The Big Bang
The first hint of the Big Bang—but a hint not understood at the time—was a discovery by an American astronomer with the improbable name Vesto Melvin Slipher. In 1914 he announced to a meeting of the American Astronomical Society in Evanston, Illinois, a remarkable finding. All of the galaxies that he had studied up to that time, about a dozen, were receding from the earth at fantastic speeds—some up to 2 million miles an hour. By 1925 he had extended his survey to include forty-five galaxies, and almost all showed the same strange behavior.
About this time, another American astronomer came into the picture: Edwin Powell Hubble. With Milton Humason, a mule-train driver and janitor turned astronomer, he used the powerful one-hundred-inch-diameter telescope at Mount Wilson Observatory in California to measure the movements of the galaxies. Not only did Hubble and Humason confirm Slipher’s findings, they went beyond him in a very important way: in addition to measuring the velocities of galaxies, they were able to estimate how far away they were. This is not an easy thing to do. One cannot stretch out a measuring stick to another galaxy. But, in a clever indirect way, Hubble and Humason were able to infer these distances, and thus they discovered two remarkable things.
The first thing was that the galaxies were much farther away than anyone had imagined. Some were millions of light-years away. (One light-year is the distance that light, which travels at 186,000 miles per second, traverses in a year. A light-year is about 6,000,000,000,000 miles.) This discovery can be seen as part of the story told in chapter 3 of the marginalization of man. This story has an interesting twist. Hubble and Humason actually, for technical reasons, misjudged cosmic distances by about a factor of ten. They therefore thought the other galaxies were closer, and smaller, than they truly are. As a result, our own galaxy—the Milky Way—seemed to be considerably larger than all the rest, a miscalculation which maintained some degree of anthropocentrism. When the mistake was later remedied, however, it was found that the Milky Way is a fairly typical-sized galaxy after all.
The second remarkable thing that Hubble and Humason found was that the farther away a galaxy is from us the faster it is receding from us. In fact, the distances and recession speeds are roughly proportional to each other—this is called Hubble’s Law. Hubble’s Law may seem to put Earth in a special location, but that is an illusion of perspective. One can show that if Hubble’s Law is satisfied by observations made at one place in the universe, like Earth, it will be satisfied by observations made at any other place. A way to see this is to imagine the galaxies painted on a rubber sheet, and then to imagine that the sheet is stretched uniformly to twice its size. Two “galaxies” that were an inch apart will recede another inch, while two that were a foot apart will recede another foot. That is, the farther apart they start, the faster they recede—Hubble’s Law. And it is apparent that this will be true for any pairs of galaxies on the rubber sheet; none are singled out as special.
Hubble’s Law has an important implication. If one traces the movements of the galaxies backward, one finds that at some point in the past all the galaxies were on top of each other. That is, the Hubble expansion suggests that all the matter in the universe is flying apart from some primeval explosion.
Hubble and Humason’s discoveries were announced in 1929. The true significance of them is far more profound than appears at first sight, and can only be understood in the light of Einstein’s theory of gravity, the so-called General Theory of Relativity. So let us now turn from observation to theory, and from 1929 back to 1916.
What Einstein had proposed in that year was that space and time, instead of being merely some static backdrop against which events played out, were themselves something dynamic and changing. Whereas Newtonian space and time were like a rigid screen on which things moved around, Einsteinian space-time was a flexible fabric, which could bend and quiver in response to the matter and energy that moved around on it. Einstein wrote down a set of equations that describe exactly how space-time bends and quivers:
Gμν = 8πGN Tμν
This looks like one equation, but actually contains several equations packaged within it. Therefore, physicists often talk about “Einstein’s equations.” The quantity Gμν is called the “Einstein tensor” and describes the curvature or flexing of space-time, while the quantity Tμν is called the “stress-energy tensor” and describes how matter and energy are distributed in space and time. GN is Newton’s constant, a number characterizing the strength of the force of gravity, which also appeared in Newton’s law of gravitation. To understand Einstein’s equations one has to know a great deal of physics and mathematics, but just looking at them one can see that there is something rather simple about them. Even so, apples falling from trees, planets orbiting the sun, the vast whirlpool motion of galaxies, which take hundreds of millions of years to go around once, the behavior of “black holes”—objects so strange that time and space get mixed up inside them, even the expansion of the universe itself, are all described by this single, simple-looking set of equations.
