Your Place in the Universe

by Paul M. Sutter

  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 fo
r 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.

  You're in the market for a house. You tell the real estate agent your budget and start the shows. Naturally, the agent has one in mind that's just a titch above your budget…but I know you'll love it. As you pull up, you get your first good look at the house. It doesn't have that much curb appeal, but it has a certain charm. The entryway is neat and tidy and organized, and the tour begins in earnest.

  The agent takes you through the house, and you begin to get suspicious. A gigantic kitchen. Formal and informal rooms. A mudroom. What's a mudroom, anyway? More bathrooms than you can count on one hand. Guest rooms galore. A movie theater. A parlor. A reading room. A breakfast nook. Rooms that only serve as waiting areas for other, larger rooms.

  This place is huge. Way bigger than you would have guessed from the street. The agent hasn't told you how much the seller is asking, but you're realizing that this is way outside your budget. There's not a single chance you'd be able to afford even the down payment, let along the mortgage.

  You're not even listening to the agent blather on about the granite this and upholstered that. A single, solitary thought occupies your entire consciousness, beating your mind like a drum: Just how big is this place, anyway?

  That, my friends, was the collective state of mind by the close of the nineteenth century and the opening decades of the twentieth. There was still heaps of angst over the nature of stars and nebulae and the causes of light spectra, and we'll get to that resolution later, but the central vexing astronomical question in the pre–Great War world was the size of our home.

  Astronomers had begrudgingly come to accept the fact that our universe is uncomfortably large—parallax measurements repeated on multiple stars had hammered home the vicious point th
at our solar system, including the mysterious, distant homes of the icy comets, was just a small, isolated island within the grand ocean of our galaxy. What's more, that galaxy—the massive collection of stars that we can and can't see with the naked eye—filled up the volume of said universe.

  Or did it?

  It's the natural assumption. Everywhere we look, we see stars. Yes, there might be vast gulfs of night separating the warming fires, but our universe is flooded with those fireballs. Interspersed among them are the nebulae, the vast clouds of dust and gas that serve as nesting homes or violent ends for those ferocious points of light.

  The challenge with astronomy a hundred-odd years ago was that parallax is only so good. It took centuries of laborious mental exertion for the instruments and techniques to become sophisticated enough to pin down the first extrasolar distance, and once the community cracked that method, thousands upon thousands more distances were measured, verified, and cataloged. The scale of the universe was just beginning to open up before us.

  But there's a limit to what we can measure with photographic plates and polished mirrors, sitting on the surface of the Earth buried under sixty miles of turbulent atmosphere. At a certain point, around a few tens of thousands of light-years, we can't get a reliable parallax measurement.

  A light-year, by the way, is the distance light travels in a single (Earth) year. It's a handy unit, credited to Bessel himself for popularizing,1 because it gets severely exhausting describing even the nearest stellar distances in “hundreds of thousands of Earth-sun distances.” Instead, a typical star is, say, forty light-years away. Forty. That's much more nimble to express, so we'll keep it.

  So what is a hapless astronomer to do when even the light-years stretch too far and reliable distance measurements are just a fond memory? Well, that old Newtonian idea of using the brightness of a star never quite died away, but with the stunning variety of stars on display in the sky, it always seemed like a pipe dream.