Cosmology— the first second
In my last post, we rewound the expanding universe to the first million years of its existence, learned about the cosmic scale factor a, and that as we go back in time, the universe gets smaller, hotter, and more dense. If we were to go all the way back to t=0, it would seem that the universe would become a geometric point of zero size, infinite density, and infinite temperature—in other words, a singularity. In my March 11 post, I noted that an initial singularity would result in a universe that is inconsistent with observations. So what do we think happened?
Cosmologists generally divide the history of the early universe into distinct epochs based on the temperature of the universe and which physical forces or particles were dominant at the time. Here’s an overview of these early epochs, with a bit of additional structure for perspective:
The quantum and unified eras— these are highly theoretical, as the energy levels exceeded what we can currently test in particle accelerators
The Planck epoch (speculative)— our current laws of physics break down here, so this is a bit of a placeholder; gravity might be a quantum force, unified with all the other forces
The grand unification epoch (speculative)— gravity split from the unified force; the strong, weak, and electromagnetic forces acted as one single force; finally, the strong force splits off
The inflationary epoch— the universe expanded exponentially, growing by a factor of at least 10^26 in a fraction of a second, smoothing out space
Reheating— the energy from inflation decayed, filling the universe with a hot, dense plasma of particles
The fundamental particles eras— the universe was a soup of subatomic particles
The electroweak epoch— the electromagnetic and weak forces were still unified; particles were massless
The quark epoch— The Higgs field “turned on,” giving particles mass; the universe was a quark-gluon plasma, too hot for quarks to bind together
The hadron epoch— the universe cooled enough for quarks to bind together into hadrons (protons and neutrons)
The nucleosynthesis and radiation eras— the universe began to look recognizable
The lepton epoch— most hadrons had annihilated; the universe was dominated by light leptons (electrons, neutrinos) and photons
The photon epoch— the universe was a hot, opaque plasma of nuclei and free electrons; photons dominated energy density
Big Bang Nucleosynthesis (BBN)— the first atomic nuclei formed (mostly hydrogen and helium); fusion occurred across the entire universe
The matter-dominated epoch began— structure formation began
Recombination— electrons and nuclei formed neutral atoms, and the universe became transparent, releasing the cosmic microwave background (CMB)
(The photon epoch and the matter-dominated epoch started well after t=1 second, and though I didn’t call them out by name, they were both discussed in my previous post on the first million years.)
Note that epoch boundaries can overlap and their timing can vary, depending on models; the sequence above reflects the dominant physical processes at each stage.
Now, some details....
The Planck epoch
It turns out that our best theories break down at what’s called the Planck epoch, which is any time before ~t=5×10^−44 seconds. This time represents the “Great Wall” where our two most successful pillars of physics—general relativity and quantum mechanics—violently disagree with one another. So we simply have no idea what to say about physics at or before the Planck epoch.
If we had a Theory of Everything (ToE), a hypothetical coherent theoretical framework of physics containing all physical principles—or at least encompassing all four fundamental interactions: electromagnetism, strong and weak nuclear forces, and gravity—then we might have some ideas. But we don’t.
Nonetheless, there is quite a lot that can be said about times just after the Planck epoch. According to our best theories so far, all the forces in physics might have been unified into a single force at this time, with the possible exception of gravity. According to general relativity, gravity isn’t a force in the same sense as the other forces (the strong and weak nuclear forces, and electromagnetism), it’s an artifact resulting from the warping of spacetime by mass-energy (remember E=mc^2). So gravity may or may not have been unified with the other forces at the end of the Planck epoch. If it was, it probably split off from the unified force at the end of the epoch, or shortly thereafter.
The grand unification epoch
Slightly after the Planck epoch, the period from around t=10^−43 to 10^-36 seconds is called the grand unification epoch, or the GUT epoch (GUT stands for grand unified theory). During this period, the strong and weak nuclear forces, and electromagnetism were all unified into a single force. The temperature ranged from ~10^32 K down to ~10^29 K. (These “temperatures” are really indications of the prevailing energy levels, and physicists prefer GeV to Kelvins for such high energies, but I’ll continue to use temperatures most of the time, as they are more familiar.) As the universe cooled through this epoch, a GUT phase transition occurred (analogous to spontaneous symmetry breaking, which you may remember from my Feb 19 post), and the strong force separated from the unified electroweak-strong interaction, ending the GUT epoch.
All of the preceding discussion is strictly theoretical, with no observational evidence to support it. These are educated guesses. Most of what follows is more firmly established, both theoretically and observationally, but I will try to point out where there are important exceptions.
There are multiple different theories regarding the timing of the GUT transition that ended the GUT epoch relative to what I’m going to describe next: cosmic inflation. This is one of the open problems in early universe cosmology. For the level of detail in these posts, it doesn’t matter much, but some theories have inflation coming before, during, or after the GUT transition. For this series of posts, I will treat the GUT epoch and cosmic inflation as separate and independent.
The inflationary epoch
Cosmic inflation, or in the context of cosmology, simply inflation, is a proposed period of exponential accelerated expansion of the universe, during which the universe expanded by a factor of at least ~10^26 (and possibly much more) in an extremely short time, smoothing and flattening space itself. During inflation, the theory says that the universe was essentially empty, cold, and expanding so fast that no particles could form.
The mechanism proposed for inflation is rather esoteric, and doesn’t really enlighten the reasons it was proposed, which are more relevant here. Inflation was introduced to fix several otherwise puzzling features of the Big Bang:
The horizon problem
The flatness problem
The monopole problem
(For more details on these problems, see my March 11 post.)
In addition, quantum fluctuations in the universe were stretched to cosmic scales and became density perturbations that later showed up as tiny anisotropies in the CMB that were stretched through later cosmic expansion, and became the seeds of galaxies and other large-scale structure of the universe.
