by Katie Mack
But because we need to quantify exactly how confused we are, and at what moment that confusion sets in, we call this the Planck Time,XIII and it encompasses the time from zero to about 10-43 seconds. If you’re not familiar with the notation, 10-43 seconds is equal to one second divided by 10000000000000000000000000000000000000000000 (that’s 1 followed by 43 zeros). Suffice it to say, this is an unimaginably small amount of time. And, to be clear, it’s not that we necessarily can explain everything from the Planck Time on, but that we currently definitely cannot explain anything before it.
To sum up where we’ve gotten so far: there may have been a singularity. If there was, it was immediately followed by an era we can’t really say much about called the Planck Time.
Truthfully, the whole timeline of the early universe is still very much an extrapolation and, I will readily admit, one that we shouldn’t entirely trust. A universe that starts with a singularity and expands from there goes through an unimaginably extreme range of temperatures, from basically infinity at the singularity to the cool comfortable environment of the cosmos today, sitting at about 3 degrees above absolute zero. What we can do is make inferences about what physics would be like in all those environments, which is how we get the ordering I present in this chapter. And though the standard Big Bang theory of steady expansion from a singularity has some major problems (which we’ll get to imminently), we can still learn a lot about how physics works by thinking about what might have happened if the standard theory is right.
THE GUT ERA
According to the standard Big Bang story, after the Planck Time comes the GUT era. (I’m using the term “era” here to refer to something that lasts about 10-35 seconds, and the term GUT to mean something unrelated to human anatomy.) “GUT” stands for Grand Unified Theory, which is the physics-utopian ideal of a “unified” theory that describes how all the forces in particle physics worked together under the extreme conditions of the universe at this early stage. Even though the universe was cooling rapidly, it was still so hot that the amount of energy at every point in space was over a trillion times higher than the energy generated by the most powerful collisions in our most advanced particle colliders. Unfortunately, and partly because of that factor of a trillion preventing us from doing experimental tests, the theory is currently still very much under construction. But for a theory we do not currently have, we can say quite a lot about it, and how it’s different from what we see today.
In everyday life in the modern universe, each of the fundamental forces of nature has a distinct role. Gravity holds us to the ground, electricity keeps our lights on, magnetism holds our shopping list to the fridge, the weak nuclear force makes sure our backyard nuclear reactor keeps glowing a nice steady blue, and the strong nuclear force prevents our bodies’ protons and neutrons from decomposing into their component parts. But the physical laws governing how these forces work, how they interact with each other, and even whether or not it’s possible to tell them apart, depend on the conditions under which they’re measured. Specifically, the ambient energy, or temperature. At high enough energies, the forces start to merge and combine, rearranging the structure of particle interactions and the laws of physics themselves.
It’s been known for some time that, even under everyday circumstances, electricity and magnetism are aspects of the same phenomenon, which is why electromagnets are a thing and why dynamos can generate electricity. This kind of unification is like candy to physicists. Any time we can take two complex phenomena and say, “Actually, when you look at it this way, they’re THE SAME THING,” we basically explode with physics joy. In some ways, this is the ultimate goal of theoretical physics: to find a way to take all the complicated messy stuff we see around us and rearrange it into something pretty and compact and simple that just looks complicated because of our weird low-energy perspective.
Where the forces of particle physics are concerned, this quest is called Grand Unification. Based on theory and extrapolations from what we see in lab experiments, it’s thought that at very high energies, electromagnetism, the weak force, and the strong force all come together to be something else entirely, such that there’s no way to distinguish them—they’re all part of the same kind of big particle-energy mix governed by a Grand Unified Theory. There have been a few GUTs developed and proposed, but the difficulty of accessing the energy scales where the unification occurs makes them hard to confirm or rule out, so we’ll just call this “an area of active research” that would very much appreciate any more funding you’d like to throw at it.
You might notice that gravity is not invited to the GUT party. To bring gravity into the picture, we need something grander and more unifying than even a Grand Unified Theory—we need a Theory of Everything (aka TOE). There’s a general belief among physicists that sometime around the Planck Time, gravity was somehow unified with the other forces (along with the dragons or whatever else was happening then). But, as we discussed before, general relativity and particle physics don’t like to work together in their current form, and so we’ve made even less progress toward a TOE than a GUT. Many people are placing their bets on string theory being the ultimate TOE. But if you thought GUTs were hard to verify experimentally, TOEs may actually be impossible to test, at least with anything like the technology we can currently conceive of. Arguments rage from time to time about whether or not this is the case, and whether or not untestable theories should even be called science. I don’t think the situation is quite as dire as that. Cosmology may offer solutions (and no, I’m not just saying that because I’m a cosmologist). In certain cases, with a bit of creativity, there are some tantalizing possibilities for testing predictions of string theory and related ideas with observations of the cosmos. If we make it through a couple of apocalypses in the next few chapters, we’ll see how cosmology might be able to show us more about the ultimate, tying-it-all-up-with-a-bow fundamental structure of the universe than any particle experiment can.
