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.
Our fifth question has to do with the temperature in the universe. How hot is it out there? Let us consider a kiln, such as potters use, to understand the situation. Take a kiln and heat it up. First you can see no light, but the kiln radiates microwaves. When it gets hotter it radiates infrared radiation, which we do not see but can feel as heat radiation. At higher temperatures it becomes red, then yellow and white, then ultraviolet; at millions of degrees it will radiate X-rays.
Today, in the immediate surroundings within a few million light years, the temperature is very low in space. It was measured a few decades ago when two Princeton physicists, A. Penzias and R. Wilson, found a very cool microwave radiation in space corresponding to heat radiation of only five degrees above absolute zero—the lowest possible temperature, which is minus 460°F. An appropriate measure of very low temperatures is the Kelvin scale. Zero degree Kelvin is absolute zero. The Kelvin scale uses Celsius degrees above absolute zero. Thus, the space temperature in our neighborhood is 3°K. This is the temperature in space between the stars. The stars are much hotter inside, but there is so much space between them that their higher temperature does not count.
Was the temperature always 3°K? No, it was much warmer at earlier times, a fact that is related to the expansion of the universe. Let us go back to the kiln again. Imagine a kiln made in such a way that we can expand or contract its volume at will. The laws of physics tell us that the temperature of a kiln drops when it expands and rises when it contracts. Thus, we must conclude that the expansion of the universe lowers the temperature. It must have been hotter at earlier times. For example, about six million years ago, the temperature was roughly twice as high—that is, near 6°K. At the very beginning, about 12 billion years ago, when space was extremely contracted, the temperature must have been extremely high. This has interesting consequences.
We know from the physics of radiation that matter is transparent for light when the temperature is below 1,000°C. This is true only for very dilute matter, such as that found in the space between the stars. Matter of ordinary density, such as a piece of iron or wood, is not transparent, of course. But if the temperature is raised from 1,000°C, even very dilute matter becomes opaque. Thus light from those outer regions near the cosmic horizon, which are so young that the temperature is over 1,000°C, cannot penetrate space and will not reach us. We should emphasize that these regions are very near the cosmic horizon. A temperature of 1,000°C was reached when the universe was about 300,000 years old, an age that is very young compared with 12 billion years. Hence, light reaches us not from the cosmic horizon but from a distance that is almost as far as the cosmic horizon. We see only matter older than 300,000 years, which is nevertheless pretty young. Even younger matter, younger than that, is hidden by the opaque space.
Figure 3 illustrates this schematically.
The outermost circle is the cosmic horizon, where matter is just born at extreme density and extreme heat. But already a little nearer to us at the center, the temperature has fallen to and below 1,000°C, and we can see it, since space inside that second circle is transparent.
But why do we not see that part of the universe glowing white-hot at 1,000°C? The reason is the famous Doppler effect. That part of the universe moves away from us at a terrific speed according to the law of expansion, which states that the greater the distance of an object, the faster it moves away from us. The Doppler effect reduces the frequency of light if the emitting object moves away from us. Everybody has observed how the whistle of a fire engine lowers its pitch as the engine moves away. Reducing the frequency is equivalent to lowering the temperature. Red’s frequency is lower than yellow’s and much lower than violet’s. Therefore, the heat radiation from that faraway region of 1,000°K is much cooled down because it moves away from us so fast. Indeed, it is cooled down from 1,000°K to 3°K. Thus, the cool radiation that Penzias and Wilson have observed is indeed the radiation from the hot universe 300,000 years after the Big Bang. The 3°K radiation can be considered the optical reverberation of the Big Bang. This is not quite correct, because it was emitted a little later. That is the explanation of the cool radiation of today.
So far we have discussed the present state of the universe and what one can deduce from it as to its past. Now we will recount the speculations and hypotheses as to the history of the universe from the Big Bang to today, and perhaps also what was before the Big Bang. Usually history does not enter physics. One studies the properties of matter as it is today. Other sciences, such as geology, anthropology, and biology, are historical sciences; the first one deals with the history of the earth, anthropology with the history of the human animal, and biology with the history of animal and plant species.
