Fundamentals

by Frank Wilczek

  Cosmic history includes, in principle, an enormous range of things, including the history of life on Earth, the history of China, the history of Sweden, the history of the United States, the history of rock and roll, and so forth. But no sane person would expect to get an understanding of those subjects based on physical fundamentals.

  What fundamentals-based cosmic history does provide is three things. First, it offers a profoundly strange yet informative and convincing account of what the early universe was like. This account is a good answer to an interesting question, and it proves to be a rich source of surprising, observable consequences. Second, it provides a broad scenario for how the structures we observe around us—including, for example, our solar system—can have emerged. Third, it suggests exciting new questions, such as what “dark matter” is.

  WHAT HAPPENED

  Strangely Simple Beginnings

  Everything should be made as simple as possible, but no simpler.

  —Albert Einstein

  As we’ve already discussed, Hubble’s discovery, which we can loosely describe as “expansion of the universe,” practically begs us to consider what happened earlier.

  We seem, on the face of it, to be living out the aftermath of a universal explosion. If we can understand the beginning, then we can hope to leverage our understanding to illuminate later events.

  As a first attempt to reconstruct the beginning, we can imagine “running the movie backward.” To do this, in our minds, we simply reverse the velocities of all the galaxies and let the laws of physics play out.* The galaxies rush together. As they approach, they begin to attract one another gravitationally, and their accelerated motion releases energy. The matter gets mixed up and heats up. The temperature rises. Atoms get stripped of their electrons, and rapidly moving charges radiate like mad. Tightly packed, rapidly moving protons and neutrons boil into a soup of quarks and gluons. Finally, our hard-won knowledge of fundamental interaction pays off. Asymptotic freedom, in particular, implies a great simplification—at high energies, the formidable complications of the strong interaction go away. Extremely hot, dense material is surprisingly simple to understand, directly from fundamentals.

  But before accepting this reconstruction of the past, we must face up to a major conceptual problem. The history of the universe depends on it. The problem is this: The simple picture I just sketched, as an account of cosmic expansion run backward, is desperately unstable. What we really should expect as the matter rushes together is that stars, planets, gas clouds, and whatever else is out there will merge, through the inexorable attraction of gravity, into gigantic black holes. The nongravitational interactions do, indeed, want super-dense, energetic matter to become a hot, homogeneous gas. That is their favored equilibrium, which they will try to enforce. But gravity abhors homogeneity. Gravity wants things to clump, in general, and gravity wants super-dense matter to clump into black holes. Running our cosmic movie backward, if we didn’t know better, and were honest about it, we’d “predict” that gravity wins, and the early universe devolves into big black holes rushing together and merging into bigger black holes. But if the early universe really were like that, then—running the movie forward again—we’d have a universe with essentially all the matter locked up in black holes. Once you’ve fallen into a big black hole, it’s quite difficult to get out!

  The universe we actually observe is nothing like that prediction. Our observed universe, averaged over intergalactic scales, is remarkably homogeneous. Wherever we look in the sky, if we sample a reasonably large chunk, we find the same sorts of galaxies, distributed with the same density. This was another of Hubble’s pioneering discoveries. Since gravity tends to make things less homogeneous, the fact that we observe large-scale homogeneity today implies that the universe was even more homogeneous early in its history. This means, in terms of our backward-running movie, that the matter comes together “just so,” in a way that is delicately orchestrated to avoid gravitational mergers.

  The big bang theory of cosmic history uses the naïve picture of the early universe as a hot homogeneous gas that I sketched originally, before I raised worries about its stability. The big bang theory simply puts those worries aside. Fundamentally, therefore, the big bang theory is a strange hybrid of two opposing ideas. It postulates complete equilibrium for the nongravitational interactions, but maximal disequilibrium for gravity. Running Hubble’s expanding universe backward in time suggests the former, while running Hubble’s quasi-homogeneous universe backward in time suggests the latter. In the big bang theory, we follow both suggestions.

  The Expanding Fireball

  We start, then, with a very hot, very homogeneous gas. We also assume that space, which (according to general relativity) might be curved, is actually flat.* For a first draft of physical cosmology, that’s all we need to know.

  The ingredients in a hot gas move around so rapidly, and interact so often, that they reach a dynamic balance, known as thermal equilibrium. At the extremely high temperatures we contemplate in the earliest moments of the big bang, thermal equilibrium is especially powerful, because so many things can—and do—happen. Many kinds of particles, from photons to gluons, quarks, antiquarks, neutrinos, antineutrinos, and more, are getting produced and destroyed (or, equivalently, radiated and absorbed). In equilibrium, all are present, with predictable abundances. H. G. Wells caught the spirit of thermal equilibrium memorably: “If anything is possible, then nothing is interesting.” In thermal equilibrium at extremely high temperatures, we find a completely predictable mixture of all the elementary particles.