But let’s get back to our story. We have just escaped the throes of Planck Time–quantum-gravity confusion and are enjoying the fundamental-force unity of the very slightly less speculative GUT era.
COSMIC INFLATION
What happened next is still a matter of debate, but the near-consensus in cosmology is that sometime around this moment the universe suddenly experienced the mother of all growth spurts—a process we call cosmic inflation. For reasons we’re still trying to understand, the expansion of the universe suddenly went into very high gear, with the region that would someday become our entire observable universe increasing in size by a factor of more than 100 trillion trillion (i.e., 1026). Of course, that only brought it up to about the size of a beach ball, but given that the starting point was unimaginable tininess smaller than any known particle, and the growth happened over the course of something like 10-34 seconds, we have reason to be impressed.
The theory of inflation came about to solve a few really perplexing problems in the standard Big Bang model. One has to do with the weird uniformity of the cosmic microwave background, another with its tiny imperfections.
The uniformity problem is that the standard Big Bang cosmology doesn’t offer any explanation for how the whole observable universe, including parts on totally opposite sides of the sky, ended up being the same temperature at early times. When we look at the afterglow of the Big Bang, we see it as the same everywhere to extremely high precision, which is, when you think about it, a really strange coincidence. Normally, two things can become the same temperature if they’re in what we call thermodynamic equilibrium. This just means there’s a way for them to exchange heat, and time for them to do so. If you leave a cup of coffee in a room for long enough, the coffee and the air in the room will interact, and eventually you’ll have room-temperature coffee and a very slightly warmer room. The problem with the standard picture of the early universe is that it doesn’t include a situation in which two distant parts of the universe could interact and come to an agreement about a t
emperature. If we take two points on opposite sides of the sky and work out how far apart they are now, and how far apart they were at the very beginning, 13.8 billion years ago, it becomes clear that there was never a time in the history of the universe when they were close enough that light beams could have traveled back and forth between them to carry out the equilibrium process. A beam of light that started at one of those points at the beginning of the universe would not have had time even in 13.8 billion years to cover the distance necessary to get to the other. They are, and always have been, outside each other’s horizons: unable to communicate in any way.XIV So either it’s the most massive coincidence in the universe, or something happened early on to let the equilibrium happen.
The problem with the imperfections is a little simpler to articulate. It’s just this question: where did those minuscule density fluctuations in the cosmic microwave background come from, and why are they patterned the way they are?
Cosmic inflation solves both these problems, along with a few others. The basic idea is that there was a time in the early universe, after the singularity but before the end of the primordial fireball stage, when the universe was expanding astonishingly fast. This helps by allowing for a period early on when a very small region could come into equilibrium, after which the rapid expansion would take that little nicely settled region and stretch it out to cover our entire observable universe. Imagine taking a complicated abstract painting and blowing it up so large that your whole view is just one color. The expansion essentially zoomed in on a part of the universe that was small enough to have already become the same temperature, and made the whole observable universe out of just that region.
Inflation also conveniently explains the density fluctuations, if we invoke a bit of quantum physics. The essential difference between the physics of the subatomic world and that of everyday life is that on the scale of individual particles, quantum mechanics imbues every interaction with an intrinsic, inescapable uncertainty. You may have heard of Heisenberg’s Uncertainty Principle: it’s the idea that there’s a limit to the precision of any measurement, because the uncertainty built into quantum mechanics will always smear out the result in one way or another. If you measure a particle’s position very precisely, you won’t be able to determine its speed, and vice versa. Even if you just leave a particle alone, all of its properties will be subject to some amount of random shifting, and every time you measure it you could get a slightly different answer.
How does this tie into the cosmic microwave background? The hypothesis is that inflation was driven by a kind of energy field subject to quantum fluctuations: random jumps up and down. These fluctuations, which would normally only be ephemeral blips on a microscopic scale, change the density on the tiny scales where they happen, and then get stretched out to large enough regions to become substantial hills and valleys in the density distribution of the primordial gas. The little splotches we see in the cosmic microwave background make perfect sense if they are the natural evolution, over hundreds of thousands of years, of the fluctuations set down in the first 10-34 seconds of the cosmos. And those same little splotches are what eventually grew into all the galaxies and clusters of galaxies we see today.
The fact that the distribution of the largest structures in the universe can be exactly patterned on the minute wiggling of a quantum field is something that never fails to blow my mind. The links between cosmology and particle physics are never clearer, or more visually striking, than when we look at the cosmic microwave background.
But we’re getting ahead of ourselves. The CMB is still, on these terms, eons away. We’ve only covered 10-34 seconds, and there’s still plenty of story to tell.
Figure 6: Cosmic timeline. The size of the observable universe increased rapidly during inflation, just after the very beginning. The universe has been expanding (at a slower rate) ever since. Labeled here are some of the important moments in the history of the cosmos.
When inflation ended, the super-stretched-out baby universe was left much colder and emptier than when it began. A process called “reheating” brought it back up to a high temperature everywhere, at which point the ordinary march of steady expansion and cooling carried on.
