Dancing With Myself

by Charles Sheffield

  It’s fascinating to note that at first, Hubble’s work seemed to re-introduce the crisis of the early nineteenth century (Earth and Sun older than universe). Using the estimated galactic distances of the 1920’s led to an age for the universe of only 2 billion years, compared with the required 4.5 billion year age of the Earth and Sun. However, over the next 30 years the estimated distances of the galaxies was gradually increased. Today’s age for the universe, of ten to twenty billion years, is totally consistent with the age of the Earth, Sun and stars.

  Answering one question, How old is the universe, inevitably leads us to another: What was the universe like, ten or twenty billion years ago, when it was compressed into a very small volume?

  In particular, can we say if the 60 and more elements heavier than iron that we know today were created in the distant past?

  Surprisingly, we can deduce a good deal about the early history of the universe, and the picture is a coherent one, consistent with today’s ideas of the laws of physics. We can also, quite specifically, say something about the formation of elements during those earliest times. We will do that, before we go back to explore the fate of our fuel-depleted massive star, poised on the brink of nuclear burn-out.

  4.EARLY (BUT NOT TOO EARLY) TIMES

  After Albert Einstein had developed his general theory of relativity and gravitation, he and others used it in the second decade of this century to study various simplified theoretical models of the whole universe.

  Einstein could construct a simple enough universe, with matter spread through the whole of space. What he could not do was make it sit still. The equations insisted that the model universe either had to expand, or it had to contract.

  To make his model universe stand still, Einstein introduced in 1917 a new, and logically unnecessary, “cosmological constant” into his own general theory. With that, he could build a stable, static universe. He later described the introduction of the cosmological constant, and his refusal to accept the reality of an expanding or contracting universe, as the biggest blunder of his life.

  When Hubble’s work showed the universe to be expanding, Einstein at once recognized its implications. However, he himself did not undertake to move in the other direction, and ask about the time when the contracted universe was far more compact than it is today. That was done by a Belgian, Georges Lemaître. Early in the 1930’s Lemaître went backwards in time, to a period when the whole universe was a “primeval atom.” In this first and single atom, everything was squashed into a sphere only a few times as big as the Sun, with no space between atoms, or even between nuclei. As Lemaître saw it, this unit must then have exploded, fragmenting into the atoms and stars and galaxies and everything else in the universe that we know today. He might justifiably have called it the Big Bang, but he didn’t. That name seems to have been coined by Hoyle (the same man who did the fundamental work on nucleosynthesis) in 1950. It is entirely appropriate that Hoyle, whose whole career has been marked by colorful and imaginative thinking, should have named the central event of modern cosmology.

  Lemaître did not ask the next question, namely, where did the primeval atom come from? Since he was an ordained Catholic priest, he probably felt that the answer to that was a given. But there are other answers, which we will get to in due course.

  Lemaître also did not worry too much about the composition of his primeval atom—what was it made of? It might be thought that the easiest assumption is that everything in the universe was already there, much as it is now. But that cannot be true, because as we go back in time, the universe had to be hotter as well as more dense. Before a certain point, atoms as we know it could not exist, because they would be torn apart by the intense radiation that permeated the whole universe.

  The person who did worry about the composition of the primeval atom was George Gamow. In the 1940’s, he conjectured that the original stuff of the universe was nothing more than densely packed neutrons. Certainly, it seemed reasonable to suppose that the universe at its outset had no net charge, since it seems to have no net charge today. Also, a neutron left to itself has a fifty percent chance that it will in about thirteen minutes decay radioactively, to form an electron and a proton.9 One electron and one proton form an atom of hydrogen; and even today, the universe is predominantly atomic hydrogen. So neutrons could account for most, if not all, of today’s universe.

  An old name for the prime material out of which the universe was made is ylem. At the time of the 1952 American Presidential campaign, dedicated Republicans announced their preference for candidate General Dwight Eisenhower, nicknamed “Ike,” by wearing lapel buttons that said, “I Like Ike.” George Gamow, as colorful, interesting and opinionated a physicist as the twentieth century has produced, went around that year wearing a button that said “I Like Ylem.”

  (He also wrote popular treatments of science. My own favorite of those works is Thirty Years That Shook Physics [see Bibliography] in which Gamow comments on the rise of the quantum theory by describing the lives and personalities of the physicists who created it. He naturally, but regrettably, omits anecdotal descriptions of one of the major spirits of that scientific revolution, George Gamow.)

  If the early universe was very hot and very dense and all hydrogen, some of it ought to have fused and become helium, carbon, and other elements. The question, How much of each?, was one that Gamow and his student, Ralph Alpher, set out to answer. They calculated that about a quarter of the matter in the primeval universe should have turned to helium, a figure very consistent with the present composition of the oldest stars. They published their results on April 1,1948. In one of physics’ best-known jokes, Hans Bethe (pronounced Bayter, as Americans pronounce the Greek letter Beta) allowed his name to be added to the paper, although he had nothing to do with its writing. The authors thus became Alpher, Bethe, and Gamow.