  Another aspect of super-high temperature conditions is that structures cannot hold together—molecules break up into atoms, atoms break up into electrons and nuclei, nuclei break up into quarks and gluons, and so on. In short, we get down to fundamentals.

  Given that starting point—a predictable mix of fundamental ingredients—we can use our knowledge of fundamental laws to predict what happens next. The result is simple: Our omnipresent fireball expands under its own pressure, working against its own gravity and cooling as it does.

  As the fireball cools, two especially notable things happen. One is that some reactions occur more rarely, and then effectively stop. This results in lingering afterglows. For example, once the temperature gets low enough, the photons in the fireball cease to interact significantly with the other matter. In plain English, the sky clears up, so that light travels more or less freely from one end of the universe to another, as it does today. The photons that were part of the fireball don’t disappear, though. They become the so-called cosmic background radiation, a lingering afterglow that fills the universe.

  Another result is that particles begin to stick together. Quarks combine into protons and neutrons, electrons bind to atomic nuclei, and so forth. In this way, matter in the form we know it begins to take shape.

  That is our first draft of cosmic history.

  HOW WE KNOW IT

  The past is never dead. It’s not even past.

  —William Faulkner

  The cosmic past is never dead. It leaves relics, which we can observe today. The cosmic past is not even past. Thanks to the finite speed of light, when we receive light from far away it carries the past to us.

  Reconstructing what happened in the early universe is a lot like reconstructing a crime. We survey the evidence, form a theory of the case, and look for corroborating evidence. If we find surprises, then we must refine our theory, or change it.

  Cosmic Census

  With better telescopes and cameras, and more powerful ways of handling data, astronomers have been able to survey the universe far deeper and more fully than was possible for Edwin Hubble. His work made the big bang a prime suspect; their work could sustain a conviction.

  You may recall that Hubble discovered that distant galaxies are moving away from us, with their velocity proportional to their distance. This relati
onship, run backward in time, suggested the big bang. It holds accurately for nearby galaxies, but we should not expect it to work for the most distant ones. Velocity proportional to distance will not bring distant galaxies together at the same time, because (in our movie played in reverse) gravitational forces come into play and modify the motion. Given the big bang as a starting point, it is possible to predict how the expansion rate changes in time. That prediction translates into a refined projection for how the redshift of galaxies depends on their distance, which can be compared with observations. It works.*

  By running the expansion backward in time, we determine what is commonly called the “age of the universe.” What that phrase refers to is the length of time that has passed since the universe was a much hotter, denser, more homogeneous place than it is now—or, more loosely speaking, the time that has passed since the big bang occurred. In the earliest moments following the big bang, stars and galaxies could not have held together. But we can estimate when such structures should have begun to form. And we can also estimate the ages of some very old objects in quite different ways, using radioactivity and the theory of stellar evolution, as we discussed in chapter 2. Those very different ways of assessing cosmic antiquity are found to agree quite nicely. In short, the universe is about as old as the oldest objects within it—as it should be.

  Lingering Glow

  The lingering glow of photons present when the fireball first cooled enough to become transparent was first detected in 1964, by Arno Penzias and Robert Wilson. Those photons have been drastically redshifted, and now they are primarily microwave radiation (the same kind of electromagnetic radiation as is used in microwave ovens). They form the so-called cosmic microwave background, or CMB. The CMB is a snapshot of the early universe, spread over the sky in invisible “light.” The big bang picture not only predicts the existence of the cosmic microwave background, but also has a lot to say about the details of its composition—specifically, the intensities of its various radiation frequencies. Here, too, the observations agree with the predictions.

  Relics

  As the raging fireball, including quarks, antiquarks, and gluons, cools, those particles start to stick together into protons, neutrons, and other atomic nuclei. One can calculate, within the big bang model, the relative abundance of the different nuclei that emerge. It turns out that the overwhelming majority of potential nuclear material emerges from the big bang as ordinary hydrogen (1H—a lone proton) and helium (4He—two protons and two neutrons). There are also small admixtures of deuterium (2H—one proton and one neutron, an isotope of hydrogen), Helium 3 (3He—two protons and one neutron, an isotope of helium), and lithium (7Li—three protons and four neutrons). These different isotopes have all been detected, using the techniques of spectroscopy, to occur with the predicted abundances within appropriate “unprocessed” environments.*

  All other kinds of nuclei got formed in stellar processes, at a much later stage in cosmic history. Observing and understanding their abundances is a wonderful subject, but its connection to fundamentals is less direct.

  THE FUTURE OF COSMIC HISTORY

  Inflation

  As I emphasized above, the big bang theory is profoundly strange. It assumes a starting point that is actually unstable, and postulates that matter in the early universe was extremely fine-tuned—specifically, that it was uniform—to avoid triggering its gravitational instability.