When physics becomes historical, it deals with the history of matter—that is, with the history of the universe. It is then called cosmology. It must be emphasized that most of the conclusions are much less reliable than those in other fields of physics. Facts are scarce and not known with any accuracy. The Russian physicist Lev Landau said that the cosmologists have very weak facts to work with but very strong convictions about what they think is going on. Whatever will be told here may turn out to be wrong in the near future. Nevertheless, it is so impressive that it is worth reporting.
As we have seen, our universe is expanding and cooling down. We are in principle able to see parts of the universe in earlier periods just by looking at distant objects. We have seen that this is possible to a point in time in the past 300,000 years after the Big Bang. Let us therefore call the time from 300,000 years after the Bang up to today the period of observable history. Of course, the history is observable only in principle. Actually, our instruments are not good enough to get detailed information regarding very distant objects.
We will not say much about that period; the preobservable history is more interesting. At the beginning of observable history the temperature was around 1,000°K, which was low enough so that atoms were not destroyed and robbed of their electrons. Therefore, space was filled mainly with hydrogen and helium atoms forming a hot gas. The density of things never was completely uniform. There were gas accumulations here and more dilute parts somewhere else. The accumulations grew because of gravity. They had more concentration of mass and therefore attracted the surrounding gas more strongly than the dilute parts. The further this accumulative process went, the more effective the gravitational pull became. Such accumulations finally formed “protostars” of much higher density than elsewhere. These protostars also became much hotter than the rest, since compression produces heat. When it became hot enough at the center of such protostars, nuclear reactions were ignited, producing even more energy. The protostar became a real star like the sun, whose radiation energy comes from the nuclear reactions at the center. Furthermore, the early deviations from complete uniformity caused the stars to be not uniformly distributed but to form agglomerations that we see today as galaxies.
The nuclear reactions inside a star produce helium out of hydrogen. When a star has used up its primary nuclear fuel—hydrogen—at its center, other nuclear processes form heavier elements, such as carbon, oxygen, up to iron. Finally the star explodes and becomes a supernova. In this process most other elements are formed and expelled into space. Then new accumulations and protostars are formed from the gases in space, which now contain traces of other heavier elements, such as oxygen, carbon, iron, gold, and uranium. The sun is an example of a “second-generation” star. Some of the stars are surrounded by planets like the sun. In some of the planets, such as the earth, heavier elements are present in higher concentration. This is because most hydrogen and helium atoms escape from smaller planets, since those atoms are light and planets exert only weak gravitational pull. The hydrogen found on earth is bound in molecules to heavier atoms. Life may develop under the mild warming of the nearby star. So much for the observable history.
Now let us turn to the period between the Big Bang and the onset of observable history at about 300,000 years. Let us call it preobservable history. Nothing about that period can be observed; space was opaque because the temperature was higher than 1,000°C. But we are able to conclude from our knowledge of physics what happened during that period, at least for times that are not too near to the Big Bang. Pursuing the picture of an expanding and cooling universe, one comes to the conclusion that a microsecond after the Big Bang, the temperature must have reached about 10 trillion degrees, or a thermal energy of a billion electron volts. Our present knowledge is good enough that we can guess what has happened in the universe between a microsecond and 300,000 years. But conclusions about events at earlier times, when the energy concentrations were higher, are very uncertain.
Let us tell the story in reverse, going back in time from 300,000 years to a microsecond. In that inverse sense, the universe must be regarded as contracting and getting hotter. When the temperature was hotter than 10,000°K, the atoms were decomposed and formed a “plasma,” a dense gas of nuclei and electrons. The plasma was bathed in shining light, visible light, during the time when the temperature was between a thousand and a few ten thousand degrees.