THE QUARK ERA
While the pre-inflation cosmos was likely ruled by a Grand Unified Theory, the post-inflation cosmos was moving closer to the laws of physics we see today. There was still a way to go, though. At this point, while the strong nuclear force had broken away from the GUT all-in-one particle physics party, electromagnetism and the weak nuclear force had not yet distinguished themselves; they were still somehow merged together as a single “electroweak” force. But particles were starting to emerge from the primordial soup—specifically, quarks and gluons.
Quarks, these days, are most commonly encountered as the components of protons and neutrons (which, together, are called hadrons). Gluons are the “glue” that binds quarks together via the strong nuclear force, and they are aptly named. They’re so good at binding quarks together that while quarks have been found in twos or threes or even occasionally quads and quintuples, finding a single quark in isolation has so far proved impossible. It turns out that if you have two quarks bound together (in an exotic particle called a meson), you have to put in so much energy to separate the quarks that before you can get them apart, the energy you just expended spontaneously produces two more quarks. Congrats! Now you have two mesons.
In the very early universe, however, the usual rules didn’t apply any more to singleton quarks than they did to anything else. Not only were the forces of nature operating under different laws, the universe contained a different mix of particles, and temperatures were so high that bound states of quarks could not exist in a stable form. Quarks and gluons bounced around freely in a hot roiling mix called a quark-gluon plasma—kind of analogous to the inside of a fire, but nuclear.
This “quark era” lasted until the universe reached the ripe old age of a microsecond. Meanwhile, somewhere in there (probably around the 0.1 nanosecond mark) the electroweak force split into electromagnetism and the weak nuclear force. Also around that time, something happened to create a distinction between matter and antimatter (matter’s annihilation-happy evil twin), allowing most of the universe’s antimatter to annihilate away.XV Exactly how and why that happened is still a mystery, but as matter we can be glad it occurred, so we’re not constantly running into antimatter particles and vanishing in a puff of gamma rays.
In contrast to the GUT era, we actually know quite a lot about the quark era and the quark-gluon plasma. The theory is pretty well developed and less of a departure from standard particle physics than a GUT, and experiments confirm the predictions we make when we start from electroweak theories and extrapolate from there. But the real coup is that we can actually re-create quark-gluon plasma in the lab. Particle colliders like the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) can, by smashing together gold or lead nuclei at extremely high speeds, momentarily produce tiny fireballs so hot and dense that they smush together all the particles and momentarily fill the collider with a quark-gluon plasma state. By watching the debris “freeze” out into ordinary hadrons, scientists can study the properties of this exotic matter as well as the way the laws of physics act under those extreme conditions.
If seeing the CMB gives us a glimpse of the Big Bang, high-energy particle colliders are giving us a taste of the primordial soup.XVI
BIG BANG NUCLEOSYNTHESIS
After the quark-gluon plasma phase, the universe started to cool down enough for some familiar particles to form. At around a tenth of a millisecond, the first protons and neutrons formed, followed shortly by electrons, laying down the building blocks of ordinary matter. Somewhere around the two-minute mark, the universe cooled to a comfortable billion degrees Celsius, hotter than the center of the Sun but cool enough to allow the strong force to bring those shiny new protons and neutrons together. They formed the first bonded atomic nucleus: a form
of hydrogen called deuterium (one proton bonded to one neutron; technically a single proton can be considered a nucleus too, since it’s the center of a hydrogen atom). Soon, nuclei were forming left and right. Some fraction of the protons and neutrons started joining together to make helium nuclei, tritium, and a smattering of lithium and beryllium. This process, called Big Bang Nucleosynthesis, went on for about half an hour, until the universe cooled down and expanded enough that the particles were able to get away from each other instead of fusing together.
One of the great validations of the Big Bang theory is the fact that we find a close match between our observations in the cosmos and the calculated abundance of elements we expect from the Big Bang based on our estimates of the temperature and density of that primordial fireball. The agreement isn’t perfect—there’s some lingering confusion around the lithium abundance that may or may not be telling us about some extra weirdness going on in the early universe—but with hydrogen, deuterium, and helium, measuring how much we actually see out there and comparing it to what we calculate should happen if you shove the entire cosmos into a nuclear furnace results in some absolutely beautiful concordance.
As an aside, the fact that pretty much all the hydrogen in the universe was produced in the first few minutes means that a pretty large fraction of what you and I are made of has been hanging around the universe in one form or another for almost as long as the universe has been here. You may have heard that “we are made of stardust” (or “star stuff” if you’re Sagan), and this is absolutely true if we measure by mass. All the heavier elements in your body—oxygen, carbon, nitrogen, calcium, etc.—were produced later, either in the centers of stars or in stellar explosions. But hydrogen, while the lightest, is also the most abundant element in your body by number. So, yes, you hold within you the dust of ancient generations of stars. But you are also, to a very large fraction, built out of by-products of the actual Big Bang. Carl Sagan’s larger statement still stands, and to an even greater degree: “We are a way for the cosmos to know itself.”
