宇宙初生的第一声交响：从暴胀量子涨落到星系化石

You may have heard the phrase "in space, no one can hear you scream." But that's true only for human ears. The universe is filled with tenuous atmospheres of one kind or another, and through those atmospheres, pressure waves -- sound -- moves. Through stars, across galaxies, even between galaxies. One of the most wonderful and recently discovered examples of cosmic sound is in the very young universe, shortly after the Big Bang.

Now you may be wondering, how would we know that? Well famously, as you look very far out into space, you also look back in time, simply because it takes time for the light to get to you. Remarkably, if you look far enough past all the galaxies, you can see to a time before any galaxies had yet formed, even to a time when the universe itself had only just been born. So what do you see coming from this newborn universe? You see the light of the Big Bang's hot, glowing youth. At that time, the universe was filled with an almost uniform, hot, glowing gas of atomic nuclei and electrons and intense light. That light, on its way to us, has crossed an expanding universe. That expansion not only carried those regions far away from us, but also as the light waves crossed an expanding space, they were stretched from micron-sized waves to millimeter-sized waves. So although it is light that leaves the young universe, it’s microwaves that arrive. And they are the famous cosmic microwave background.

Now because every direction in which you look ends back in the young universe, then the microwave radiation comes to us from all directions, and for many years, its brightness was thought to be extremely uniform, around the full spherical dome of the sky, a bit like this image here. But as microwave telescopes became more sensitive, they began to see slight variations in brightness from place to place, so more recent images of the microwave sky appear completely covered in very slight patches. You can see them more clearly in this blowup of a region that's about the size of your fist, held at arm's length. It spans eight million light-years and shows us the newborn universe when it was only 400,000 years old. Now that's equivalent to a one-day-old human. So what are these patches? Why is it slightly brighter and darker in different places? Well, these are places where the temperature and pressure in the glowing gas are slightly higher and slightly lower, or stated differently, the bright and dark patches are the peaks and troughs of huge sound waves moving through the primordial atmosphere. The brightness contrast reveals 90 decibels -- that's rock-concert loudness, and their gigantic size reveals a frequency or pitch, a whopping 50 octaves below the human range. One wave might pass you by in 50,000 years.

So what causes this sound? Now don’t be fooled. It's not the bang of the Big Bang. No, it's a slowly growing sound driven by gravity. The distribution of matter at that time was slightly uneven, and where there was a denser region, its stronger gravity pulled in the surrounding gas, which compressed and bounced back out again, only to fall back in again, creating an approximately spherical sound wave. Now the landscape included small and medium and large regions, and so, a bit like a set of organ pipes of different sizes. Together, they create a wide range of pitch.

So what did the primordial sound sound like? Well to find out, you must first measure the sounds spectrum. Now a computer can do that. And here is the remarkable result. There is a fundamental tone and several higher harmonics. And just for comparison, here is the sound spectrum of a flute playing a single note with its fundamental and harmonics. Now although these two sets of harmonics arise for somewhat different reasons, nevertheless, it does seem that the young universe had qualities similar to a musical instrument or even a human voice singing. But notice how those cosmic harmonics are a little bit broad. So please don't expect the primordial sound to be too clean or musical to your human ears.

Now before I play you the sound, take a look at that green line that goes right through all the data. It comes from a detailed computer calculation that aims to include all the relevant physics. The fit is amazingly good and shows that we really do understand what’s happening in the young universe. In fact, the task of matching those computer calculations to the data basically measures quite accurately many of the fundamental properties of our universe. OK, let's now listen to the sound after up-shifting by 50 octaves so that we can hear it. Now I’ve also used those computer calculations to track the sound forward in time, starting at the Big Bang and spanning the full acoustic era, which ends at 400,000 years. The sound is accompanied by a graphic that's a bit busy, but just focus on the green line, which tracks the changing harmonics as the young universe begins to expand and age. Also, don't worry, I’ve compressed those 400,000 years into just 10 seconds. Here we go.

OK, thank you. If you weren't quite awake before that, you are now. Well as I warned you, those broad harmonics make a sound that's more like noise than music to our ears. And I'll return to that in just a minute. You obviously noticed the drop in pitch, and that's because as time passes, larger and larger regions, larger organ pipes, have had time to start sounding. And as their larger waves get included, also the overall pitch drops.

Now you might wonder whether this wonderful acoustic period back in the universe’s youth left any traces in today's old universe filled with galaxies. And the answer is yes, it has. And here's why. At 400,000 years, the cooling universe suddenly turned transparent. The sound waves ceased oscillating and froze in place, and the matter began to collapse under its own gravity to make first stars and then galaxies. Now here's a map of today's old universe, spanning four billion light-years, with us at the center. And it shows the positions of about 100,000 galaxies. And if you perform a similar wave analysis, sure enough, there's the fundamental and harmonics showing up faintly in those web-like patterns. The sound waves have turned to stone, so to speak, and become fossilized in the patterns of galaxies.

But even more remarkable, I think, is that we think all of this cosmic structure, the sound waves and the galaxy patterns, had their ultimate cause with subatomic quantum vibrations during the universe's birth process. An incredibly short time of hyperexpansion that we call inflation. Now this is a truly stunning proposition. That quantum oscillations smaller than atoms get amplified by cosmic expansion to make huge sound waves that then transform into an ever-expanding tapestry of galaxies. It's a direct connection between the quantum world and the cosmic world.

Well lastly, I wanted to introduce a project that brings this primordial sound and its changing harmonics into our own human musical world. And so after up-shifting by 50 octaves, I timed when the pitch of each harmonic matched the pitch of each note on a simple piano keyboard, and you can see the notes here, descending over time. Now this modifies the sound in two important ways. It narrows those broad, noise-like harmonics into single musical notes. And secondly, those notes get sounded at particular times. So overall, the sound takes on a more melodic and rhythmic character. So here's a brief sonification of these notes for a period lasting 150,000 years, compressed to 10 seconds. [Cosmic harmonics]

OK, thank you. To finally add artistic expression to this, I gave these notes to sound artist Ander Mikalson, who worked with Pulitzer Prize-winning composer Caroline Shaw to write a work for choir and organ, lasting about 10 minutes, and based on these note sequences. It's been performed a number of times, but here's a brief excerpt from a performance in the Catholic Cathedral in Richmond, Virginia. [Music]

I think this is one of the few times I've experienced a genuine coming together of science and art, both framed within a sacred context. It was a moment to feel deeply connected to and part of the entire universe. Those particles and atoms from that first acoustic era went on to make stars and galaxies and planets and, of course, us. And right now, some of those atoms are inside your brain, somehow witnessing and understanding their own ancestry. They were out in that hot, glowing gas, participating in that first symphony. Nowhere else, as far as we know, can the universe appreciate itself. The stars and galaxies are not themselves sentient, but we are. We are an exceedingly rare part of the universe. Perhaps the only part through which the universe can witness and understand and marvel at itself. Thank you.