February 23 1961 - The Evolution of the Universe

Few problems are more important or more fascinating than that of the origin of the universe. Formerly it was supposed that the universe began at a definite moment and is now evolving towards its death, but different views are held by H. Bondi, T. Gold, F. Hoyle and their colleagues, who believe that the universe is in a 'steady state'; that it had no beginning, and will never come to an end.

The controversy between these rival theories reached a new crescendo in February 1961, when, at a meeting of the Royal Astronomical Society, Professor Martin Ryle, of Cambridge, presented a paper in which the evolutionary hypothesis was supported; new results were given, and reports in the Press on the following day suggested that the steady-state idea had been definitely disproved. This is not the case, and since then there have been counter-arguments; the whole question is still very open.

 

 

 

 

 

 

 

 

 

 

 

Left Sir William McCrea who died 25th April 1999 and the author

During the programme I was accompanied by Professor W. H. McCrea, of the Royal Holloway College, London University, formerly President of the Royal Astronomical Society.

At a meeting of the Royal Astronomical Society in February, Professor Martin Ryle stated that new radio observations gave strong support to an evolutionary theory of the origins of the universe as opposed to an alternative hypothesis, that of the 'steady state' theory, due mainly to F. Hoyle, H. Bondi, and T. Gold. Let us consider the basis of these theories.

The Sun is a star, and is one of perhaps 100,000 million suns making up our star-system or Galaxy. It is in no way exceptional; it is of special importance to ourselves only because we live upon one of its nine planets. The distance of the Sun is 93,000,000 miles; the distance of the nearest of the so-called 'fixed stars' is over four light-years. (A light-year is the distance covered by a light-signal in one year. Since the velocity of light is 186,000 miles per second, this amounts to rather less than 6 million million miles.)

The term 'fixed stars' is a misnomer, since the stars are in fact moving about in space at high velocities. Our Galaxy has a general rotation in which the Sun naturally shares. Since the war, radio astronomy studies have shown that the Galaxy is spiral in form - a fact which had been previously suspected, but not proved.

Far beyond the boundaries of the solar system may be seen other galaxies, essentially similar in type to that of our own Galaxy. One of the closest of them is the Great Spiral in Andromeda (Messier 31), which is faintly visible to the naked eye. Its distance is about 2,200,000 light-years, and it is a system larger than the Galaxy in which we are situated. Together with various other galaxies, it makes up the so-called Local Group.

For many years there were doubts about whether the Andro­meda Spiral and others of its kind were genuine external systems, or whether they formed part of our Galaxy. The question was cleared up in 1923 by E. P. Hubble, at Mount Wilson Observatory, who used the 100-inch Hooker reflector to study certain interesting variable stars in the Spiral. These variables, the famous Cepheids, fluctuate regularly in brightness, and it is known that their real luminosities are linked with their periods. Cepheids also exist in our own Galaxy, and so the variables in the Andromeda Spiral could be used as 'standard candles'. It was at once evident that the Spiral itself lay at a truly immense distance.

More remote galaxies naturally appear fainter, and many of them show up on the photographs merely as tiny specks of light. The 200-inch Hale reflector at Palomar has been invaluable in studying them, and has added much to our knowledge.

Apart from the members of the local group of galaxies (less than two dozen in all), all these external systems appear to be receding from us, so that the whole universe is expanding. This conclusion has been reached from spectroscopic studies. With a receding body, there is a measurable shift of the spectrum lines toward the red end; with an approaching body, the shift is toward the violet. These Doppler shifts, as they are termed, seem to be conclusive as to the recession of the galaxies. Attempts have been made to explain them in other ways, but without success.

The recessional velocities become greater with increasing dis­tance. The most remote galaxy yet measured, 3C-295 in the con­stellation of Bootes, is about 5,000,000,000 light-years away, and is receding at perhaps 90,000 miles per second, or half the velocity of light. (It is hardly necessary to add that these values are approximate only, and may be subject to considerable error, but at least they are of the right order.) *

If this state of affairs continues at still greater distances, there must come a point at which a galaxy is receding from us at the full velocity of light. In this case its light will never reach us, and we will be unable to see it; it will have passed beyond the boundary of the visible universe. Optical astronomy cannot yet penetrate nearly so far, but the relatively new science of radio astronomy has come to the aid of cosmologists. Radio waves may be received from immense distances, and it is known that some galaxies are very strong radio emitters.

One of the first men to draw up a modern-type theory of the creation of the universe was a Belgian priest, Georges Lemaitre. He supposed that originally all the material was contained in a very dense 'primeval atom', which exploded, sending its material outward in all directions. Expansion began, and continued for thousands of millions of years. Two forces were acting against each other: gravitation on the one hand, and the so-called 'comical repulsion' on the other. According to Lemaitre, cosmical repulsion became dominant, so that the expansion of the universe is still going on. Various modifications of this theory have been made, but in any case it assumes that the universe began at a definite moment in the past. The universe is evolving, and must eventually die, so that it may be compared with a clock which is running down. We can understand why the whole conception has been nicknamed the 'big bang' hypothesis.

