Frontiers of the Astronomy. Fred Hoyle

Frontiers of Astronomy. Fred Hoyle.

The Expanding Universe

Elements other than hydrogen are built up by nuclear reactions occurring inside evolving stars. Such elements are constantly being blown out into the interstellar gas by the exploding stars. Heavy element content of a star can be interpreted as a measure of its age. Old stars are very poor, while young stars are comparatively rich (only to the tune of a percent or so in metals though).

Gas and dust become concentrated in spiral arms of the Galaxies. The metal content of the disk population is about a third of the metal content of spiral arm population.

Disk stars proportion of double stars is less than the proportion among the spiral arm population. The disk population does not contain blue giants or any supergiants. These extremely luminous stars are confined to the regions containing gas and dust, to the spiral arms.

The Continuous Origin of Matter

Olbers' paradox is resolved in a far subtler way it would be natural to discount entirely the idea of a singular origin were it not that the expansion of the Universe apparently gives support to it from an unexpected direction.

Expansion takes the clusters of galaxies apart from each other. Space is therefore (it seems) getting more and more empty as time goes on. Space must accordingly (it seems) have been more densely occupied in the past than it is today. Indeed if the Universe has always been expanding as it is at present, space must (it seems) have been jammed tight with matter not so very long ago. Let us formulate the argument a little more precisely. Suppose we take the distance of a particular cluster of galaxies and divide by the rate at which its distance is increasing. Because of the linear property the result is essentially the same whatever cluster we elect to use for this purpose. The result is a period of time known as the Hubble constant, constant because it is the same for all clusters. Perhaps the best value consistent with all present-day knowledge is about 7,000 million years.

Then if the clusters of galaxies have always had their present rates of recession the manner of derivation of Hubble's constant requires all the clusters of galaxies to have been jammed on top of each other at that time, giving a density of matter in space that rises inordinately high, indefinitely high, infinitely high. This state of affairs represents, according to the view of some astronomers, the singular start of the Universe. On this view such important features of the Universe as its expansion and its large scale uniformity of composition were impressed on the Universe at the start by the manner of creation. Creation could have occurred quite differently, matter might have been distributed lopsidedly without large scale uniformity, but it isn't because it wasn't created that way. Indeed the Universe might have been created in any of an infinity of other ways but it wasn't. It was created to have just the properties of expansion and of uniformity that we observe. If we ask why so, no answer can be given. At the time of creation the density of material was very high, much higher than the density of water.

As expansion proceeded the density became steadily less: it decreased to the density of water; then steadily down and down until it reached a millionth of the density of water; then steadily further down to a million millionth of the density of water; down and ever down to a million million millionth of the density of water; further down and still further down to a million million million millionth of the density of water; and so down to about a thousand million million million millionth of the density of water, when at long last something happened the clusters of galaxies were formed, presumably as a result of some such process as was described in the previous chapter. Once the clusters of galaxies had condensed, the expansion continued by way of increasing the distances between the clusters, this being the stage of the proceedings that we now observe. Let us see whether this argument is really an inescapable one. What are the alternative possibilities? One alternative is to deny that the Universe has always been expanding. This can be done in a consistent way by postulating that the real nature of gravitation differs from classical Newtonian ideas.

Instead of requiring attraction always to occur between two particles as in the Newtonian theory it can be argued that attraction occurs only if the distance between two particles is not too great, otherwise attraction is replaced by repulsion. And if instead of considering just two particles we consider a whole cloud of matter the modified situation is that gravitation produces a condensation of the cloud only if its density is sufficiently high, otherwise a general repulsion and dispersal occurs. The densities at which repulsive gravitation thus becomes operative are so low that there is no question of the ordinary attractive form of gravitation being appreciably modified in our solar system or in the Galaxy or in other galaxies. Similar arguments can be applied to the Universe at large. If the average density in the Universe is less than a certain critical value (fixed by hypothesis) then the Universe will start to expand even if it is not expanding to begin with.

If on the other hand the average density in the Universe is just equal to the critical value, the Universe remains static if it is initially static. But this state of balance is unstable give the Universe a slight expansion and it continues to expand with ever increasing speed, give it a slight contraction and it contracts with ever increasing speed.

The object of thus altering the law of gravitation is to explain the observed expansion of the Universe without any need for an initially explosive state. On this view when we go back into the past the density of matter does not pile up indefinitely because the expansion was then slower than it is now; and sufficiently back in the past there was no expansion at all because the Universe started from the balanced state just described.

A special feature of this theory is that it provides a better way of forming galaxies. It can be shown that clusters of galaxies could condense in the balanced initial state. Although on the very large scale it must be supposed that some unknown cause disturbed the Universe in such a way as to set it off expanding instead of contracting, in local regions the reverse situation might have occurred local regions might have started contracting instead of expanding, thereby forming clusters of galaxies. In the explosion theory the formation of clusters of galaxies has to be introduced as an ad hoc process that takes place for no good reason at just the stage where the density of matter falls to a thousand million million million millionth part of the density of water (or perhaps somewhat less than this). But in a theory with gravitation modified along the lines indicated above the origin of the clusters of galaxies is afforded a more natural explanation. A state of balance implies the possibility of the balance being tipped in localised regions towards contraction.

To end the present chapter a second flaw in the argument for a superdense singular explosive origin of the Universe will be discussed. Without any modification of gravitation of the sort contemplated above it is still incorrect to argue that expansion necessarily implies a superdense singular explosive origin of the Universe. This inference is not valid unless all the matter now in existence was also in existence in the past. It is therefore important to examine the idea that many of the atoms now in existence were not in existence in the past, and that many of the atoms of the Universe that will be in existence in the future are not in existence today. This idea requires atoms to appear in the Universe continually instead of being created explosively at some definite time in the past. There is an important contrast here. An explosive creation of the Universe is not subject to analysis. It is something that must be impressed by way of an arbitrary fiat. In the case of a continuous origin of matter on the other hand the creation must obey a definite law, a law that has just the same sort of logical status as the laws of gravitation, of nuclear physics, of electricity and magnetism. This distinction is very important and is worth a rather more detailed exposition.