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