  What Gamow and Alpher could not do, and what no one else could do after them, was make the elements heavier than helium after the Big Bang that began the universe.10 In fact, Gamow and colleagues proved that heavier element synthesis did not take place. It could not happen very early, because in the earliest moment, elements would be torn apart by energetic radiation. At later times, the universe expanded and cooled too quickly to yield the needed temperatures.

  Heavier element formation, as we have seen, had to be done by stars, during the process known as stellar nucleosynthesis. The failure of the Big Bang to produce heavier elements confirms something that we already know, namely, that the Sun (and with it the whole Solar System) is much younger than the universe. Sol, at maybe five billion years old, is a second, third, or even fourth generation star. Some of the materials that make up Sun and Earth derive from older stars that ran far enough through their evolution to produce elements up to iron, by nuclear fusion.

  Apart from showing how to calculate the ratio of hydrogen to helium after the Big Bang, Gamow and his colleagues did one other thing whose full significance probably escaped them. In 1948 they produced an equation that allowed one to compute the present background temperature of the universe from its age, assuming a universe that expanded uniformly since its beginning in the Big Bang. Whether Gamow was skeptical of his own equation or not is difficult to say, but the fact remains that he never agitated for experiments to look for such background radiation—and when he had a bee in his bonnet, Gamow was as good as anyone at stirring up action. The background radiation, corresponding to a temperature of 2.7 degrees above absolute zero, was discovered by Arno Penzias and Robert Wilson in 1964—discovered by accident, at the very time when a Princeton group a few miles away were planning to look deliberately. Penzias and Wilson head the list of “lucky” Nobel Prizewinners, since if they had not received their 1978 award for discovery of the leftover (or “relict”) cosmic background radiation, it is hard to believe they would have received the award for anything else.

  If our biography of the universe is to be complete, we
need to explore the time of its birth in more detail, turning the clock back well beyond the time when hydrogen fused to helium. That happened, we now believe, when the universe was between three and four minutes old. Before exploring such early times, and trying to make the extraordinary statement of the previous sentence plausible, let us take off the hook our massive star, ten or more solar masses, that had run out of fusion fuel at the end of Section 2 and seemed to have nowhere to go next.

  5.BIG BANGS (BUT NOT THE BIG BANG)

  We have a star, of ten or more solar masses, running out of energy. The supply provided by the fusion at its center of silicon into iron is almost done, radiating away rapidly into space. In the middle of the star is a sphere of iron “gas” (technically, a plasma) about one and a half times the mass of the sun and at a temperature of a few billion degrees. It acts like a gas because all the iron nuclei and the electrons are buzzing around freely. However, the core density is millions of times that of the densest material found on Earth. Outside that central sphere, like layers of an onion, sit shells of silicon, oxygen and carbon, helium and neon and hydrogen, and smaller quantities of all the other elements lighter than iron.

  When the source of fusion energy dries up, iron nuclei capture the free electrons in the iron gas. Protons and electrons combine. The energy that had kept the star inflated is sucked away, and the core collapses to become a ball of neutrons, only a few miles across.

  That near-instantaneous gravitational collapse unleashes a huge amount of energy, enough to blow all the outer layers of the star clear away into space. What is left behind is a “neutron star”; a solid sphere of neutrons.

  How much is a “huge” amount of energy? Well, when a star collapses and then blows up like this, in what is known as a supernova, it shines as brightly as a whole galaxy—which is to say, its brightness can temporarily increase by a factor of one hundred billion. If that number doesn’t tell you much, try it this way: if a single candle in Chicago were to “go supernova” and increase in brightness one hundred billion times, you would easily be able to read a newspaper by its light in Washington, D.C.

  The explosion of the supernova also creates pressures and temperatures big enough to generate all the elements heavier than iron that could not be formed by standard nucleosynthesis in stars. So finally, after a long, complex process of stellar evolution, we have found a place where substances as “ordinary” as tin and lead, or as “precious” as silver, gold, and platinum, can be created.

  For completeness, I want to point out that there are actually two types of supernova, and that the other can also produce heavy elements. However, the second kind cannot happen to an isolated star. It occurs only in binaries, pairs of stars, close enough together that material from one of them can be stolen gravitationally by the other.

  The star that does the stealing must be a small, dense star of the type known as a white dwarf, while its partner is usually a larger, diffuse, and swollen star known as a red giant. As more and more matter is stolen from the more massive partner, the white dwarf star shrinks in size, rather than growing. When its mass reaches 1.4 times the mass of the Sun (known as Chandrasekhar’s limit11) it collapses. The result is again a huge explosion, with a neutron star left behind as a possible remnant. The outgoing shock wave creates heavy elements, and ejects them along with the rest of the star’s outer layers away from the binary system. If the nearest stellar system to us, the triple star complex of Alpha Centauri, were to turn into a supernova (it can’t—at least according to current theory) the flux of radiation and high-energy particles would probably wipe out life on Earth.

  I think of supernovas as rather like nuclear power stations. What they produce is immensely important to us, but we prefer not to have one in our own local neighborhood. If you want a physics question which must have a definite answer, but one which today we cannot even approach, consider this: Where was the supernova explosion or explosions that produced the heavy elements on Earth?

