I want you to imagine visiting a hotel, one familiar but with some odd properties that you usually don't encounter. Feel free to call it Hotel Dirac if you want. If you don't, you'll get the joke later.

Here's how the hotel works. There are multiple floors and rooms on each floor, as usual. But there can only be one person assigned to any room at any one time, and the rooms are filled starting with the first room on the first floor, then the second room, and so on, until the lowest floor is filled; then rooms get assigned on the second floor. There's technically an infinite number of floors, but that's not really relevant.

Sometimes a guest will feel like moving up in the world and bump themself up to a room on a higher floor. But they only get to visit that penthouse suite for a little bit of time—as soon as one of the hotel staff notices (and they're very diligent about these sorts of things), the guest gets scolded and pushed back down to a lower room.

Now here's where the hotel gets especially strange. The rooms don't stop at the ground floor—there's a basement, and that basement is full of rooms. And beneath that is another subsurface floor, equally stuffed full of rooms. And down and down and down—there is technically an infinite number of rooms beneath the surface, but that's not especially important for now either.

When you arrive at Hotel Dirac, the hotel may look empty, but really all the subsurface floors are already occupied with guests, one per room. So what you see as an unoccupied ground floor actually sits on top of countless millions of underground guests.

When I say that any guest in the hotel occasionally moves up to a random higher level, I really mean it—that includes the underground rooms. Someone down there can get motivated, find the nearest elevator, and briefly get to enjoy the views out the window—until the surly hotel staff finds them and bounces them back down.

When such a guest gets bumped up to a higher room, they may find that all the room doors are unlocked—they can wander from room to room on their floor, flicking on the lights as they go, checking out the layout and seeing all the possible views. They can't change floors without permission, but the rooms on each floor are fair game.

What about the empty room they left behind, on one of the underground floors? Down there all the rooms are unlocked too, and curious looky-loos will float in and out of that unoccupied room, one at a time.

What does Hotel Dirac look like from a distance when an underground guest gets bumped to a higher floor? That guest, floating from room to room, turns on the lights when they enter and off when they leave. From far enough away, you'd see a single light on an upper floor, shifting from room to room as the guest explores.

If you could see through the ground, what would the subterranean floors look like? As the guest left, they dutifully turned off the light—that room looks like a hole in an otherwise unbroken sea of lights. But as other nosy guests switched to the unoccupied room, the “hole” would appear to shift around.

For as long as the guest got to remain upstairs, a light would move on the aboveground floors, and a hole would move on the underground floors. Once the staff noticed the discrepancy, however, the guest would reluctantly slink back down beneath the ground level, find the nearest unoccupied room, and turn on the light, closing the hole and returning life to normal.

This is the picture of the world as painted in the 1920s by Paul Adrien Maurice Dirac, who accidentally discovered antimatter as a result of trying to solve another problem—reconciling the burgeoning description of quantum physics with the already-established theory of special relativity.1

This marriage was attempted and then abandoned by Erwin Schrödinger, who instead settled for a less general formalism—his eponymous wave equation, which students across the world grapple with in frustration on a daily basis.² But Dirac managed to nail it; all it took was a completely new mathematical description of the world and a serious rethink about the nature of reality, so you can see why Schrödinger shied away.

And buried within the mathematics of the theory was a surprising little symmetry: a new kind of particle, positioned like a mirror to our everyday world. Every fundamental particle, like an electron, was matched by a new particle with identical properties (mass, spin, etc.) but with perfectly opposite charge. For the electron, its antimatter twin is called the positron. Exactly like the electron, but with a positive sign in front of its charge.

The positron was experimentally discovered a couple of years later, followed shortly by the twins (antitwins?) of all the other known particles.

The scenario of Hotel Dirac is a useful way to explain a surprising situation: if you have a bit of light—a photon—at high enough energies, it can spontaneously split into an electron and a position. After traveling for a bit, the two particles will find each other again, collide, and disappear in a flash of light, releasing back the original photon.

