How did the universe begin about 12 billion years ago? The question concerns the very large—space, galaxies, etc.—but also the very small, namely the innermost structure of matter. The reason is that the early universe was very hot, so that matter was then decomposed into its constituents. These two topics hang together, and this is what makes them so interesting.
One must start with a few words about the innermost structure of matter. The sketch in Figure 1 indicates, on the very left, a piece of metal.
It is made of atoms. To the right of it you see one of the atoms symbolically designed with a nucleus in the middle and with electrons around it. Here we proceed toward the innermost structure of matter in steps. That’s why I call it the quantum ladder. Further to the right you see the nucleus, consisting of protons and neutrons, which I will call nucleons from now on. We have found out that the nucleons themselves are composite; they are made up of quarks, as seen in Figure 1.
Let us look at the forces that keep the constituents together in the four steps of the quantum ladder. The deeper you go, the stronger the forces become. In the piece of metal, the chemical force that keeps the atoms together has the strength of a few electron volts (this is a measure of force strength). In the atom, the electrons are bound to the nucleus by a few tens of electron volts. The protons and neutrons are bound within the nucleus by millions of electron volts, and the forces between the quarks in a nucleus are in the billions of electron volts. This leads us to the concept of conditional elementarity. When we apply small amounts of energy, we cannot overcome the forces that keep the constituents together. For example, if energies of less than a few electron volts are available, atoms cannot be decomposed into electrons and nuclei. They seem to be elementary, which means stable, or unchangeable. When energies above a few hundred but below a million electron volts are available, atoms may be decomposed, but nuclei and electrons seem elementary. For energies over a million electron volts, nuclei are decomposed, but the protons and neutrons are elementary. At a billion electron volts, the nucleons appear to be composed of quarks. Electrons, so far, have never been shown to be composite.
It will be important later on to understand the connection between energy and temperature. Heating a piece of material is equivalent to increasing the energy of motion of the constituents of that piece, be they atoms or electrons or other particles. In a hot material, the atoms or the electrons perform all kinds of motions, oscillations, straight flights, etc. The greater the temperature, the higher the energy of the motions. Thus, temperature is equivalent to energy. For example, one electron volt corresponds to about 12,000 degrees Celsius (about 22,000 degrees Fahrenheit). The temperature at which atomic nuclei decompose is about 20 billion degrees. A billion electron volts would be about 20 trillion degrees Celsius.
On the last rung of the quantum ladder, when billions of electron volts are available—by means of accelerators or when the universe was very hot—new phenomena appear. Let us call it the subnuclear realm. Antimatter plays an important role at that stage. What is it? In the last fifty years it was discovered that there is an antiparticle to every particle; an antielectron called a positron, an antiproton and antineutron, an antiquark. They carry the opposite charge of the actual particle. Thus there ought to exist antiatoms, antimolecules, antimatter of all sorts, made of antielectrons and antinuclei. Why do we not find antimatter in our environment? Because of an important fact: when an antiparticle hits a particle, they “annihilate.” A small explosion occurs, and the two entities disappear in a burst of light energy or other forms of energy. This is in agreement with the famous Einstein formula E = mc2, which says that mass—in this case, the masses of the particle and the antiparticle—is a form of energy. The opposite process also occurs: a high concentration of energy can give rise to the birth of a particle and antiparticle. This is called pair creation.
To summarize the quantum ladder, let me quote a prophetic statement by Newton, who wrote three hundred years ago; it describes Figure 1 from the right to the left, as it were:
Now the smallest particles of matter may cohere by the strongest attractions, and compose bigger particles of weaker virtue. And many of these may cohere, and compose bigger particles whose virtue is still weaker. And so on for diverse successions, until the progression ends in the biggest particles on which the operation in chemistry and the colors of natural bodies depend, which by cohering compose bodies of a sensible magnitude…
…like that piece of metal. He foresaw the ideas of the structure of matter that were developed centuries after his time.