Although there is no direct evidence yet for inflation (e.g., primordial gravitational waves are still unconfirmed), there is strong indirect evidence, including predicting the nearly scale-invariant spectrum of perturbations in the CMB, and solving the three problems mentioned above. However, some physicists have argued inflation is not falsifiable (i.e., it can’t be disproved) in any meaningful sense given the wide variety of models; this is generally considered a shortcoming.
Reheating
As inflation was ending, through a process called reheating, inflation’s energy transferred into other quantum fields. Reheating started with a phase called preheating, which created a chaotic spray of relativistic, high-energy particles, forming a hot, dense, near-perfect fluid of massless bosons and fermions. (Bosons are elementary subatomic particles that carry forces, like photons and gluons; fermions are elementary subatomic particles that are usually associated with matter, like electrons and neutrinos.) All of these these particles were massless because the Higgs field—that gives mass to matter—had not yet “turned on;” and since they didn’t have mass, they were all traveling at the speed of light! These particles interacted and scattered off of each other, finally reaching thermal near-equilibrium (a final phase called thermalization), and a temperature of ~10^27 K, marking the beginning of a measurable temperature of the universe.
At the end of inflation and reheating, ~t=10^–32 seconds, the boson-fermion mixture had become a quark-gluon plasma (QGP). (Quarks and gluons are elementary particles that combine to form hadrons, which are composite subatomic particles like protons, neutrons, and mesons. During this timeframe, the universe was still too hot to form hadrons.) This was the transition into the true “Hot Big Bang“ we usually envision.
You may have noticed that I have sometimes used terminology like the “beginning of the universe” and “t=0“ instead of referring to such a time as the Big Bang. Most cosmologists now consider inflation to have preceded the Big Bang, so the Big Bang really wasn’t the beginning of the universe. Thermalization also marks the beginning of the radiation-dominated era (more on this later).
The ordering of GUT transition, inflation, and reheating is model-dependent, and many events have uncertain timing, even within a given model. I’m going to skip many of these details and controversies so I can focus on more of the big-picture stuff.
The electroweak epoch
The electroweak epoch was the period, ~t=10^–32 until ~t=10^–12, when the electromagnetic force and the weak nuclear force were still unified. It represented a critical transition where the universe shifted from an exotic, high-energy state into the world of recognizable particles. During this epoch, the universe was so hot (~10^28 K down to ~10^15 K) that there was no distinction between light (electromagnetism) and the force responsible for radioactive decay (the weak force).
Instead of the photons, W bosons, and Z bosons we see today, the force was carried by four massless gauge bosons: the W^1, W^2, W^3, and the B boson. All the elementary particles (quarks and electrons) were still massless and traveled at the speed of light.
Baryogenesis
This is important phase whose timing is model-dependent— it occurred either before, during, or after the electroweak epoch, and is generally not given the status of an epoch. In any case, baryogenesis was a hypothetical process during the early universe that produced baryonic asymmetry, the observation that only matter (baryons) and not antimatter (antibaryons) are detected in the universe today (other than in cosmic ray collisions). A number of theoretical mechanisms have been proposed, and some have favorites, but there’s no clear winner.
The quark epoch
As the universe cooled following reheating, at ~t=10^–12 seconds, falling to a “critical temperature” of ~1.8×10^15 K, and the electroweak symmetry was broken, with the following results:
The unified electroweak force split into the weak nuclear force and electromagnetism— this is generally called the electroweak transition,
The Higgs field gradually took on a non-zero value throughout space, and
Fermions (quarks and electrons), and the W and Z bosons acquired mass.
Subsequently, all the fundamental interactions we know of—gravitation, electromagnetic, weak and strong interactions—took their present forms, and fundamental particles took on their expected masses.
But the temperature of the universe was still too high to allow the stable formation of many of the composite particles we now see in the universe. (More precisely, any composite particles that formed by chance almost immediately broke up again due to the extreme energies.) So the universe was still filled with a QGP.
The hadron epoch
As the universe continued to cool, at ~t=10^–6 seconds (one microsecond), the temperature dropped to ~1.7×10^12 K, a critical threshold called the Hagedorn temperature. This was the moment the universe transformed from a hot, chaotic QGP into a collection of hadrons—composite particles like protons and neutrons. It’s called the hadron transition, or the QCD phase transition (QCDstands for quantum chromodynamics, the study of the strong interaction between quarks mediated by gluons).
This era was incredibly violent. Because the universe had cooled so much, it could no longer spontaneously create new quark-antiquark pairs from pure energy. Quarks and antiquarks began to find each other and annihilate into pure high-energy photons. And because of baryogenesis, there was a tiny surplus of hadrons (vs. antihadrons) that became all the matter in the universe today—though it was still a very dense, hot gas.
The lepton epoch
After the electroweak transition, quarks and gluons were still present, but were becoming less dominant as the universe cools, and there was a gradual transition from the hadron epoch to the lepton epoch. This period lasted from t=~10^–12 to ~1 second, and the temperature fell from ~10^15 K down to ~10^10 K. The epoch was leptonic in the sense that leptons and antileptons were the dominant matter component, in thermal equilibrium with the photon bath, carrying most of the non-radiation energy density, and dominated the contribution from hadrons.
This period had significant internal structure as successive lepton species (electron, tau, muon, neutrino) dropped out of equilibrium, and annihilated with their antiparticle counterparts. Most of this annihilation was done by t=~1 second, but the electron-positron annihilation may have lasted until t=~10 seconds. By the end of this process, the universe is a photon-dominated plasma with a small number of baryons.
In subsequent posts, I plan to return to discussing these kinds of changes that took place during times after the first million years. But to understand these later times, we first need to take a short detour to discuss stars, their formation, their evolution, and their fate.