In this picture, what we view as the “ground state” of the electron in any situation—the lowest possible energy state—actually sits on top of an infinite pancake stack of negative energy states, already occupied by a subterranean hotel full of electrons. A photon of sufficient energy can knock one of these negative-energy electrons into a positive-energy state, where it runs around doing all the things that electrons do. But it leaves behind a “hole” in the sea of negative-energy electrons, and that hole looks, acts, and smells like a typical electron—except it has the opposite charge.

Eventually the electron gets tired of wandering in the positive-energy world and falls back down into its hole, releasing the energy that originally promoted it. The photon returns, and everything is back to status quo.

This picture isn't exactly correct—our more modern view of the process is different in some subtle and important ways, and I'll get to that in a another chapter, but it does serve a very useful point here: matter and antimatter are symmetric. Or at least, ought to be symmetric. For every piece of matter—an electron over here, an atom over there—there ought to be a matching twin with the opposite charge out there, somewhere.

This symmetry of antimatter is baked into the same mathematics that predicted its existence in the first place. It appears completely unavoidable.

But look out there, somewhere, anywhere. See any antimatter? No, you don't. From one end of the Milky Way to the other, from the earliest moments of the universe that we can observe to the present day, matter rules the cosmos. Almost all the stuff is normal, not anti. If there were, say, a galaxy composed entirely of antimatter, then as it swam through the thin soup of particles between the galaxies, it would be releasing enormous amounts of energy—the most energetic events ever known.

One ounce of antimatter annihilating with one ounce of normal matter would release the energy equivalent of about a good-sized H-bomb. One galaxy of antimatter annihilating with one galaxy of matter would release—let me see here—ah, right, a lot of energy.

We don't see it. We don't have the suspicion of seeing it. We don't even have a hint of a suspicion of seeing it.

Matter, not antimatter, dominates the universe, and has for an incredibly long time—essentially its entire history. We're obviously misunderstanding something.

Where did all the antimatter go?

When we last left the story of the universe, it had reinvigorated itself after exhausting its energy in the most rapid expansion yet known—and ever to be known. Inflation had ballooned the observable cosmos to the preposterous dimensions of an apple, in the slightest sliver of a second. That inflation was triggered by the splitting of the strong nuclear force away from the others, and the process spread out all the matter into a cold, thin soup.

This soup was somehow reinvigorated as whatever drove inflation shook itself off (cosmologists are still working out that detail of the story). But that reinvigoration itself creates a new problem (sensing a common theme here?). Let's say the universe at this stage is filled with high-energy radiation at a temperature of 10¹⁵ K. Some of those photons can transform into pairs of electrons and positrons (and a host of other particles), but they will always do so symmetrically. For every bit of matter that pops into existence, a matching bit of antimatter will be along for the ride. Eventually they'll find each other, tragically end their lives in a fury of mutual self-destruction, and return back to radiation.

But our universe, today, is not filled with only radiation. There's matter all over the place. To create the amount of matter that we observe, the imbalance in the newborn universe didn't have to be much: just one part in a billion extra in the ratio of regular matter to antimatter would do it.3 That's a small number, but even small numbers are huge compared to the totally zero predicted by particle physics.

Long side note: Unfortunately, when dealing with high-energy physics, which is the realm of the early universe, the jargon comes fast and furious. We have to keep track of the names for all the particles, the antiparticles, the hypothetical particles, the forces, the hypothetical forces, groups of particles, families of particles, and on and on. Plus it's all twisted up because some processes got their names assigned before we fully settled on a definition. Hence I've been trying to avoid most of that messiness, but just in case you want to look this stuff up on the internet later (you masochist), the name given to the process of the domination of matter over antimatter is baryogenesis, and no, my spell-checker doesn't recognize that as a real word either. Baryon in most contexts means particles made of three quarks, like the familiar proton and neutron. And quarks are—well, I'll just save that for later.