* In 1964, it was announced that one of the extraordinary "quasi-stellar" objects, described on pages 202-8 - 3C-147 - is still more remote, at perhaps 6,000 million light-years.

Then, little over a decade ago, Hoyle, Bondi, and Gold, working at Cambridge, proposed an entirely different scheme. They sug­gested that there never was a definite moment of creation, so that the universe has always existed — and will moreover exist for ever. As old galaxies die, new matter is created, and the universe may be likened to a clock which is being continually re-wound.

Direct observational tests are out of the question, since it would be impossible to detect this newly created matter; the task would be far harder than detecting the formation of a single new grain of sand in the whole Sahara. As R. A. Lyttleton has pointed out, the rate of creation would be such that only a fraction of a gramme would have appeared over a volume the size of the Earth during the last 4,500,000,000 years, which is the best available estimate of the Earth's age. Neither is there any explanation of this continuous creation; the suggestion is simply that the material is produced out of nothingness.

We have no idea how matter could be so created - but this in itself is no drawback to the steady-state hypothesis. The galaxies exist; their matter must have come from somewhere; and if we return to the 'big bang' idea we are still at a loss to explain how the primaeval atom appeared in the first place.

The only practicable test appeared to be obtainable from studies of very distant galaxies. It must be remembered that on the evolu­tionary theory, the density of material in the universe used to be much greater than it is now, so that many thousands of millions of years ago the galaxies were closer together than is at present the case. If the steady-state theory is correct, the universe has always had much the same aspect as it has now, since galaxies which pass beyond the observable horizon will be replaced by new galaxies formed from the continually created matter. Consequently, we could decide between the rival hypotheses if we could look back­ward in time to see the universe as it used to be thousands of millions of years ago.

In effect, this is precisely what can be done. Take, for instance, the galaxy 3C-295, recently studied by Minkowski in the United States. Its distance is about 5,000,000,000 light-years; we are there­fore looking at it as it used to be 5,000,000,000 years ago, before the Earth came into existence as a separate body. We are looking through time as well as through space.

By the methods of radio astronomy it should be possible to study galaxies still more remote than this, and a positive test may be made. On the evolutionary theory, the galaxies were relatively crowded together in past ages - say 8,000,000,000 or 9,000,000,000 years ago; on the steady-state theory, they were not. If, then, galaxies at a distance of 8,000,000,000 or 9,000,000,000 light-years are seen to be closer packed than those in our own region of space, then the evolutionary idea is valid. If not, then the steady-state theory is vindicated.

At Cambridge, Ryle and his colleagues have carried out ex­haustive studies of very weak radio sources, which are regarded as associated with remote galaxies. It is maintained that some of these lie at least 8,000,000,000 light-years from us, which should be far enough for any increase in 'crowding' to become noticeable. The crux of the matter is, therefore: are these weak, distant sources more numerous than they should be on the steady-state theory ? The answer, according to Ryle, is a definite 'yes'. If so, then the steady-state theory must be either modified or else abandoned.

It is hardly necessary to add that Ryle's work has been carried out with the greatest care, and is a magnificent technical achieve­ment. Every possible source of error has been taken into account, and the whole research programme has already taken years. Yet it is certainly premature to say that the problem has been definitely solved in favour of the 'big bang' idea, and it is still essential to keep an open mind.

One point is, of course, that the new results depend not upon a full analysis, but upon samples; the magnitude of the task can be emphasized by remembering that the Palomar 200-inch reflector can show 1,000,000,000 galaxies at least, and, as we have seen, radio methods can probe still farther into space. This has already been stressed by F. Hoyle, one of the originators of the steady-state theory. It is also suggested that there are dangers in regarding the number of radio sources as a definite clue to the number of galaxies. Some galaxies are much more powerful radio emitters than others and there is at least a possibility that very old galaxies contain a higher proportion of strong radio emitters. According to some workers, this would have a profound effect upon the whole analysis. In addition, it must be said that the new evidence is indirect, so that the margin of doubt may be greater than appears at first sight.

Further research will probably enable us to decide between the two theories, but this is not the same thing as solving what is often termed 'the mystery of the creation'. We still have no idea how the material of the universe came into being, whether it did so at one particular moment or as a continuing process, and in this respect we are as yet completely in the dark.

Let us now do our best to sum up the arguments which have been carried on so energetically during the last few weeks.

There are two theories of the universe: the evolutionary or 'big bang' theory, in which all the matter was created at one moment, and the steady-state theory, in which "the universe has always existed, so that new material is being created out of nothingness all the time. On the former hypothesis, very distant galaxies will be more crowded than closer ones; on the latter, they will not. A first observational test has been made, and the steady-state theory has apparently been contradicted. Yet there are so many unknown and uncertain factors that it would be most unwise to jump to conclusions, and studies carried out in the near future may well cause the pendulum of scientific opinion to swing once more. The arguments will continue, and the last word has by no means been said.