That light was more and more ultraviolet (that is, of a higher frequency) at earlier times, when the temperature was higher. This radiation should be considered the same as today’s 3°K radiation but enormously compressed at the early seconds of the expansion. Compression makes light hotter and of higher frequency. Going back in time, we come to a moment at about one second after the Big Bang, when the temperature was about 10 billion degrees, corresponding to an energy concentration of about a few million electron volts. At that point the thermal energy is high enough for creating pairs of electrons and antielectrons (positrons). This is the process of matter-antimatter formation mentioned before. Hence, at one second and earlier, when the temperature was even higher, space was filled by a plasma composed not only of hydrogen and helium nuclei and their electrons but also of a rather dense gas of electrons and positrons.
At a fraction of a second after the Bang, the temperature was high enough to split the helium and nuclei into neutrons and protons. Finally, when our backward history reaches the microsecond after the Bang, the heat and the corresponding energy concentration were high enough not only to decompose protons and neutrons into quarks but also to produce quark-antiquark pairs. At this point of our backward journey in time, the universe was filled with hot, dense gases of quarks and antiquarks, electrons and positrons, and a very intense, high-frequency thermal light radiation. There was also a hot, dense gas of neutrinos, which survived the whole evolution and should be present even today, though much less hot and dense, together with the cool three-degree light radiation. We stop at this point, which is a millionth of a second after the Big Bang. We are practically at it anyway.
The prehistory described here is based on a relatively firm knowledge of the properties of matter at energy concentrations up to several billion electron volts. This knowledge stems from experiments made with accelerators producing particle beams at these energies. The largest of these machines, the ones in Geneva, Switzerland, and in Batavia, Illinois, have reached energies of several hundred billion electron volts. It would be hard to guess what happened much earlier than a microsecond, since the energy concentrations were much higher than the ones reached with our accelerators, and we have no way to know how matter behaves at these enormous compressions and temperatures.
We now have reached the point where we should ask the great questions: What was the Big Bang? What caused it? And what existed before? When facing these exciting questions, it must be said that we have no reliable answers. There are speculation, guesswork led by intuition, and a great deal of imagination that may turn out to be wrong in a few years. However, the answers that are discussed in these days are so unusual and impressive that it is worthwhile to describe them in simple terms. The underlying ideas came mostly from four persons whom one might call the four apostles of the new story of Genesis: Alan Guth of MIT, Alexander Vilenkin of Tufts University, Andrei Linde in the USSR, and Stephen Hawking in England. Paul Steinhardt of the University of Pennsylvania also contributed to it.
In order to understand the basic ideas, we must introduce a concept that is suggested by some of the latest developments in particle physics. It is the so-called false vacuum. According to these ideas, there are two types of vacuum: the true vacuum and the false vacuum. The true one is very much what one would imagine: it is empty space, empty of matter and empty of energy. The false vacuum, however, is also empty of matter, but not of energy. The energy of the false vacuum is supposed to be none of the ordinary forms of energy, such as electric fields or gravity fields. It is imagined to be a new kind of field, of a type encountered in the current theories of radioactive processes. The most characteristic feature of the false vacuum follows directly from Einstein’s general relativity theory. A region filled with energy but not with matter is bound to expand suddenly and explosively, filling more and more space with false vacuum. Alan Guth has called it, succinctly, an inflationary expansion, with a speed very much faster than the previously considered expansion of our universe at any time in its development. According to our four apostles, this sudden explosion is nothing else but the Big Bang.
How does this sudden inflationary expansion of a false vacuum start? Before the event, all space was in the state of a true vacuum. “The world was without form and void, and darkness was upon the face of the deep,” as the Bible says. Now we must introduce a concept that is typical for quantum mechanics. According to the fundamental tenets of this well-established theory, there is nothing in nature that remains quiet. Everything, including the true vacuum, is subject to fluctuations—in particular to energy fluctuations. The field that provides the energy to the false vacuum is absent in the true vacuum, but not completely. There must be fluctuations of the field. Thus, at one moment a small region somewhere in space may have fluctuated into a false vacuum. It would happen very rarely but cannot be excluded. That region almost instantly expands tremendously and creates a large space filled with energy according to the properties of a false vacuum. That is supposed to be the Big Bang!