  There’s also another uncanny aspect, which I mentioned only in passing, because a full explanation would have interrupted my narrative.* The big bang theory assumes that space is Euclidean, or “flat.” Spatial flatness is consistent with Einstein’s general relativity, but not required by it. Relativity is ready to accommodate spatial curvature. We need some other idea to explain why Nature does not make use of that opportunity.

  My MIT colleague Alan Guth introduced a brilliant and promising idea, which addresses those issues elegantly. He proposed that the universe underwent a tremendously rapid expansion early in its history, which he calls “inflation.”

  It is easy to appreciate intuitively how inflation can help with our issues. If the universe inflates, then inhomogeneities in matter are diluted, and curvature is expanded away.*

  But did inflation actually happen? I’d like to think so, but it would be good to have more specific ideas about how it happened, and more specific evidence in its favor.

  Inflation is not a consequence of the fundamental laws we know today. It requires something more—additional forces and fields, presumably. Andrei Linde and Paul Steinhardt proposed some forces and fields that could do it, but there is no independent evidence for them. A good model of inflation might enable us to test the basic idea more rigorously and draw out new consequences. As yet there is no such model. There’s a big opportunity for discovery here.

  Reaching Further Back

  The cosmic microwave background is a lingering afterglow of the big bang, which gives us a direct window on the universe’s early history. It arises, you may recall, from photons that were present in the cosmic fireball at the time when it first cooled down enough to become transparent. That happened about 380,000 years after the big bang. While this is impressively early, relative to the 13,800,000,000-year age of the universe, a lot of fascinating events happened earlier, and we’d like to look into those, too.

  Investigating those will be challenging, but there are some real prospects for accomplishing it. For instance, there are at least two other afterglows that ought to surround us. Their origins resemble that of the cosmic microwave background. They are composed of neutrinos and of gravitons.*

  Since neutrinos interact feebly with other sorts of matter, and gravitons more feebly still, the fireball becomes transparent to them much earlier than it does for photons. In consequence, the lingering afterglows of neutrinos and gravitons carry messages that are much older than the ones that the cosmic microwave background carries. Gravitons, in particular, can give us a glimpse of events that occurred only small fractions of a second after the big bang. There’s plenty of room for surprises there. A graviton-based snapshot could show us what was going on at temperatures and other conditions far more extreme than anything that occurs in terrestrial laboratories, or most likely anywhere else in the present-day universe. We might get to see a burst of gravitational radiation spit out by material moving rapidly, during cosmic inflation, for example.

  The challenge of observing these more exotic lingering afterglows arises from the same feature that makes them so fascinating, namely that they interact very feebly with other kinds of matter. We will need new and highly sensitive kinds of antennas and telescopes in order to see them at all. Those antennas and telescopes will almost certainly bear little resemblance to the ones developed for photons. Here there is much room for creativity.

  There may also be other lingering afterglows, arising from particles whose existence is not yet established. After all, the basic thing about cosmic afterglows is that they arise from particles which interact so feebly with matter that the universe becomes transparent to them.

  “Dark matter” could be just such an afterglow. I, and most of my colleagues, suspect that it is. Specifically, I suspect it is an afterglow of axions. I’ll be spelling this out, and making the case, in chapter 9.

  The Very Beginning

  Because our vision gets blinded as we approach the big bang, it is not possible to speak with confidence of “the very beginning.” The concept might be misguided, or even senseless. Saint Augustine made a brilliant suggestion about this in his Confessions, which I suspect is on the right track. A parishioner asked Augustine, “What was God doing, before He created the universe?” Augustine records that he considered answering, “Preparing hell for people who ask too many questions.” But he had too much respect for his parishioner, for himself, and for God to do that. Instead he thought hard about the problem, and he prayed for an answer, as it preyed on his mind. This led him in
to a deep meditation on time.

  Augustine reached a conclusion about the nature of time very similar to ours, in chapter 2. Basically, he concluded that time is what clocks measure—neither more nor less. That thought led him to a better answer to his parishioner’s question. Before God created the world, Augustine reasoned, there were no clocks—and therefore no time, and therefore no such thing as “before.” Thus, the question “What happened before God created the universe?” when carefully considered is devoid of meaning.

  The essence of Augustine’s answer survives translation into the language of modern physical cosmology. Nothing precedes the origin of the universe, because in that context, time— the thing that clocks measure—has no meaning.

  7

  COMPLEXITY EMERGES

  The physical world is complicated. Rain forests, the internet, and the collected works of William Shakespeare are all contained within it. Yet our fundamentals promise to build it all up from a few ingredients, a few laws, and a strangely simple origin.

  This poses a challenging question: How does complexity emerge, fundamentally? This chapter explores that question. At its close, I’ll discuss the long-term prospects of cosmic complexity and how apparent complexity can exist within profound simplicity.

  HOW THE UNIVERSE GOT INTERESTING