  6.ALL THE WAY BACK

  “Oh, call back yesterday,” said Salisbury, in Shakespeare’s Richard the Second. “Bid time return.”

  Let us do that. We are going to run the clock backwards, towards the real Big Bang, as opposed to the trifling explosions known as supernovas. For although most new-born babies look much alike and are subjects of limited interest except to parents, the universe by contrast appears more and more interesting, the closer we get to its origin.

  How far back do we want to start the clock? Well, when the universe was smaller in size, it was also hotter. As we pointed out in Section 4, in a hot enough environment, atoms as we know them cannot hold together. High-energy radiation rips them apart as fast as they form. A good time to begin our backward running of the clock might then be the period when atoms could form, and persist as stable units. Although stars and galaxies would not yet exist, at least the universe would be made up of familiar components, hydrogen and helium atoms that we would recognize.

  Atoms form, and hold together, somewhere between half a million and a million years after the Big Bang. Before that time, matter and radiation interacted continuously. After it, matter and radiation “decoupled,” became near-independent, and went their separate ways. The temperature of the universe when this happened was about 3,000 degrees. Ever since then, the expansion of the universe has lengthened the wavelength of the background radiation, and thus lowered its temperature. The cosmic background radiation discovered by Penzias and Wilson, at 2.7 degrees above absolute zero, is nothing more than the radiation at the time when it decoupled from matter, now grown old.

  Continuing backwards: even before atoms could form, helium and hydrogen nuclei and free electrons could be created; but they could not remain in combination, because radiation broke them apart. The form of the universe was, in effect, controlled by radiation energetic enough to prevent the formation of atoms. This situation held from about three minutes to one million years A.C. (After Creation).

  If we go back to a period less than three minutes A.C., radiation was even more dominant. It prevented the build-up even of helium nuclei. As noted earlier, the fusion of hydrogen to helium requires hot temperatures, such as we find in the center of stars. But fusion cannot take place if it is too hot, as it was before three minutes after the Big Bang. Before helium could form, the universe had to “cool” to about a billion degrees. All that existed before then were electrons (and their positively charged forms, positrons), neutrons, protons, neutrinos (a mass-less, charge-less particle) and radiation.

  Until three minutes A.C., it might seem as though radiation controlled events. But this is not the case. As we proceed farther backwards and the temperature of the primordial fireball continues to increase, we reach a point where the temperature is so high (above ten billion degrees) that large numbers of electron-positron pairs can be created from pure radiation. That happened from one second up to 14 seconds A.C. After that, the number of electron-positron pairs decreased rapidly. Less were being generated than were annihilating themselves and returning to pure radiation. After the universe cooled to ten billion degrees, neutrinos also decoupled from other forms of matter.

  Still we have a long way to go, physically speaking, to the moment of creation. As we continue backwards, temperatures rise and rise. At a tenth of a second A.C., the temperature of the universe is 30 billion degrees. The universe is a soup of electrons, protons, neutrons, neutrinos, and radiation. As the kinetic energy of particle motion becomes greater and greater, effects caused by differences of particle mass are less important. At 30 billion degrees, an electron easily carries enough energy to convert a proton into the slightly heavier neutron. Thus in this period, free neutrons are constantly trying to decay to form protons and electrons; but energetic proton-electron collisions kept right on re-making neutrons.

  We will keep the clock running. Now the important time intervals become shorter and shorter. At one ten-thousandth of a second A.C.,
the temperature is one thousand billion degrees. The universe is so small that the density of matter, everywhere, is as great as that in the nucleus of an atom today (about 100 million tons per cubic centimeter—a fair-sized asteroid, at this density, would squeeze down to fit in a matchbox). Modern theory says that the nucleus is best regarded not as protons and neutrons, but as quarks, elementary particles from which the neutrons and protons themselves are made. Thus at this early time, 0.0001 seconds A.C., the universe was a sea of quarks, electrons, neutrinos, and energetic radiation.

  We can go farther, at least in theory, to the time, 10-35 seconds A.C., when the universe went through a super-rapid12 “inflationary” phase, growing from the size of a proton to the size of a basketball in about 5 × 10-32 seconds. We can even go back to a time 10-43 seconds A.C. (called the Planck time), when according to a class of theories known as supersymmetry theories, the force of gravity decoupled from everything else, and remains decoupled to this day.

  But at this point I want to pause, and ask, Does it make any sense to go back so far? We are already far beyond the realm in which the physical laws that we accept—and can test—today can be expected to apply. Are we, at this stage, any more plausible than Archbishop Ussher, who was presumably quite convinced that he had pinned down the time of creation?

  More to the point, does that early history of the universe make any difference to anything today?

  Oddly enough, it does. That early history of the universe was crucial in deciding the whole structure of today’s universe.

  The universe is expanding. Every cosmologist today agrees on that. Will it go on expanding forever, or will it one day slow to a halt, reverse direction, and fall back in on itself to end in a Big Crunch? Or is the universe poised on the infinitely narrow dividing line between expansion and ultimate contraction, so that it will increase more and more slowly, and finally (but after infinite time) stop its growth?