by Katie Mack
THE SURFACE OF LAST SCATTERING
After Big Bang Nucleosynthesis, things in the infernoverse started to settle down, relatively speaking. At that point, the mix of particles was more or less stable, and would remain so until the time of the first stars, millions of years later. But for hundreds of thousands of years, the cosmos was still a hot, humming plasma composed mostly of hydrogen and helium nuclei and free electrons, with photons (particles of light) bouncing around between them.
Over time, the expansion of the universe gave all that radiation and matter room to spread out. I sometimes imagine experiencing this phase of the early universe like a journey from the center of the Sun outward, but instead of moving through space, you’re moving through time. You start in the center of the Sun, where the heat and density are so high that atomic nuclei are fusing together to make new elements. The solar interior is opaque with light, with photons continually bouncing off electrons and protons so violently that it can take hundreds of thousands of years of constant scattering for a photon to reach the surface. Eventually, as you move outward, the plasma becomes less dense and light is able to travel farther between scatterings. At the surface, it can stream freely out into space.
In a similar way, a journey through time from the first few minutes of the universe to about 380,000 years later takes the entire cosmos from that hot dense plasma to a cooling gas of protons and electrons that can finally come together to make neutral atoms, allowing light to travel freely between them instead of constantly bouncing off the charged particles. We call the end of this fireball stage of the early universe the “surface of last scattering,” because it’s a kind of surface in time at which light goes from being trapped in plasma to traveling long distances across the cosmos.
This is what we see when we look at the cosmic microwave background: the moment that delineates the end of the Hot Big Bang, and the transition to a universe in which space is dark and silent and light travels through it. It is also the beginning of the cosmic Dark Ages—the time when the gas is slowly cooling and condensing into clumps, drawn in by the tiny density blips set up by those initial fluctuations. Sometime around the hundred-million-year mark, one of those clumps becomes so dense it is able to ignite into a star, and Cosmic Dawn has officially begun.
COSMIC DAWN
The transition from a dark, gaseous universe to one shimmering with the light of galaxies and stars was driven, largely, by a kind of matter so exotic that we haven’t been able to re-create it even in the most powerful particle colliders. In the mix with the radiation, hydrogen gas, and a sprinkling of other primordial elements was a substance we know today as dark matter. It’s not really dark, but rather invisible: seemingly unwilling to interact with light in any way. No emission of radiation, no absorption, no reflection. A light beam heading toward a clump of dark matter, as far as we can tell, passes right through. But where dark matter really shinesXVII is in its ability to gravitate. When regular matter tries to condense in a clump drawn in by its own gravity, that matter has pressure, and pushes back. But dark matter can condense without feeling this force. A side effect of not interacting with light is not interacting much with anything at all, since, under most circumstances, collisions between particles of matter come from electrostatic repulsion, which needs interaction with light to take place. (Photons are particles of light, but they’re also the carriers of electromagnetic force, so if something is invisible, it doesn’t experience electromagnetic attraction or repulsion.) No electromagnetism, no pressure.
The first little blips of higher-density matter, set down by the fluctuations at the end of inflation, contained a mix of radiation, dark matter, and ordinary matter. Because the ordinary matter had pressure, and mixed with the radiation, at first only the dark matter was able to clump together due to gravity without immediately bouncing back. Later on, when the universe expanded more and the radiation streamed away from the cooling matter, gas was able to fall into these gravitational wells and begin to condense as stars and galaxies. Even today, the structure of matter on the largest scales—the cosmic web of galaxies and clusters of galaxies—is scaffolded by a network of dark matter clumps and filaments. At Cosmic Dawn, those invisible clumps and filaments first started to light up, as stars and galaxies ignited and shone, sparkling along the network like fairy lights in the darkness.
THE ERA OF GALAXIES
The next big transition for the universe came when so much starlight was coursing through space that it was able to ionize the ambient gas that had become neutral at the end of the cosmic fireball stage. This intense starlight broke hydrogen atoms back apart into free electrons and protons, creating giant bubbles of ionized hydrogen gas surrounding the brightest collections of galaxies. Those bubbles expanding through the cosmos marked the Epoch of Reionization (“re-” because the gas had been ionized during the Big Bang at the beginning, and was now being ionized again by the stars). That transition, which was completed at about the billion-year mark, is now one of the frontiers of observational astronomy, and we are only just beginning to understand how and when it occurred. In the almost 13 billion years since that time, things have carried on in much the same way, with galaxies forming and combining, supermassive black holes building up mass in galactic centers, and new stars being born and living out their lives.
