Download Frontiers of Astronomy. Fred Hoyle. The Expanding Universe

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.