Most comets, however, continue their lonely orbits out in the cloud, never approaching the Inner System. They do not have to be small to be invisible to us. The amount of sunlight a body receives is inversely proportional to the square of its distance from the Sun; the apparent area it presents to our telescopes is also inversely proportional to the square of its distance from Earth. For bodies in the Halo, the reflected light that we receive from them thus varies as the inverse fourth power of their distance from the Sun. A planet with the size and composition of Uranus, but half a light-year away, would be seven trillion times as faint. And we should remember that Uranus itself is faint enough that it was not discovered until 1781, when high-quality telescopes were available. So far as present-day detection powers are concerned, there could be almost anything out there in the Halo.

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:

1. 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;

2. 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.