Anyway, to solve this riddle, we have a few options. Take your pick:

Option 0: Asymmetry is a lie! There is actually tons of antimatter out there; we just live in a little patch of regular matter. But as I talked about earlier, the implications of that kind of universe seem kind of (a) violent and (b) obvious, so essentially nobody finds this palatable.

Option 1: The universe had an asymmetry between regular matter and antimatter baked in since the beginning. I know, I know, there may not be a “beginning,” but in this argument, there is some overarching rule that says the two kinds of matter are not really in balance, and it's been in place for the entire history of the universe, in a similar vein as a rule that says, “By the way, there's a force of gravity.” This is generally unappealing because it's totally just sweeping the problem under the rug and pretending it doesn't exist, and because we see no evidence of this grand law operating in the present-day universe. You would think something that was that big of a deal ought to hang around for longer than a second.

Option 2: Hey, I know, there were lots of crazy physics happening in the preinflation madhouse era, so maybe that's the key! We don't really understand the physics, so maybe tucked into an equation here or slipped into a term over there is an imbalance, and that will do the trick. In this story, before inflation even got rolling, the stage was set for baryons to dominate. Our understanding of this epoch is fuzzy enough to accommodate lots of wacky ideas (something the theorists among us love) but not clear enough to actually separate one idea from another (something the theorists hate, because none of them can get the validation to win a Nobel Prize). While a valid choice, this option is basically the community saying, “Let's have the next generation of scientists solve this one.”4

Option 3: Maybe the imbalance came later, after inflation, as the universe was steadily expanding and cooling. It's still a crazy mess of a place, and there's plenty of particle wiggle room to get up to some funky stuff. For example, the weak nuclear force still hasn't split off from the electromagnetic force, and while we largely understand that process, there might be something interesting there.

Just for fun, and because it's our most solid lead, let's follow Option 3.

I really need to introduce the weak force properly to show how it might play a rather unexpected role in disrupting the delicate balance between matter and antimatter. Let's face it: nobody treats the weak force with any respect. I mean, just look at the name! The other forces have ancient, complex, or assuredly self-descriptive names. The weak nuclear force is indeed weak, but it's far, far stronger than gravity. And it does play a role in nuclear reactions, but not in the same way as the strong force.

In essence, the strong nuclear force is a binder: it glues things together (except when it repels—it's complicated5). The weak nuclear force is a transformer: it can change one kind of particle into another. That may not seem impressive, but it lies at the heart of radioactive decay and the synthesis of heavy elements. So yeah, kind of important.

And when it comes to matter versus antimatter, the weak nuclear force has a favorite. It's not immediately obvious, and for the effect to show up it requires piles of particle collision data. It also came as a big surprise to particle physicists when it was discovered in the 1950s and ’60s, but that's just life.

I won't go into the details here, since I'm trying my best not to make this a textbook on particle interactions,⁶ but in a collider you can make some exotic combinations of particles. These exotic combinations don't hang around for long—they're unstable and quickly decay into a shower of smaller, longer-lasting particles. Two of these bizarre characters, called the pion and kaon (pro tip for any wannabe particle physicists: if you need to name something new, just take letters from another alphabet and add “on”), decay into various children particles with various rates.

But they don't decay into exactly the same particles at the same rates every time. They show an ever-so-slight preference for decaying into one combination of charges versus the opposite. In the jargon that we are now enmeshed in, their decays violate C-symmetry, or symmetry of charge. These decays also violate another apparent symmetry of our universe called parity, which means that all fundamental interactions at the deep particle level look the same in a mirror. Well, almost all: these pion and kaon decays are an exception.

So both C (charge) and P (parity) are not essential symmetries in the cosmos, even though for a long time we thought they were. If you're curious, the ultimate combo of charge-parity-time (CPT) is thought to be persistent: if you take a particle interaction of your choice, flip all the charges, run it in a mirror, and run it backward in time, you shouldn't see any difference. But let's not get carried away here.