One might wonder where the energy comes from that fills the expanding false vacuum. There is no need to worry about conservation of energy. According to Einstein, energy is subject to gravity. The newly created energies interact via gravity, an effect that produces negative energy, so that the net energy remains essentially constant.
When a certain large size is reached, the inflationary explosion stops and a true vacuum emerges. But the vast amount of energy contained in the false vacuum must have shown up in some form. It filled the true vacuum with hot light, quark-antiquark pairs, electron-antielectron pairs, neutrinos, etc.—in other words, with all the stuff we have described as filling the space at a microsecond after the Big Bang. Our universe is born, the slow expansion takes over, the temperature falls, and the preobservable history develops and is followed by the observable history.
In short, the history of our universe started with a fluctuation of the empty true vacuum into a small region of false vacuum, which exploded, almost immediately, into a very much larger region of false vacuum. That was the primal Bang. Then it changed to a true vacuum, but the energy of the false vacuum created all light, all particles and antiparticles, which developed into what existed at about a microsecond after the explosion. Then the ordinary expansion of the universe took over; it cooled down; quarks and antiquarks as well as electrons and antielectrons were annihilated, but a few supernumerary quarks and electrons remained. The quarks formed protons and neutrons. Then some of these nucleons formed helium nuclei. After 300,000 years it was cool enough that the protons and helium nuclei could grab and retain electrons and become atoms. A hot gas of hydrogen and helium appeared. The gas of atoms condensed to protostars, which became hot inside, allowing nuclear processes to start. Stars were born, grouping themselves in galaxies. The nuclear reactions in the center of the stars and in exploding supernovas produced heavier elements. The expelled gases of exploding stars condensed to protostars and then to stars containing traces of all elements, not only hydrogen and helium. The sun is one of these second-generation stars. It is surrounded by planets, some of which—such as the earth—are special concentrations of heavier elements, benignly supplied with energy from the nearby sun, so that life can start and develop the strange human animal that pretends to understand the whole process.
An interesting conclusion follows from this view of the birth of our universe, as the consequence of an energy fluctuation in the true vacuum. Such intense fluctuations creating a speck of false vacuum are very rare, but it may have happened at other places in infinite space at other times and may have developed into other universes. Thus, we may conclude that our universe is not the only one. It is not the center and the stage of everything in this world. There may be other universes much older or much younger or even not yet born somewhere else. Remember that our universe today is most probably considerably larger than our present cosmic horizon of about 12 billion light years, but there is room and time enough for many other universes. Maybe, in a few billion years, another universe will penetrate ours. Until then we cannot check this hypothesis. Our own universe, of which we see only a small part today, may not be unique. Its beginning is not the beginning of everything. Other universes may exist at an earlier or later stage.
It must be emphasized again that these are unproven hypotheses. They may turn out to be pure fantasies, but the ideas are impressively grandiose.
The origin of the universe is not only of scientific interest. It always was the subject of mythology, art, and religion. Such approaches are complementary to scientific ones. Most familiarly, the Old Testament describes the beginning of the world with the creation of light on the first day. It seemed contradictory that the sun, our terrestrial source of light, was only created on day four, but it turns out to be in line with current scientific thought, according to which the early universe was full of various kinds of radiation long before the sun appeared.
Those first days have been depicted in various forms, in pictures and poetry, but to me, Franz Josef Haydn’s oratorio The Creation is the most remarkable rendition of the Big Bang. At the beginning we hear a choir of angels singing mysteriously and softly, “And God Said Let There Be Light.” And at the words “And There Was Light” the entire choir and the orchestra explode into a blazing C major chord. There is no more beautiful and impressive presentation of the beginning of everything.
February 16, 1989