It is the last application of Einstein’s equations that concerns us now. What Einstein realized is that not only could the fabric of space-time bend and stretch in one locality, but that the whole of space, the entire universe indeed, could stretch like an expanding balloon. In this balloon analogy, one is not supposed to be thinking of the air or space inside the balloon (or outside the balloon, for that matter). It is the elastic sheet of the balloon itself that is supposed to represent the space of our universe, and physical objects are—as we spoke of the galaxies before—like little spots of paint on the balloon’s surface. Of course, the balloon is only a two-dimensional sheet, while the space of our universe, which it is supposed to represent, is three-dimensional. But do not worry, the mathematics of differential geometry, which is used to study curvature, can be applied to any number of dimensions. For the purposes of analogy, two dimensions are quite adequate. (One might ask whether there is anything about the real universe that corresponds to the air or volume inside—or outside—the balloon. That is, is our three-dimensional universe embedded in some higher-dimensional space, just as the two-dimensional sheet of the balloon is contained in the air around it? No, not at least in the standard cosmological theory. It is not easy, but one is supposed to imagine just the surface of the balloon without imagining the inside and outside of it.)
When a balloon is expanded, the area of the rubber sheet increases—there is more balloon surface. In the same way, the universe can expand so that there is more universe. The actual volume of the universe can increase (or, for that matter, decrease). At this point, I am describing what is called a “closed universe,” which has a finite volume at any given time. It is possible for the universe to have a finite volume, just as the balloon has a finite surface area; and the universe can curve around on itself, just as the surface of the balloon curves around to form a finite area that has no edges. In a closed universe, if one were to extend a line in any direction, it would eventually come back around near to where it began, in the same way that a line drawn on the rubber sheet of the balloon will come around back to where it started. On the other hand, there are also so-called “open universe” solutions to Einstein’s equations, in which the universe has an infinite volume at any given time. An open universe would be like a rubber sheet that does not curve back on itself, but extends to infinity in all directions. These “open universes” will be important in some of our later discussions.
A very important point to realize is that in Einstein’s theory an expansion of the universe does not simply involve an expansion of the matter, in the sense of the matter sliding through space away from other matter. It is the space separating the matter that is expanding. In the balloon analogy, it is not that the painted spots move along the rubber surface of the balloon, but that the balloon itself is stretching to put more space between the spots. If one traced the motion back one would not come to a time when all the galaxies were crowding on top of each other surrounded by empty space.1 Rather, one would come to a point where all the space was on top of itself, so to speak, where the volume of space was zero. That is, the whole balloon started off being of zero size.2
The amazing fact, then, is that the time at which the expansion began—if such a time existed—was also the time at which space and time themselves began. In an expanding universe described by Einstein’s equations, time and space themselves had a beginning. But we are getting ahead of the history here. We are still in the year 1916.
After he discovered his equations, Einstein quickly realized that they cannot, in the form he originally wrote them down, describe an eternal static universe. In Einstein’s equations, the shape and movement of space is described by the Einstein tensor, Gμν. What the matter—galaxies and the rest—is doing is described by the stress-energy tensor, Tμν. In essence, through Einstein’s equations the matter tells the space what to do. The important point is that if the universe is filled with matter, as it is, space cannot remain static. It must be expanding or contracting. A static universe filled with matter is impossible, because all the matter pulls on all the other matter gravitationally.
There is an analogy with a ball that is up in the air. If there is no gravitational pull from the earth, a ball could remain suspended at a fixed height above the ground. But because of gravity, you know that if you see a snapshot of a ball up in the air, it is either on its way up or on its way down. It can be motionless, at most, for an instant, if it happens to be at the top of its motion.
The realization that the universe must be dynamic profoundly disturbed Einstein. Like most materialists, he had naturally assumed that the universe was static and eternal. Some may object to calling Einstein a materialist, since he often spoke of “God.” However, the word God as he used it did not signify a personal being. He believed, he said, in the same kind of God as the philosopher Spinoza, an immanent, somewhat abstract deity. Perhaps one can come close to Einstein’s ideas by saying that “God” for him stood for the orderliness and harmony of the universe. (Nevertheless, he often spoke in very personal terms of his God, referring to him as “The Old One.” A famous statement of Einstein’s was, “Subtle is the Lord, but he is not malicious.”3) In the final analysis, for Einstein the universe with its beauty and harmony is really all there is; it encompasses all of reality, and therefore he conceived of it as being eternal.
In order to avoid the strange conclusion that the universe was expanding or contracting, Einstein modified his equations in 1917. He added a new term to them so that they looked like this:
Gμν = 8πGN Tμν + 8πGN Λ gμν
The new term is the one containing the parameter Λ. The curious thing about this term is that it has no effect whatsoever on ordinary gravitational phenomena. It has, for example, no effect on the way apples fall, or planets orbit, or galaxies spiral around. In fact, it has one and only one physical consequence—it affects the way the universe as a whole expands or contracts. For this reason Λ is called the “cosmological constant.” The cosmological constant (if it is positive) gives a repulsion that tends to counteract the mutual gravitational attraction of the matter. If the cosmological constant has precisely the right value, then the new equations do admit solutions in which the universe is static and eternal, with an exact counterbalancing of the effects of matter and the new cosmological term.