This symmetry violation in the weak nuclear force is important because it provides a known channel for favoring one kind of electric charge. And since the weak nuclear force doesn't get to become a player in cosmic history until after inflation, that's why we think it's a prime candidate for making the universe—wait for it—matter.

Violating this central symmetry, however, isn't the only part to the story. I know, just as you thought we were getting out of the woods. Two other conditions must be satisfied if you want more matter than antimatter, and they are harsh conditions indeed.

One is that there must be a process that produces a raw excess of matter over antimatter. Wait, what? Why didn't we just, you know, start with that? You might think this is the only condition you need, and you're almost certainly questioning my decision to regale you with tales of kaons and symmetry violations. However, you need both conditions (a favor for matter and a favor for charge) to get the desired result.

Now would be a good time to take a break. Go on, I'll wait.

You can have a process that makes an abundance of matter all you want, but if charge symmetry is enforced, it must be matched by a process that generates more antimatter than the regular kind. In other words, you might think you're cleverly generating an imbalance, but nature will sneakily slip in some back-channel reaction when you're not looking to make sure everything evens out. Then all the particles will end up back in balance, despite your best efforts, and you'll be stuck with a radiation-only universe again. So you need a channel for generating extra matter, and you need to make sure that channel isn't negated by its evil charge twin. Only then can you flood the universe with regular matter.

Unfortunately, creating excess matter doesn't happen in our normal everyday universe. Fortunately, the initial moments into the history of the cosmos aren't our normal everyday universe.

The culprit is once again the crafty weak nuclear force. We know that every once in a while, a weak interaction can produce an excess of matter, but the channels available to do it are highly suppressed—they are so rare that they essentially never happen at low energies. But at high energies, especially energies high enough to merge the weak and electromagnetic forces, these processes can operate at full blast. So it's certainly possible to transmute radiation into a matter-filled early universe, using hidden tricks and trapdoors built into the nature of the weak force itself.7

Unless your universe is in equilibrium. If you have a hot ball of gas or plasma, and it's left totally to its own devices, then all allowable processes and interactions within that hot ball will happen, canceling out each other. That's the very definition of equilibrium. So if the young universe were in such a state, any method that produced extra normal matter would be competing against other methods that produced extra antimatter, and nobody would win, ending in a draw (i.e., no matter at all, anywhere).

Back to square one.

Ever ready for a three-peat, the humble weak force comes to the rescue to satisfy this last of the necessary conditions to tip the cosmic scales in favor of normal matter. But not the force itself; here, the splitting of the unified electroweak interaction, while not quite as violent as the inflation-inducing cataclysms of earlier epochs, was just as spontaneous and scattered. The cooling from electroweak to electromagnetism-plus-weak didn't happen all across the universe simultaneously, but rather as bubbles sparked at random places, each spreading outward.

It's like bubbles in boiling water. Except this happens at a temperature of a thousand trillion Kelvin in the first picosecond into the history of the universe as we know it. Outside the bubbles, the universe is in equilibrium. Inside the bubbles, evenness prevails as well, but in a new state. But the boundaries of the bubbles are different beasts altogether, and here a fully nonequilibrium state (basically by definition) occurs.

And it's there, in these exotic bubble boundaries, that all the conditions can be met within the realm of known physics: excess matter is produced, it's preserved by asymmetries in charges, and there are no competing processes there to fight against it.

Problem solved! Except that our best guess at the details of this process predict about one-billionth the expected amount of matter. Whoops.

So, yeah. After all the buildup and explanation and excruciating jargon, we don't have much. Or do we?

At first blush, this chapter so far, plus the earlier chapter discussing the earlier epochs, seems like it could be three words: “We don't know.” But I hope, if I've spun this tale the way I intended, you're starting to see something interesting emerge. The further we get into the history of the universe—the older, larger, and cooler it becomes—more recognizable shapes and patterns begin to emerge from the mist.

