It is fair to say that our Universe is on the whole well represented by one or other of these simple homogeneous and isotropic cosmological models. They certainly provide a basis for understanding the gross features of the Universe. But what about the galaxies and stars that are the basic building blocks of the Universe, what does our simple model have to say about these ? In the preceding discussion, we tacitly assumed that the early Universe was a gas or plasma, and based many of our arguments on this premise. Can we support this assumption in view of the fact that what we see through our telescopes are mainly stars and galaxies ? Two arguments can be given which suggest that the gaseous models are a reasonable assumption at early times. According to the simple models, the density of matter at the epoch of neutralization (a red-shift of about 1000) is of the order of 107 atoms per litre. This is at least a thousand times the density of a typical galaxy, so we can argue with confidence that whatever the Universe was made of at such early times, it was not galaxies as we know” them now. The argument against stars at this time is less direct: if the Universe had been made up entirely of stars, it would be difficult to understand the origin of the cosmic background radiation. Of course, it is yet more difficult to rule out a mixture of stars and gas except by saying that we do not know how stars could have formed in the early Universe! In the face of such problems, we sidestep the issue by asking a slightly different question: how can we account for the existence of galaxies and stars within the framework of the simple model universes ?

The question of the origin of galaxies is a crucial one for any cosmological hypothesis. We would like not only to explain the fragmentation of the Universe into sub-units, but also to explain why galaxies have the mass and rotation we observe. These para¬meters should arise in a natural way in any cosmological theory. Let us concentrate on the question of the origin of galaxies in a I general sense, without worrying for the moment about explaining their properties. There are two views one can take about this: firstly, the Universe was, at all times in our past, at least as lumpy as it is now; or secondly, the Universe was, in the past, fairly smooth, and by some physical process became lumpy. The first view, represents a significant departure from the simple Big Bang theory, but no doubt a theory of this kind can be constructed and may be quite difficult to refute observationally. Perhaps the strongest argument is that such a lumpy Universe would not look like the simple model at the time of nucleosynthesis, and so is likely to give the wrong amounts of deuterium and helium. This is not a problem if we take the second view, but of course there we have the problem of finding a mechanism by which galaxies can condense out of the Universe. Two such mechanisms have been proposed. There is the GRAVITATIONAL INSTABILITY picture, where the force of gravity acts so as to enhance any slight irregularities, and there is the COSMIC TURBULENCE picture where lumps are created by hypothetical eddies crashing into one another.

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