The addition of this cosmological constant term was a perfectly logical step for Einstein to take. In the first place, the original mathematical and physical reasoning that led Einstein to his equations allowed for the possibility of such a term. There was no theoretical reason, therefore, not to include it. Moreover, at that time there was no evidence that the universe was expanding or contracting. Nevertheless, Einstein himself came to regard his addition of the cosmological constant term to his equations as a major mistake. In fact, he called it the greatest blunder of his life. He felt that he had needlessly complicated his equations and somewhat marred their beauty. Had he instead stuck with the original form of his equations he could have predicted the expansion of the universe and the Big Bang.
Once the expansion of the universe was discovered by astronomers the cosmological constant fell by the wayside. It was no longer needed, and physicists generally came to believe that it was not there. By a curious twist of fate, however, evidence has come from astronomical observations in just the last few years that suggest strongly that there might be a cosmological constant after all. And its value seems to be very roughly what Einstein proposed. One could say then that Einstein’s “greatest blunder” has turned out instead to be a masterstroke. And yet, even though there may well be a cosmological constant, it does not fulfill the purpose for which Einstein introduced it. It does not exactly counterbalance the attraction of ordinary matter to produce a static universe. In fact, in 1930 Arthur Eddington showed that such an exactly balanced static universe would not be stable.4 So while Einstein turned out in the end not to be wrong to introduce a new term into his equation, he was wrong in thinking that the universe was static and eternal.
The first people really to take seriously the idea of an expanding universe were Alexander Friedmann, a Russian meteorologist and mathematician, and Georges Lemaître, a Belgian physicist and Catholic priest. Friedmann, in 1922, and Lemaître, independently in 1927, discovered the solutions to Einstein’s equations that described an expanding universe filled with matter.5 At first, Einstein did not believe Friedmann’s result, and published a proof that it was wrong. Shortly afterward, however, he admitted that his proof was in error, and that Friedmann was correct.
It was Lemaître who tied it all together by relating his expanding-universe solutions of Einstein’s equations to the ongoing observations of Hubble and Humason at Mount Wilson Observatory. Lemaître also proposed that all the matter in the universe was originally concentrated into an incredibly dense “primeval atom” that exploded to produce the world we see. He thus laid down the basic outlines of our present “hot Big Bang” theory.
Only in 1930 was Einstein forced to admit that what he had so long resisted was probably true—fourteen years after he proposed his theory of gravity, and eight years after Friedmann’s work. He said, “New observations by Hubble and Humason concerning the red shifts of distant nebulae [i.e., galaxies] make it appear likely that the general structure of the Universe is not static.”6
Einstein’s initial attitude to an expanding universe had been expressed in letters to Willem de Sitter, a Dutch physicist: “One cannot take such possibilities seriously.” Such feelings were shared by many others, even after the discoveries of Hubble and Humason. Eddington, in 1931, wrote, “The notion of a beginning is repugnant to me.… I simply do not believe that the present order of things started off with a bang … the expanding Universe is preposterous … incredible … it leaves me cold.” The German physicist Walter Nernst wrote, “To deny the infinite duration of time would be to betray the very foundations of science.”7
In the words of the cosmologist Andrei Linde, “The nonstationary character of the Big Bang theory [based on the] Friedmann cosmological models seemed very unpleasant to many scientists in the 1950s.”8 As late as 1959, a survey of leading American astronomers and physicists found that two-thirds of them believed that the universe had no beginning.9 This was forty-three years after Einstein’s equations were discovered, and thirty years after Hubble and Humason’s results.
There can be no question that the aversion that some scientists felt for the Big Bang Theory stemmed largely from philosophical prejudices, and in particular to the fact that the reality of a beginning seemed to sit much better with religious views than with their own materialism.10
ATTEMPTS TO AVOID THE BIG BANG
While reluctance to accept the Big Bang Theory was largely a matter of philosophical prejudice, it was not entirely that. The theory faced, for a while, a significant problem. As mentioned, Hubble had gotten the distance scales wrong by a factor of ten. The galaxies were therefore thought to be ten times closer to each other than they really are. As a result of this mistake, the age of the universe was miscalculated by the same factor of ten: it was estimated to be not about 15 billion years old, as now believed, but closer to 2 billion years old. This was a problem, because there were also estimates of how old the earth was and how old stars were that were several times greater than this. Obviously, the universe itself cannot be younger than the objects it contains.