We've gone from the Planck era, which at this stage can barely be conceived of even by hints of mathematics, into the GUT era, which has some plausible inroads that physicists have begun to explore. Then comes inflation, which Isn't That Crazy Of An Idea™, and now baryogenesis. Even though, admittedly, we don't fully grasp how matter won out over antimatter in the infant cosmos, we have a language for grappling with the problem. Weak nuclear forces, breaking of charge and parity symmetry, phase transitions, electroweak unification.

This is physics. We can do this.

Heck, even the words I'm using are beginning to change. I'm finally able to drop exponential notation and discuss temperatures (as insanely high as they are) and ages (as achingly short as they are) with familiar Greek prefixes. Instead of the barest hints of theoretical guidance, we have laboratory experiments providing clues. The fuzzy mathematics are replaced with replications of the conditions in particle colliders around the world.

Traditional cosmology books usually start with the bits we know really, really well before introducing the early-universe stuff, for good reason, because it seems like we're just making it up as we go along. But this is a book about the limits of our knowledge and the mysteries that confront us in the universe—and how we come to terms with it. And the universe prior to a picosecond in age experienced profound and fundamental changes; compared to the timescales of typical interactions, more happened in the first second of the universe than the following billions of years of cosmic history.

The first picosecond may be the dark waters where krakens lurk, but as the instants turn to full seconds, the seconds turn to minutes, the minutes to days and years, our understanding starts to crystallize. There are still plenty of mysteries out there—and don't worry, we'll get to them all—but you'll be seeing fewer maybes and possiblies and more “This is what we know happened.”

Our first encounter with something far more familiar happens after the four forces are finally cleanly separated from one another. The universe is still swamped with high-energy radiation, but the chaos of the earlier epochs has simmered down into a state of matter affectionately called the “quark soup.”

It's too hot in that boiling cauldron for atoms to form. Too hot for nuclei. And too hot for protons and neutrons themselves. The energies here are so extreme that even those tiny particles are ripped apart into their constituent parts.

The future history of the universe will be dominated by ever-slower and ever-less-energetic transitions. Nothing will ever reach such incredible energies again, except in isolated pockets like supernova explosions and collider experiments. The chain of events at the global scale will be dominated by pure, simple, unadulterated expansion.

As the universe grows larger, it continues to cool. Through this cooling, phases of matter can maintain their state, but eventually a thin red line is breached, and pop, the cosmos switches to a new form. We've already encountered a few exotic transitions as the forces of nature themselves splintered off from the crucible that proceeding them.

The phase transitions that will mark the boundaries of succeeding epochs, while still completely transformative, are much less violent. Instead of a massive cataclysm signaling the birth of a new era, they will be condensations.

Let's look at something familiar, like water, as an example before I start slinging more jargon at you. At high enough temperatures and pressures, water takes the form of a gas—water vapor. If you take a snapshot of, say, your backyard on a hot summer day and examine it at a microscopic level, you will see lots of gross bugs, but also some interesting behavior of water. Water molecules from the air will naturally condense to form a liquid on a surface, but because of the high temperature, it will immediately evaporate into the air again. The party-hearty water molecules would just love to settle down, buy a house, and start a family, but their raucous ways overwhelm the better angels of their nature.

But if you cool your yard below a certain threshold, called the dew point, the vapor-to-liquid transition will begin to overwhelm the liquid-to-vapor process, and droplets will begin to appear. Take a mass of water vapor, for instance, and start cooling it. It will remain as “water vapor but colder” for a while until the dew point is reached, when it will undergo a phase transition and become “water, as normally in a liquid.”

So it's like this in the early universe, except, as you might have guessed, at much higher temperatures than you typically encounter in your backyard. From the perspective of a quark living in the first picosecond, our present-day cosmos is bone-chillingly cold and in the impossibly distant future. Keep that in mind for when I get to the chapters on the future of the cosmos from our perspective.

