The Compleat McAndrew

by Charles Sheffield

  One of the things that may be there is life. In a carefully argued but controversial theory developed over the past thirty years, Hoyle and Wickramasinghe have advanced the idea that space is the natural place for the creation of “pre-biotic” molecules in large quantities. Pre-biotic molecules are compounds such as carbohydrates, amino acids, and chlorophyll, which form the necessary building blocks for the development of life. Simpler organic molecules, such as methyl cyanide and ethanol, have already been observed in interstellar clouds.

  Hoyle and Wickramasinghe go further. They state explicitly: “We shall argue that primitive living organisms evolve in the mixture of organic molecules, ices and silicate smoke which make up a comet’s head.”

  The science fiction of the fourth chronicle consists of these two assumptions:

  The complex organic molecules described by Hoyle and Wickramasinghe are located in a particular region of the Halo, a “life ring” that lies between 3,200 and 4,000 AU from the Sun;

  The “primitive living organism” have evolved quite a bit further than Hoyle and Wickramasinghe expected, on at least one body of the Oort cloud.

  Missing matter and the beginning of the Universe.

  Today’s so-called “standard model” of cosmology suggests that the Universe began in a “Big Bang” somewhere between ten and twenty billion years ago. Since we have been able to study the Universe in detail for less than four hundred years (the telescope was invented about 1608), any attempt to say something about the origin of the Universe implies considerable extrapolation into the past. There is a chance of success only because the basic physical laws of the Universe that govern events on both the smallest scale (atoms and subatomic particles) and the largest scale (stars, galaxies, and clusters of galaxies) appear not to have changed since its earliest days.

  The primary evidence for a finite age for the whole Universe comes from observation of distant galaxies. When we observe the light that they emit, we find, as was suggested by Carl Wirtz in 1924 and confirmed by Edwin Hubble in 1929, that more distant galaxies appear redder than nearer ones.

  To be more specific, in the fainter (and therefore presumably more distant) galaxies, every wavelength of light emitted has been shifted toward a longer wavelength. The question is, what could cause such a shift?

  The most plausible mechanism, to a physicist, is called the Doppler effect. According to the Doppler effect, light from a receding object will be shifted to longer (redder) wavelengths; light from an approaching object will be shifted to shorter (bluer) wavelengths. Exactly the same thing works for sound, which is why a speeding police car’s siren seems to drop in pitch as it passes by.

  If we accept the Doppler effect as the cause of the reddened appearance of the galaxies, we are led (as was Hubble) to an immediate conclusion: the whole Universe must be expanding, at a close to constant rate, because the red shift of the galaxies corresponds to their brightness, and therefore to their distance.

  Note that this does not mean that the Universe is expanding into some other space. There is no other space. It is the whole Universe—everything there is—that has grown over time to its present dimension.

  And from this we can draw another immediate conclusion. If expansion proceeded in the past as it does today, there must have been a time when everything in the whole Universe was drawn together to a single point. It is logical to call the time that has elapsed since everything was in that infinitely dense singularity the age of the Universe. The Hubble galactic redshift allows us to calculate how long ago that happened.

  Our estimate is bounded on the one hand by the constancy of the laws of physics (how far back can we go, before the Universe would be totally unrecognizable and far from the place where we believe today’s physical laws are valid?); and on the other hand by our knowledge of the distance of the galaxies, as determined by other methods.

  Curiously, it is the second problem that forms the major constraint. When we say that the Universe is between ten and twenty billion years old, that uncertainty of a factor of two betrays our ignorance of galactic distances.

  It is remarkable that observation of the faint agglomerations of stars known as galaxies leads us, very directly and cleanly, to the conclusion that we live in a Universe of finite and determinable age. A century ago, no one could have offered even an approximate age for the Universe. For an upper bound, most nonreligious scientists would probably have said “forever.” For a lower bound, all they had was the age of the Earth.

  Asking 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?

  That question was tackled by a Belgian, Georges Lemaître. Early in the 1930s Lemaître went backwards mentally 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 Fred Hoyle, whom we met in the previous chronicle.

  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. 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 them 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 1940s, 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. 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.

  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 Bay-ter, like 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.

  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. The background radiation, corresponding to a temperature of 2.7 degrees above absolute zero, was discovered by Arno Penzias and Robert Wilson in 1964, and made the Big Bang theory fully respectable for the first time.

  We now believe that hydrogen fused to form helium when the Universe was between three and four minutes old. What about even earlier times? Let us run the clock backwards, as far as we can towards the Big Bang.

