The First Sounds in the Universe.
There are supposedly no sounds in space, because space is a vacuum. After all, sound is a pressure wave that needs air in which to travel, so space must be completely quiet.
But as it turns out, there are sounds in space. Space might be more rarefied than anything we can produce in a laboratory here on Earth, but it is certainly not empty. In a typical part of the Milky Way, far from any stars, planets or nebulae, every cubic metre of space contains about a million atoms. This is more than 10 million trillion times fewer atoms than in a cubic metre of air at sea level, but it is still not a vacuum. Correspondingly, the pressure of the gas in space is extremely low. But because the pressure is not zero, the movements of stars, planets and other celestial bodies through the cosmos will produce upwards or downwards variations in this pressure. And these pressure fluctuations will then travel through space as sound waves.
As a result, the Universe is full of noise: the deep roaring of giant black holes, the sharp cracks of supernova explosions, and a myriad of other sounds. One way or another, all these sounds are produced by the actions of stars, black holes and galaxies. But these constituents of the cosmos have not always existed. We know that the Universe is 13.8 billion years old, and we know that there were times, very early on, when no stars or galaxies had yet formed.
So before the first star and before the first galaxy, were the vast stretches of the Universe filled with nothing but silence? Or was there a cosmic song long before there were individual singers? What was the first sound in the Universe?
These questions sound like the sort of thing best left to philosophers. But incredibly, astronomers can answer them with considerable precision.
There is very strong evidence that space and time both began with an event known as the “Big Bang”, which from our current best estimates occurred 13.8 billion years ago. But despite its name, the Big Bang is thought to have been utterly silent. The distributions of matter and energy created in this sudden cataclysmic event were almost perfectly smooth – there were no oscillations in pressure that could correspond to any noise.
However, after much less than a trillion trillionth of a second, when the observable Universe had expanded to about the size of a beach ball, the cosmos had become decidedly lumpy. As time passed, and the Universe continued to expand, the denser clumps of material used their gravitational attraction to pull in more mass toward them. These clumps then grew in pressure as the gas in them became more tightly squeezed, forcing the gas to expand. As these clouds of gas expanded, their pressure dropped and their expansion slowed. Gravity then began to exert itself, and the process repeated.
By less than a millisecond after the Big Bang, gas clouds over a whole range of sizes had begun collapsing and expanding, their pressure rising and falling as a consequence. Oscillations of pressure had been established – the Universe had found its voice!
These first sound waves were special. Rather than travelling from point A to point B, like my voice sending sound through the air to your ears, these waves oscillated up and down in pressure without actually going anywhere. These are known as “standing waves”, and are very similar to the stationary sound waves set up inside a flute or organ pipe.
The length of an organ pipe determines the tone of the sound it produces: the smallest organ pipes produce the highest notes. In an analogous way, the age of the observable Universe at these early times dictated the pitch of the primordial tune. When the Universe was very young, only clumps of matter that were relatively small, and for which the gas was able to expand and contract rapidly, had had enough time to complete one full cycle of pressure oscillations. Correspondingly, the cosmic choir was comprised only of sopranos. As the Universe aged, increasingly slower oscillations were completed, and correspondingly deeper notes were added to the chorus.
Furthermore, as time went on, the music became louder. This is because the overall level of clumpiness in the Universe increased as gravity began to exert its grip. As the clumps grew in size, the contrast between expansion and contraction of gas clouds was higher, and the pressure waves became stronger.
So what did the standing waves in the early Universe sound like? Just 10 years after the Big Bang, the dominant note in the Universe was F-sharp (but 35 octaves lower than the lowest note a human ear can perceive), at a volume of 90 decibels (about as loud as standing next to a lawnmower). Over the next hundred thousand years, a whole new set of larger gas clouds were able to begin oscillating: more than 13 octaves of even deeper notes were added to the celestial pipe organ, with the volume increasing by a factor of 20.
At any moment in time, just as the largest possible gas cloud was completing its first cycle of collapse and expansion, there were other gas clouds, exactly half the size, which had completed two full cycles, and yet more clouds, half again as large, which had oscillated four times. As a consequence, the loudest note was accompanied by a whole set of fainter harmonics and overtones.
However, do not envisage some pleasant sounding barbershop quartet. This set of harmonics was not the pure timbre of a musical instrument, but a blurry blend of overlapping notes. The result, if you could hear it, would be a fuzzy hiss, steadily descending in pitch and gaining in volume as the Universe aged.
This celestial song lasted for 380,000 years, but then abruptly ceased, never to resume. What happened to mute this enormous cosmological pipe organ? And how do we know that these sounds ever happened, if they vanished billions of years ago?
At very early times the Universe was a dense fog, because a ray of light was unable to travel even a short distance before colliding with a sub-atomic particle. It was throughout this period, known as the “pre-recombination era”, that clumps of gas expanded and collapsed, producing these first sounds.