After all the craziness of the force-splitting phase transitions, we're left with our quark soup. Just like a real soup is made of chopped-up bits of larger things immersed in a broth, so is our universe at this time. Your molecules are made of atoms are made of electrons around a nucleus, the nucleus is made of protons and neutrons, and the protons and neutrons are each composed of three tiny little quarks glued together with—well, they're called gluons. That's what they do—they carry around the strong nuclear force.

We're pretty sure that quarks are the tiniest thing as tiny things go, so as far as we know, there's no “pre-quark stew” at earlier epochs, but be warned that picture might change.

You need to be at relatively cool temperatures to actually have a proton; otherwise, like the analogy with water, any time a trio of quarks assemble to form one, they get blasted apart by their energetic neighbors. But the expansion of the universe is inevitable, and the subsequent cooling inexorable. About a microsecond in, the first protons and neutrons begin to condense out of the early-morning fog.

It's a furious frenzy of activity. Heavy particles recondensing and reevaporating, particles and antiparticles emerging and obliterating in continuous showers of activity. But the imbalance laid down in the previous epochs persists, with normal matter having a slight edge, coming out of the fray the victor.

And it's over in a second. A single tick of the clock and our universe has gone from an incomprehensibly dense unknown state, through the splitting of the forces and a period of inflation, into a sea of familiar protons and neutrons (plus some other friends) and radiation. A state of matter, while extreme indeed, that we can recreate—briefly—in particle accelerators. So while we don't fully understand the physics of this epoch (as per usual), at least we can test our ideas. While “quark soup” is its cute nickname, it does have the more formal moniker of quark-gluon plasma, and it's something that we can cook up in labs around the world.

Even now, one second into the history of the universe, the GUT era is a relatively distant memory. Almost the entire history of the cosmos, 99.9999 percent of the time until this point, has been taken up by the formation of the first heavy particles.

And not just the first—all. Almost every single proton and neutron that we see in the universe, including the ones this book and your brain are made out of, was forged in these moments.

By the way, during this phase, the observable universe grows from about the size of the sun to about the size of a small galaxy.

The close of the first second is an important milestone in our cosmic history. The physics from here onward is even more familiar than what can be accessed deep in the hearts of particle colliders. It becomes accessible to much lower-energy devices: nuclear reactors. You could, given enough time, materials, and dedication (and access to restricted ingredients), construct in your very own backyard a device that could recreate this age of the universe. You'll also probably give yourself and your neighbors radiation poisoning, so let's just talk about it instead.

And what you would find is a nuclear maelstrom. A swarm of neutrons, protons, radiation, and neutrinos. We haven't met neutrinos yet in our story, and this isn't really their tale,8 but something important occurs at the one-second mark concerning them.

Neutrinos themselves are nearly massless (so much so that for decades we thought they were massless) particles that interact with normal matter only via the weak (here we go again) force. There are billions-with-a-b neutrinos streaming through you right now. But they don't talk to your electrons or quarks or anybody else—except exceedingly rarely—so you don't really notice. To even get a hint of them we need literally gigantic detectors.

Neutrinos are produced in all sorts of nuclear reactions near and far, from the local power plant to the sun to distant supernovae. And the early universe. There are, at this very moment, relic neutrinos left over from this tumultuous era of the first second that have been streaming through the cosmos ever since.

Earlier than the first second, the temperatures and densities were so high that neutrinos, despite their ghostly let's-just-be-friends character, were compelled to interact with matter. But once the densities dropped, they could stream freely, liberated to live their lives as they saw fit.

Neutrinos are, therefore, one of the only ways we could directly access this epoch of the universe. No laboratories. No theories. Straight-up raw observational data. The downside is that these ancient neutrinos are diluted to a thin, low-energy soup here in the present-day universe, so it's incredibly challenging. But not impossible, which is important for those of you who think I'm still making all this up on the spot. This is science, folks.

Shortly after the release of the neutrinos, the leptons (yet another family of particles, this time comprising the light guys like electrons) fully separate out, in a similar telling of the story as the baryons (the heavy guys) but at lower, slower, energies, using more familiar physical processes.