* * *
So, here we are. The cosmos as we see it today is a vast, beautiful web of galaxies shining in the darkness. Our own pretty blue-and-white world orbits a moderately sized yellow star in a galaxy that is, in every meaningful way, fairly close to the average. While we have yet to find clear signals, this unremarkable galaxy might be teeming with life, as the debris of long-exploded supernovae creates the basic ingredients of biology on each of the worlds scattered around a hundred billion stars. By current estimates, as many as one in ten star systems has a planet of just the right size and distance from its star to sustain liquid water on its surface—a hint, though not a certain one, that life could find a way to thrive. In the trillion other galaxies visible across the observable universe, there could be countless other species, with their own civilizations, arts, cultures, and scientific endeavors, all telling their stories of the universe from their own perspectives, slowly discovering their own primordial past. On each of those worlds, creatures like, or unlike, ourselves might be detecting the faint hum of the cosmic microwave background, deducing the existence of the Big Bang and the staggering knowledge that our shared cosmos does not go back forever in time, but had a first moment, a first particle, a first star.
And those other beings, like us, might be coming to the same realization: that a universe that is not static, that had a distinct beginning, must also, inevitably, have an end.
I. Science fiction loves to ignore this. There’s an early episode of Star Trek: The Next Generation in which they accidentally travel a billion light-years in a few seconds and the place they end up in is some kind of abyss of shimmering blue energy and thought, which, if it really existed, we’d totally be able to see in telescopes.
II. We can thank relativity for this. Special relativity says time passes more slowly for us when we are moving quickly; general relativity says it slows down when we are close to a massive object.
III. When Doc Brown in Back to the Future proclaimed, “You’re not thinking fourth-dimensionally!,” he was talking to you.
IV. I’m being a bit flippant here but this is a rather important point. So far, in physics, most of what we’ve done is describe the universe with mathematical constructions we call models, and use experiments and observations to test and refine those models, until we arrive at a model that seems to fit the observations better than any of the others. And then we start trying to break the model. It’s not that we just trust that math is fundamental to the universe, it’s that there doesn’t seem to be any other way to approach these things that makes any sense.
V. “Our WHOLE universe was in a hot dense state, then nearly 14 billion years ago expansion started�
��” Yes, the Barenaked Ladies got it right: the beginning of the theme song for the TV show The Big Bang Theory is actually a very good summary of the theory itself.
VI. Of course, this is before “years” were a thing, because it is before there was a planet orbiting a star, defining a unit of time. But we can take our own units and extrapolate back and just label all the seconds as they count up to years and give them numbers, for our own convenience.
VII. I just now invented this term and I’m feeling very proud of myself.
VIII. Sadly, this line of investigation did not end well for the pigeons, who were in fact innocent of all wrongdoing.
IX. Without knowing anything about this story except the bit about the pigeons, I happened to run into Bernard Burke a few years ago at MIT. We were just chatting as physicists tend to do, and he was telling me about some past work I didn’t really follow and at some point I realized he was talking about his phone call with THE Penzias and just casually dropping the fact that he was a catalyst for one of the most important discoveries in the history of physics. A similar thing happened to me at a conference several years ago when I met Tom Kibble, who built much of the theory around the Higgs boson. Moral of the story: listen to the emeritus profs; they might have quietly revolutionized your whole field of study.
X. In the course of writing this book, I was thrilled to hear that Peebles won a 2019 Nobel Prize for, in part, the theory side of this discovery. So maybe there is some justice in the end. Just not for the pigeons.
XI. The name “blackbody” comes from the idea of an object—a “body”—that perfectly absorbs all the light that hits it and reemits it as pure heat. Most objects don’t do this perfectly, of course; they reflect a bit of the light, and some is absorbed without being reemitted. But most materials when heated up will glow at some level in a way that is recognizable as an approximate shape of a blackbody curve.
XII. The light is all in the microwave part of the spectrum, so “redder” means lower-frequency microwave radiation, and “bluer” means higher-frequency microwave radiation, but when we make maps we do actually use colors like red and blue, because, you know, human eyes.
XIII. Named for Max Planck, an early originator of quantum theory. There’s also a Planck energy, length, and mass, all defined through various combinations of fundamental constants, one of which is the Planck constant, which is central to anything with a quantum nature. If you find the Planck constant in your equations, you know things are liable to get weird.
XIV. There’s a subtlety here in this simplified explanation that has always bugged me. I’m telling you on the one hand that these regions have never communicated in the history of the cosmos, but I’m also telling you that the universe started with a singularity where, one might suppose, all the distances between things were zero. The reason this doesn’t solve the problem is this. Take two points on opposite sides of the sky now. For the sake of argument, we’ll say they were at zero distance at time zero. The problem is, at every time AFTER zero, those parts were not in contact—they couldn’t have had any information exchange (like a light beam carrying information about temperature). What about, you say, zero itself? While we can label the first moment zero, it is literally zero time. Time began at the singularity. So there wasn’t any time for the information exchange (because there wasn’t any time) and every moment after that has the “too far apart to communicate” problem.
XV. Today, we find antimatter in certain kinds of particle reactions, but mostly notice it because when an antimatter particle meets its matched regular matter particle, they annihilate, destroying each other and creating a burst of energy.