Let us now turn to our main subject: the universe. Let’s first look at the universe as we see it today. There are six facts that are important to us. First, most of the stars we see in the universe consist of 93 percent hydrogen, 6 percent helium, and only 1 percent all other elements. This has been determined by analyzing the light from the stars. Here on earth, things—including our bodies—consist mainly of other elements besides hydrogen. But this is a special case; the stars are made mostly of hydrogen. I have to mention something of which astronomers should be very much ashamed. It turned out that visible matter, the one that sends light to us, is only 10 percent of the total matter. Ninety percent of the matter of the universe is what is now called dark matter—dark because we don’t see it; dark because we don’t know what it is. How do we know that it is there? The dark matter, like any matter, attracts other matter by gravity. One has found motions of stars and galaxies that could not be explained by the gravitational attraction of the visible, luminous matter. For example, stars in the neighborhood of galaxies move much faster than they would if they were attracted only by the visible stars. So far the nature of that dark matter is unknown. We do not have the slightest idea of what 90 percent of the world is made of.
The second fact concerns the distribution of matter in space. We know that it is very uneven. We see stars, but nothing in between; we see galaxies and clusters of galaxies. However, if we average over a large part of space containing many stars and galaxies, we find that luminous matter is very thinly distributed, only about one hydrogen atom per cubic meter. To this we must add ten times as much dark matter.
The third fact is the expansion of the universe. The following astounding observation was made about sixty years ago, first by the American astronomer E.P. Hubble. It was Hubble who found that faraway objects like galaxies, move away from us; the greater the distance, the faster they move away. For example, a galaxy that is as far as one million light years moves away from us with a speed of about twenty kilometers per second. Another galaxy, at a distance of two million light years, moves away at forty kilometers per second; another, at three million light years, moves away at sixty kilometers per second; and so on. As a consequence, the distances between objects in space increase as time goes on. The universe gets more dilute with time. It is a kind of decompression of matter.
A most dramatic conclusion must be drawn from this: if we go backward in time, we conclude that galaxies were nearer to each other in the past. Therefore, at a certain time in the far distant past, the matter in the universe must have been extremely dense. Matter must have been highly compressed, far more than any compression achievable on earth by technical means. At that time there were no galaxies or stars: matter was so thoroughly compressed that everything merged. A little calculation shows that this happened about 12 billion years ago.
In this calculation, one has taken into account that the expansion was faster at an earlier time, since the gravitational attraction acts like a brake and slows down the expansion. Today’s rate of expansion, the so-called Hubble constant, is not very well established. It could be fifteen or thirty, instead of twenty, kilometers per second at a million light years. Therefore, the time of extreme compression—this is the time of the beginning of our universe, of the Big Bang—may not have been 12 billion years ago, but perhaps 10 or 15 billion years ago. Still, we can introduce a new chronology: the zero time is the time of extreme compression, the time of the Big Bang. Today is about 12 billion years since the beginning.
We now approach the fourth point regarding our present universe. How far can we see into space? Since the universe is about 12 billion years old, we cannot see farther than about 12 billion light years. We call this distance the cosmic horizon of today. As we will see later in more detail, the Big Bang was a tremendous explosion in which space expanded almost infinitely fast, creating matter over a region probably much larger than what is visible today. Light from those farther regions has not had enough time to reach us today but may do so in the future.
There is another interesting consequence: the farther we look within the cosmic horizons, the younger are the objects we see. After all, it took time for the light to reach us. The light we see of a galaxy, say, 100 million light years away, was emitted 100 million years ago. A picture of the galaxy shows how it was 100 million years back. Figure 2 shows this schematically.
The outer circle is the cosmic horizon. The broken circle is about six billion light years away, and objects there appear to us only six billion years old. What about objects at or very near the horizon? What we see there is matter in its first moments, matter just or almost just born. Thus, if we had very good telescopes, we could see the whole history of matter in the universe, starting far out and ending near us.
Beware of the following misunderstanding. One could wrongly argue that, say, the regions that are six billion light years away were much nearer to us when they did send out their light, and therefore we should see them earlier than six billion years after emission. This conclusion is false, because the light velocity must be understood as relative to the expanding space. Seen from a nonexpanding frame, a light beam running against the expansion—that is, toward us—moves slower than the usual light velocity. As it were, light is dragged along with the expansion.