To drive home the point that the universe is already becoming middle-aged, the era of lepton formation doesn't last a picosecond, or an attosecond, but tens of seconds. That's in the realm of human timescales—a couple of breaths and you've encompassed the formation of light particles! Still short, but an eternity at this epoch.

Once the temperature drops below ten billion degrees or so, as the universe ages over the course of the next few minutes, a remarkable process unfolds. So remarkable, in fact, that I'm about to remark upon it.

Let's set the stage: you've got a hot, dense soup of primordial particles, primarily protons and neutrons, buzzing around in a sea of high-intensity radiation and electrons, set against a backdrop of an expanding, cooling universe.

Simple question: What happens? What are the physical consequences of this scenario?

The development of the nuclear age had some serious downsides, but also an unexpected benefit. It gave us the tools we needed to answer precisely this question. Nuclear chain reactions, decay products, fusion mechanics, the whole deal. The universe at this stage is a high-powered nuclear reactor, and dang it, we know what that looks like.

It looks like a series of chain reactions: Protons and neutrons combining to form deuterons. Free neutrons decaying into protons and electrons (and neutrinos). Deuterons acquiring another neutron to generate tritium, which is radioactive but if it acquires another deuteron can generate helium-4. Free protons finding a friendly deuteron to make helium-3. Tritium meeting up with a couple of deuterium pals to make lithium-7. And so on.

These reactions can only proceed in a narrow window. Attempt to make these chains of heavier elements too early, and the sheer intensity of high-energy radiation tears everything apart, like cops at a house party. Too late, though, and the partygoers are too tired to keep dancing—the universe is too cool, too thin to sustain the nuclear fiesta.

A good solid fifteen or twenty minutes, as the cosmos cools from a billion to ten million Kelvin, is all the universe gets to form hydrogen, helium, and a little bit of lithium. While far into the future stellar furnaces will forge some additional helium and lithium, the combined might of the trillions upon trillions of stars is minuscule compared to this primordial inferno.

In other words, essentially all the hydrogen you can get your hands on, including the hydrogen literally inside your hands, and most of the helium floating around the cosmos percolated out of this epoch of nucleosynthesis.

I mentioned this earlier, but I'll reinforce it here: notice that I didn't say “maybe” or “we think” or “according to some random paper I found in the Astrophysical Journal.”

This is a prediction of this picture of the universe, made shortly after we cracked the secrets of the nuclear code in the 1950s and ’60s. The mathematics that go into this calculation have very simple results. If you happen to know the total amount of regular matter in the universe (which you can measure by counting stars and stuff), and the total amount of light (another thing that's not too hard to measure), a nuclear analysis of primordial element-making predicts relatively how much of the universe ought to be hydrogen, helium, and lithium.

It gets better: the numbers you need to know to input into the calculations don't hugely affect the results—you could not know the true amount of regular matter to within a factor of ten (that is, the real answer could be ten times smaller or ten times bigger than what you guess) and still get pretty much the same outcome in the primordial math. So you have a lot of observational wiggle room to make predictions, which are as follows: the universe ought to be about three-fourths hydrogen and one-fourth helium, with a tiny fraction of lithium.9

Boom. Write it down. It's a big deal. This is a moment of truth. A place where we can take this weird, complicated story of the early history of our universe and body slam it into observational reality. Our model of the infant cosmos is making a bold, unambiguous statement about our present-day circumstances. And we must ask, do we see it?

It's precisely what we find. Stars, gas clouds, galaxies. When you smear out all the stuff that we can see in the universe, it's three-quarters hydrogen, one-quarter helium, and a dash of everything else.

This is one of the biggest reasons we think this story is on the right track. We don't fully understand the eras that come before, but once the universe is a second or so old, it's on solid nuclear ground, ground that we've trodden on for decades. The math is complex but not intractable. The physics is difficult but not nosebleed inducing.

And it's a very straightforward, very robust prediction that agrees with the observational data.

As crazy as it sounds, it looks like this is our universe.