Partly in response to this problem, Hermann Bondi, Thomas Gold, and Fred Hoyle proposed the so-called Steady State Theory in 1948. According to their hypothesis, the universe had existed for an infinite time and had always been expanding just as it is now. In fact, their theory said that it had always looked essentially the same as it does now, which is why it was called the Steady State Theory. This sounds like a contradiction, for if the universe is expanding, the matter in it should get ever more thinned out as the galaxies get farther apart from each other. What Bondi, Gold, and Hoyle proposed was that matter was continuously being created to make up for the thinning out due to the cosmic expansion. The universe, therefore, while always expanding, could always remain of the same density.
The Steady State Theory sounds rather bizarre in retrospect. Nevertheless, it was taken seriously by many scientists for reasons that were as much philosophical as narrowly scientific. Aside from solving the problem that the age of the universe appeared too small in the Big Bang Theory, the Steady State Theory was consistent with a strong form of something called, rather grandly, the Cosmological Principle. The Cosmological Principle said that every part of the universe was pretty much the same as every other part of the universe. This was an extrapolation from the trend of discoveries since Copernicus. Earth is not at the center of the universe, as Ptolemy thought, but is in a place that is completely undistinguished. The Sun is an ordinary star, in an ordinary galaxy. If we went to any other part of the universe, we would find similar galaxies, similar stars, and similar planets.
In its weaker version, the Cosmological Principle only held that at any given time every part of the universe is similar to every other part. This is also the assumption made in the standard Big Bang cosmological model. In the jargon of the field, the universe is assumed to be “homogeneous and isotropic.” The stronger version of the Cosmological Principle, however, said not only that there is nothing special about where we live, but that there is nothing special about when we live. The uniformity of the universe extends through time as well as space. In other words, the idea is that things have always been essentially the same as they are now, and always will be. Of course, Earth itself had a beginning and will have an end. But there have always been planets like Earth and stars like the Sun, and there always will be.
This Strong Cosmological Principle was not something that came out of scientific observation or scientific theory. At root it is just a philosophical prejudice—in fact, it is really nothing but the prejudice in favor of an eternal universe. This is not to say that those scientists who found it appealing were being bigoted or unscientific. When the evidence against the Steady State Theory got too strong that theory was abandoned, more quickly by some people, but eventually even by its founders. However, while it lasted, the theory made some very strong demands on the credulity of its adherents. The continuous creation of matter that it entailed was a very radical proposal, because it meant abandoning the principle of conservation of matter and energy, which to physicists is something almost sacred. Indeed, it is somewhat ironic that a principle that had once seemed to imply an eternal universe now had to be abandoned, in the face of all evidence, to maintain belief in an eternal universe. Some of the other proposals to avoid a Big Bang, such as the “tired light theory,” involved equally radical modifications to the laws of physics.
In 1948, two students, Ralph Alpher and Robert Herman, working with George Gamow, started thinking about what the universe must have been like in its earliest moments if there had been a Big Bang. They realized that it must have been intensely, inconceivably hot. The volume of the universe was much smaller right after the Big Bang than it is today, and therefore matter was in a state of extreme compression, leading to tremendous heat. One second after the Big Bang, for example, the density of matter was several thousand times the density of lead, and the temperature was about 10 billion degrees centigrade. The early universe was a raging inferno.
The heat of this “primeval fireball” meant that the universe was filled with intense radiation. Indeed, in technical jargon, the universe was “radiation dominated” for the first several million years of its existence. Much of this radiation was in the form of light. There is some historical irony here. Once, it was a common argument against the literal interpretations of Genesis that light was created on the “first Day,” while the sun and stars were not made until later, on the “fourth Day.” It now appears that the biblical chronology was quite right in this respect. Light indeed existed virtually from the beginning,11 while stars took many millions of years to appear.
As the universe expanded, this primordial radiation cooled and became more faint. The afterglow of the Big Bang was “red-shifted” to longer and longer wavelengths. Now, 15 billion years after the Big Bang, it is no longer in the form of “visible light” but in the form of microwaves (the kind of light that heats your food in microwave ovens). The universe is filled with this ghostly remnant of the flash with which it began. Shortly after World War II, Gamow, Alpher, and Herman predicted that such a “cosmic background radiation,” as it is now called, should exist, but no one paid much attention until the mid-1960s, when a group at Princeton led by Robert Dicke had the idea of trying to detect it. However, as it turned out, someone else beat them to the punch.
In 1965, radio astronomers Arno Penzias and Robert Wilson, working at Bell Labs in New Jersey, made a series of measurements with a detector that had been specially designed for satellite communications. They found a noise, or static, that seemed to come equally from all directions in the sky. At first they thought that it was a problem with the device itself, or some local interference—they even considered the possibility that heat given off by bird droppings inside the antenna was responsible. Eventually, however, because of the work going on in nearby Princeton, the true significance of what they were seeing was realized. They were hearing a whisper from the Big Bang. Since that time, several pieces of strong confirming evidence have been found, and the fact of the “hot Big Bang” is no longer much disputed.12