XVI. They are also, incidentally, giving us clues about the other end of time: recent breakthroughs have shown us evidence that the end of the universe might come in a totally unexpected way, and it may happen at any moment. But that’s all covered later in the book; let’s not get ahead of ourselves. We’ll probably make it to Chapter 6.
XVII. I’m very sorry.
CHAPTER 3: Big Crunch
Let’s start with the end of the world, why don’t we? Get it over with and move on to more interesting things.
N. K. Jemisin, The Fifth Season
On a dark, moonless night in autumn, in the Northern Hemisphere, look up and find the wide W shape of the constellation Cassiopeia. Stare at the space below it for a few seconds and, if the sky is dark enough, you’ll see a faint fuzzy blur almost as wide across as the full Moon. That blur is the Andromeda Galaxy, a great spiral disk of about a trillion stars and a supermassive black hole, all of which are hurtling toward us at 110 kilometers per second.
In about four billion years, Andromeda and our own Milky Way galaxy will collide, creating a brilliant light show. Stars will be flung chaotically out of their orbits, forming stellar streams that stretch across the cosmos in graceful arcs. The sudden smashing together of galactic hydrogen will spark a minor explosion of star birth. Gas will ignite around the previously dormant central supermassive black holes, which will meet in the middle of everything and spiral into each other. Jets of intense radiation and high-energy particles will pierce the chaotic tangle of gas and stars, while the central region of the new Milkdromeda galaxy is irradiated with the X-ray-hot glow of a whirlpool of doomed matter falling into the new, even more supermassive black hole.
Even in the midst of this great galactic train wreck, the vast distances between stars will make head-on stellar impacts unlikely: the Solar System as a whole will probably survive, more or less. Not the Earth, though. By that time, the Sun will have already begun to swell to red-giant size, heating up the Earth enough to boil the oceans and completely sterilize the surface of all possibility of life. If, however, any outpost of human ingenuity manages to sustain a presence in the Solar System to watch, the combining of the two great spiral galaxies will be an awe-inspiring and beautiful process, playing out over billions of years. When the particle jets and supernova fires have calmed, the resulting mass will become a giant ellipsoidal collection of old and dying stars.
* * *
As cataclysmic as it may be to those in the midst of it, the merging of galaxies is an everyday occurrence in the cosmic sense, and strangely lovely if viewed from an extremely distant vantage point. Large galaxies tear apart and cannibalize smaller ones; adjacent stellar systems combine with one another. Our own Milky Way shows evidence of having consumed dozens of smaller neighbors—we can still see trails of stars tracing giant arcs around our own galactic disk like debris from an interstellar car crash.
Throughout the universe, however, collisions like this are becoming increasingly rare. The universe is expanding: space itself—that is to say the space between things, not the things in it—is getting bigger. This means isolated individual galaxies and groups of galaxies are getting, on average, farther and farther apart. Within each group and cluster, mergers can still occur. Our immediate neighborhood collection of stellar systems, members of the blandly named Local Group, are a ragtag gang of small and irregular galaxies dominated by the two giant spirals, and we are all destined to get nice and cozy sooner or later. Venture farther afield, though, beyond a few tens of millions of light-years, and everything appears to be spreading out.
The big question, in the long term, is: will this expansion continue indefinitely, or will it eventually stop, turn around, and bring absolutely everything crashing together? How do we know the expansion is even happening?
When you’re in a universe that’s expanding the same way in every direction, it doesn’t look like expansion, per se, but rather like the odd phenomenon of everything else receding from you… wherever you are. From our perspective, we see distant galaxies all moving away from us, as though we emit some kind of repulsive force. But if we were suddenly in a galaxy a billion light-years from here, we would see the same phenomenon: the Milky Way and everything else beyond a certain distance would be receding from that point. This behavior is a somewhat counterintuitive consequence of space getting b
igger in the same way, at the same rate, everywhere.
Figure 7: Illustration of cosmic expansion. Here, the increasing size of the universe at three different moments is represented by the increasing size of the square from left to right. As time goes on, galaxies move apart from one another, but they do not get bigger with the expansion of space.
The upshot is that every point in the universe is the nexus of what appears to be a powerful uniform repulsion. Technically, the universe doesn’t have a center. But we’re each the center of our own observable universe.I And from our perspective, all the galaxies farther out than our near neighbors are careening away from us as fast as they can. It’s not us; it’s cosmology.
Cosmic expansion was harder to discover than you might think. While galaxies other than our own have been visible through telescopes since the 1700s, their distances are so great and motions so slow (as judged by human timescales) that determining how they were moving relative to us, and even that they were galaxies at all, took more than two centuries. Even now, the most powerful telescopes can’t see the motion directly—the galaxies don’t appear to be farther away every time we look. But we can detect it by carefully teasing it out of a seemingly unrelated property of galaxies: the color of their light.