold in all these other effects to yield the "observed" microwave sky. By catching the calculation before this final stage, one can extract a cleaner, more accurate sound spectrum, relatively free of distortions. Sound6.wav compares the raw and corrected sounds coming from the CMB -- as one might expect, the corrected sound spectra have better separated and narrower harmonic peaks, giving a cleaner purer sound, though it is still more of a roar than a chord [Figure 12].

The second wonderful aspect of these programs is that one can follow the time evolution of the sounds, from just moments after the Big Bang, through the great transition of fog clearing, right up to the time when the first stars begin to form. It is these evolving sounds which have the characteristic downward scream descending into the deep roar which becomes drowned out by the loud hiss. This is the sound you hear in sound1.wav. In a slight variation of this, sound7.wav uses exponential time to help us follow the evolution more clearly: the first two seconds span 100 to 1000 years, the next two seconds span 1000 to 10,000 years, and so on up to 100 million years (the volume is held constant to allow the full evolution to be heard).

Quantum Hiss and Waves of Galaxies

Let's end this article with two excursions; one before the acoustic era, and one after it. First the trip before. Here's the question; ultimately, what caused primordial sound -- what struck the cosmic bell to set it ringing? Because sound began when gas fell into dense regions, the question should really be; why was the Universe born lumpy? If it had been perfectly smooth, there would be no sound and, as we shall see, no stars or galaxies. The cause of the initial lumpiness has proved a tough nut to crack, though cosmologists are beginning to feel they now have the answer. It seems that in the exceedingly early Universe, quantum fluctuations of space-time were amplified by an extraordinary period of exponential expansion, called inflation. The cosmic lumpiness is therefore a relic of quantum roughness in the sub-atomic world, writ large by inflation's amazing amplifier. This is a truly bold idea, that all structures, from stars to galaxies to the cosmic tapestry, arise out of the ghostly world of quantum foam.

We can pursue these first moments one step further by asking what the initial "quantum sound" was like. Recall that although it took time for sound proper to get started, that was only because it took time for the Universe to "wake up" as gravity's influence spread out at light speed. In a sense, all the sound was already present, but it was latent, just itching to become manifest. If we imagine cheating time and releasing all the wavelengths together -- we could hear the sound of that initial silence -- a truly pregnant silence on the brink of birthing the entire Universe. In fact, the form of that first "sound" is another holy grail in cosmology, called "The Initial Sound Spectrum". It embodies all wavelengths, all structures, all that is ultimately to come. What a sound that must be! Perhaps a real heavenly choir? Was it melodic, harmonic, beautiful to our ears?...... Alas no! It was pure noise. The initial sound spectrum is thought to be extremely simple; loudness increasing proportional to frequency, yielding a high pitched formless "hiss" [sound8.wav]. On reflection this is just as it should be, egalitarian through and through with no frequency preferred over any other. Furthermore, this kind of spectrum has a remarkable property: when clumps are allowed to grow naturally, structures of all sizes can form -- from stars to galaxies to clusters; none are omitted. Even if the initial silence wasn't "music" to our ears, Nature made by far the wisest and most creative choice.

Now let's take our trip beyond the acoustic era, and establish just how important sound was in determining all that was to come. In the 100 million years that followed fog clearing, the cosmic atmosphere gradually fell into the dark matter clumps, which were themselves growing denser all the while. Finally, the clumps became sufficiently dense to reverse their cosmic expansion and begin a true collapse. What happened to the atomic gas caught in these regions? It collapsed with them to become the first generation of newborn stars. This was a remarkable period in the life of the Universe. Those first stars were massive powerful beacons, living short lives which ended in cataclysmic explosions -- their brilliance lit up the whole Universe in a spectacular fireworks display the likes of which has never been seen since. Their searing light heralded a new dawn which ended that first cold dark 100 million year night. This period of darkness, let's not forget, had itself begun shortly after the Universe turned transparent, as the expanding fireball cooled and faded and darkness came. Of course, that night had itself brought to a close the most brilliant day of all, when the Universe burst forth in a fanfare of light and sound, announcing Nature's first dawn.

Returning to our newborn stars; in a sense, they emerged out of the highest frequencies, the growing hiss spawned from the smallest clumps. But recall that the full sound spectrum contains clumps and frequencies of all sizes. So as soon as stars had formed from the smallest clumps, the next largest clumps began to congregate to form star clusters, and these in turn were soon gathered into galaxies. In a hierarchy of gravitational clustering, the entire sound spectrum peeled off, with ever larger waves generating ever larger structures [Figure 13]. It took a billion years to form the first galaxies, and another eight billion to form the first galaxy clusters. Today, when we look out into the Universe, we see a labyrinth of galaxies and galaxy clusters which continue to assemble into ever larger patterns [Figure 14]. Indeed, the largest patterns we see today are huge intersecting sheets containing millions of galaxies. Looking at these sheets and voids of galaxies, one sees the remnants of what were once the crests and troughs of the largest primordial sound waves -- the very waves which made up the first and deepest harmonic of the cosmic chord.

Thus we learn that from cosmic sound came all of cosmic structure. Without it there would be no stars or galaxies. And without stars, there would be no elements and no planets and no people. The tapestry of galaxies, the star filled sky, the mind that holds both these, all have their roots in primordial sound. It is only fitting that one of our primary senses is sound, and one of our primary arts is music. Both can help bring us closer to an appreciation of the Universe, which for so long has yielded only to visual or abstract experience. With a little coaxing, it is now possible to listen directly to Nature whisper some of her oldest and deepest secrets.


Acknowledgements : It is a pleasure to thank the real experts here -- those who have created both the CMB maps and the computer codes. In particular, CMBFAST was written by Uros Seljak and Matias Zaldarriaga, and DASh, which is built around CMBFAST, was written by Manlo Kaplinghat, Lloyd Knox and Constantinos Skordis.