However, after 380,000 years, the Universe had cooled to a temperature of 2700 oC, cold enough that sub-atomic particles could combine to form atoms. With this soup of free-floating particles removed, the skies cleared, and the cosmos became transparent.
This process silenced the Universe, because it changed the speed of sound. Before recombination, sound waves travelled through a gelatinous mix of light and matter, for which the speed of sound was about 60% of the speed of light, or about 620 million kilometres per hour. At this high sound speed, gas clouds were able to collapse and expand relatively quickly.
However, once matter and light went their separate ways, the speed of sound plummeted essentially to zero. At the moment of recombination all the sloshing of gas in and out immediately ceased, and the Universe became silent.
The cosmic symphony suddenly halted, right at the time when the Universe opened itself up for view. So how do we even know that these sounds existed?
We know because although these sounds have long since faded, the final crescendo is forever frozen into the very fabric of the cosmos.
The moment of recombination left behind the cosmic microwave background (CMB), a faint, cold light that fills the Universe. The CMB was discovered in the 1960s, and immediately became the object of detailed study by astronomers around the world. By the 1990s, precision observations were able to show that the glow from the CMB was not completely uniform, but that some parts of the sky were 0.001% warmer or cooler than others.
As measurements have continued to improve, these tiny variations have revealed a spectacularly detailed portrait of the Universe at that moment of recombination more than 13 billion years ago, just 380,000 years after the Big Bang. Because what these small temperature variations correspond to are individual clumps of gas, frozen in time in the middle of their pressure oscillations in or out. Those oscillations have now ceased their motion, but we can see them at their final positions. It is as if we have a photograph of the orchestra as it hits its final note: the conductor’s arms are raised high, and the performers can all be seen straining with effort as they play their instruments at their loudest volumes. But the sound itself is missing.
Astronomers have analysed these temperature fluctuations in considerable detail, and have found that the CMB is not comprised of a random jumble of different-sized sized patches of hot and cold, but that regions of higher or lower temperature tend to have certain sizes. In particular, most of the temperature variations that we can see extend over extents on the sky about twice the diameter of the full moon. This implies that there is a clear fundamental tone imprinted onto the Universe (subsequent analysis has that this is accompanied by at least five higher harmonics).
We can thus state with considerable accuracy and confidence that the dominant note of the cosmos at recombination was almost exactly 54 octaves below middle C, at an ear-splitting volume of around 120 decibels. To play this note, an organ would need a pipe more than 10 trillion kilometres long!
After recombination, the Universe continued to expand and cool, but did so in absolute silence. Over the next hundreds of millions of years, clumps of gas that happened to be near maximum contraction at recombination were able to continue collapsing under the influence of gravity, and eventually coalesced into the first stars and galaxies.
There is a startling connection between the strange harmonising of the pre-recombination era and the hubbub that the cosmos experiences today.
As we can see directly from the CMB, the hottest gas clumps at recombination (i.e., those that were just completing the compression part of their pressure oscillation at the moment the Universe became transparent) all had a particular size. The size that we see on the sky, about double the size of the full moon, corresponded to a physical extent of 460,000 light years at the time of recombination. However, over the more than 13.8 billion years since then, the Universe has expanded by more than a factor of 1000. As a consequence, if these regions still existed now, they would have been stretched so that they would now be 500 million light years across.
In the early 1980s, astronomers began to measure the three-dimensional positions of hundreds of relatively nearby galaxies, and found that they are not scattered uniformly, but are clumped into complicated patterns. The realisation that the Universe is not totally chaotic but has a characteristic structure was a remarkable discovery.
But in 2005, when astronomers had expanded their catalogues of galaxy positions to many tens of thousands of objects, an even more incredible result emerged. Not only is the distribution of galaxies clumpy, but the size of these clumps is not random. How big is a typical clump of galaxies? Pretty close to 500 million light years, the same size the hot clouds of gas from recombination would be if they had survived through to the present.
The conclusion is inescapable: these hot clouds have survived, but have now evolved into galaxies, stars, planets and people. What we see all around us, and indeed ourselves are part of, is a fossil record of the oscillating sound waves from the earliest times in history, forever woven into the distribution of matter throughout the cosmos.
The first sounds in the Universe ceased long ago. The conductor and the musicians have departed the cosmic stage, taking their instruments with them. However, the performers have left behind their sheet music. By studying the cosmic microwave background and the large-scale structure of the Universe, we can recover the first music ever played, music that was never intended to be heard.
Bryan Gaensler (@SciBry) is Director of the Centre for All-sky Astrophysics at The University of Sydney. This is an edited excerpt from his book Extreme Cosmos, published by NewSouth Books (Australia/NZ) and Penguin (everywhere else).