On the very largest scales we can regard the universe as a roughly homogeneous mixture of matter and radiation. The overall density of matter is not known with certainty, but it is thought to be at least a few tenths of the critical density, about 10-26 kg/m3, above which the universe will eventually recollapse, and below which it will expand forever. The universe is apparently open, but barely so.
The matter in the universe consists of the familiar building blocks of atoms—protons, neutrons, and electrons—as well as the mysterious dark matter, whose composition is still hotly debated by astronomers. Most of the radiation in the universe is in the form of the cosmic microwave background, the low-temperature (3 K) radiation field that fills all space. 
(Sec. 26.6) Surprisingly, although the microwave background radiation is very weak, it still contains more energy than has been emitted by all the stars and galaxies that have ever existed! The reason for this is that stars and galaxies, though very intense sources of radiation, occupy only a tiny fraction of space. When their energy is averaged out over the volume of the entire universe, it falls short of the energy of the microwave background by at least a factor of 10. For our current purposes, then, we can ignore much of the first 26 chapters of this book and regard the cosmic microwave background as the only significant form of radiation in the universe!
Is matter the dominant component of the universe, or does radiation also play an important role on large scales? In order to compare matter and radiation, we must first convert them to a "common currency"—either mass or energy. Let's choose to compare their masses. We can express the energy in the microwave background as an equivalent density by first calculating the number of photons in any cubic centimeter of space, then converting the total energy of these photons into a mass using the relation E = mc2. (Sec. 16.5) When we do this, we arrive at an equivalent density for the microwave background of about 5 
10-31 kg/m3. Thus, at the present moment the density of matter (around 10-26 kg/m3) in the universe far exceeds the density of radiation. In cosmological terminology, we say that we live in a matter-dominated universe.
Was the universe always matter dominated? To answer this question, we must ask how the densities of both matter and radiation change as the universe expands. Both decrease, as the expansion dilutes the numbers of atoms and photons alike. But the radiation is also diminished in energy by the cosmological redshift, so its density falls faster than that of matter as the universe grows (see Figure 27.1). Conversely, as we look back in time, closer and closer to the Big Bang, the density of the radiation increases faster than that of matter. Accordingly, even though today the radiation density is much less than the matter density, there must have been a time in the past when they were equal. Before that time, radiation was the main constituent of the cosmos. The universe is said to have been radiation-dominated then. Given our best estimates of the present densities, the crossover point—the time at which the densities of matter and radiation were equal—occurred a few thousand years after the Big Bang, when the universe was about 20,000 times smaller than it is today. The temperature of the background radiation at that time was about 60,000 K, so it peaked well into the ultraviolet portion of the spectrum.
Figure 27.1 As the universe expanded, the number of both matter particles and photons per unit volume decreased. However, the photons were also reduced in energy by the cosmological redshift, reducing their equivalent mass, and hence their density, still further. As a result, the density of radiation fell faster than the density of matter as the universe grew. Tracing the curves back from the densities we observe today, we see that radiation must have dominated matter at early times—that is, at times before the crossover point.
Throughout this book we have been concerned exclusively with the history of the universe long after it became matter dominated—the formation and evolution of galaxies, stars, and planets as the universe thinned and cooled toward the state we see today. In this chapter we consider some important events in the early, radiation-dominated universe, long before any star or galaxy existed, that played no less a role in determining the present condition of the cosmos.
The very early universe was characterized by temperatures and densities far greater than anything we have encountered thus far, even in the hearts of supernovae. In order to understand the early universe, we must delve a little more deeply into the behavior of matter and radiation at very high temperatures.
The key to understanding events at very early times lies in a process called pair production, in which two photons give rise to a particle—antiparticle pair, as shown in Figure 27.2(a) for the particular case of electrons and positrons. In this way, matter can be created from radiation. The reverse process can also occur—a particle and its antiparticle can annihilate each other to produce energy in the form of electromagnetic radiation, as depicted in Figure 27.2(b). In other words, energy in the form of radiation can be freely converted into matter in the form of particles and antiparticles, and particles and antiparticles can be freely converted back into radiation, subject only to the law of conservation of mass and energy.
Figure 27.2 (a) Two photons can produce a particle—antiparticle pair—in this case an electron and a positron—if their total energy exceeds the mass energy of the particles produced. (b) The reverse process is particle—antiparticle annihilation, in which an electron and positron destroy each other, vanishing in a flash of gamma rays. (c) Tracks in a particle detector allow us to visualize pair creation. Here a gamma ray, whose path is invisible because it is electrically neutral, arrives from the left; it dislodges an atomic electron and sends it flying (the longest path). At the same time it provides the energy to produce an electron—positron pair (the spiral paths, which curve in opposite directions in the detector's magnetic field because of their opposite electric charges).
The higher the temperature of a radiation field, the greater the energy of the typical constituent photons, and the greater the masses of the particles than can be created by pair production. For any given particle, the critical temperature above which pair production is possible, and below which it is not, is called the particle's threshold temperature. The threshold temperature increases as the mass of the particle increases. For electrons it is about 6 109 K. For protons, which are nearly 2000 times more massive, it is just over 1013 K.
As an example of how pair production affected the composition of the early universe, consider the production of electrons and positrons as the universe expanded and cooled. At high temperatures—above about 1010 K—most photons had enough energy to form an electron or a positron, and pair production was commonplace. As a result, space seethed with electrons and positrons, constantly created from the radiation field and annihilating one another to form photons again. Particles and radiation are said to have been in thermal equilibrium—new particle—antiparticle pairs were created by pair production at the same rate as they annihilated one another. As the universe expanded and the temperature decreased, so did the average photon energy. By the time the temperature had fallen below a billion or so kelvins, photons no longer had enough energy for pair production to occur, and only radiation remained. Figure 27.3 illustrates how this change took place.
Figure 27.3 (a) At 10 billion K most photons have enough energy to create particle—antiparticle (electron—positron) pairs, so these particles exist in great numbers, in equilibrium with the radiation. (b) Below about 1 billion K, photons have too little energy for pair production to occur.
Pair production in the very early universe was directly responsible for all the matter that exists in the universe today. Everything we see around us was created out of radiation as the cosmos expanded and cooled. Because we are here to ponder the subject and we ourselves are made of matter, we know that some matter must have survived these early moments. For some reason there was a slight excess of matter over antimatter at early times. A small residue of particles that outnumbered their antiparticles was left behind as the temperature dropped below the threshold for creating them. With no antiparticles left to annihilate them, the number of particles has remained constant ever since. These survivors are said to have frozen out of the radiation field as the universe cooled.
The first hundred or so seconds of the universe's existence saw the creation of all of the basic "building blocks" of matter we know today—protons and neutrons froze out when the temperature dropped below 1013 K, when the universe was only 0.0001 s old. The lighter electrons froze out somewhat later, about a minute or so after the Big Bang, when the temperature fell below 109 K. This "matter-creation" phase of the universe's evolution ended when the electrons—the lightest known elementary particles—appeared out of the cooling primordial fireball. From that point on, matter has continued to evolve, clumping together into more and more complex structures, eventually forming the atoms, planets, stars, galaxies, and large-scale structure we see today, but no new matter has been created since that early time.