  How far back do we want to start the clock? Well, when the Uni
verse was smaller in size, it was also hotter. 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 can 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, and the Universe was almost opaque to radiation. 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 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 combine to form atoms; but they could not remain in combination, because radiation broke them apart. The content 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 chargeless particle, until recently assumed to be massless but now thought to possess a tiny mass), 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 fourteen 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 thirty 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 thirty 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 go on remaking neutrons.

  We 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 match box). 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 move on, to the time, 10-36 seconds A.C., when the Universe went through a super-rapid “inflationary” phase, growing from the size of a proton to the size of a basketball in about 5 × 10-32 seconds. We are almost back as far as we can go. Finally we reach a time 10-43 seconds A.C., (called the Plank 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.

  This may already sound like pure science fiction. It is not. It is today’s science—though it certainly may be wrong. But at last we have reached the time when McAndrew’s “hidden matter” was created. And today’s Universe seems to require that something very like it exist.

  The argument for hidden matter goes as follows: 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?

  The thing that decides which of these three possibilities will occur is the total amount of mass in the Universe, or rather, since we do not care what form mass takes and mass and energy are totally equivalent, the future of the Universe is decided by the total mass-energy content per unit volume.

  If the mass-energy is too big, the Universe will end in the Big Crunch. If it is too small, the Universe will fly apart forever. And only in the Goldilocks situation, where the mass-energy is “just right,” will the Universe ultimately reach a “flat” condition. The amount of matter needed to stop the expansion is not large, by terrestrial standards. It calls for only three hydrogen atoms per cubic meter.

  Is there that much available?

  If we estimate the mass and energy from visible material in stars and galaxies, we find a value nowhere near the “critical density” needed to make the Universe finally flat. If we say that the critical mass-energy density has to be equal to unity just to slow the expansion, we observe in visible matter only a value of about 0.01.

  There is evidence, though, from the rotation of galaxies, that there is a lot more “dark matter” present there than we see as stars. It is not clear what this dark matter is—black holes, very dim stars, clouds of neutrinos—but when we are examining the future of the Universe, we don’t care. All we worry about is the amount. And that amount, from galactic dynamics, could be at least ten times as much as the visible matter. Enough to bring the density to 0.1, or possible even 0.2. But no more than that.

  One might say, all right, that’s it. There is not enough matter in the Universe to stop the expansion, by a factor of about ten, so we have confirmed that we live in a forever-expanding Universe. Recent (1999) observations seem to confirm that result.

  Unfortunately, that is not the answer that most cosmologists would really like to hear. The problem comes because the most acceptable cosmological models tell us that if the density is as much as 0.1 today, then in the past it must have been much closer to unity. For example, at one second A.C., the density would have had to be within one part in a million billion of unity, in order for it to be 0.1 today. It would be an amazing coincidence if, by accident, the actual density were so close to the critical density.

  Most cosmologists therefore say that, today’s observations notwithstanding, the density of the Universe is really exactly equal to the critical value. In this case, the Universe will expand forever, but more and more slowly.

  The problem, of course, is then to account for the matter that we don’t observe. Where could the “missing matter” be, that makes up the other nine-tenths of the universe?

  There are several candidates. One suggestion is that the Universe is filled with energetic (“hot”) neutrinos, each with a small but non-zero mass. However, there are problems with the Hot Neutrino theory. If they are the source of the mass that stop
s the expansion of the Universe, the galaxies, according to today’s models, should not have developed as early as they did in the history of the Universe.

  What about other candidates? Well, the class of theories already alluded to and known as supersymmetry theories require that as-yet undiscovered particles ought to exist.

  There are axions, which are particles that help to preserve certain symmetries (charge, parity, and time-reversal) in elementary particle physics; and there are photinos, gravitinos, and others, based on theoretical supersymmetries between particles and radiation. These candidates are slow moving (and so considered “cold”) but some of them have substantial mass. They too would have been around soon after the Big Bang. These slow-moving particles clump more easily together, so the formation of galaxies could take place earlier than with the hot neutrinos. We seem to have a better candidate for the missing matter—except that no one has yet observed the necessary particles. At least neutrinos are known to exist!

  Supersymmetry, in a particular form known as superstring theory, offers another possible source of hidden mass. This one is easily the most speculative. Back at a time, 10-43 seconds A.C., when gravity decoupled from everything else, a second class of matter may have been created that is able to interact with normal matter and radiation, today, only through the gravitational force. We can never observe such matter, in the usual sense, because our observational methods, from ordinary telescopes to radio telescopes to gamma ray detectors, all rely on electromagnetic interaction with matter.