How can this elementary yet profound deduction be checked ? It is at this point that the theoretical cosmologist enters the picture. His task is to exploit the assumption that the force of gravity dominates the evolution of the Universe. He uses Einstein’s theory of general relativity to map out the history of the competition between expansion and gravitation. In doing so, he constructs what are known as COSMOLOGICAL MODELS. It turns out that there is a large number of evolutionary histories for the Universe which are consistent with both the known laws of physics, and the observed expansion and density of the Universe. They have one feature in common: they all possess a singularity, a finite time in our past. The existence of such a singularity – a Big Bang – is an inescapable consequence of Einstein’s theory. There is no way of avoiding the Big Bang which is suggested so strongly by a naive extrapolation of the observed cosmic expansion into our pact, without changing Einstein’s theory of general relativity, or abandoning some fundamental tenet of modem physics. We have already mentioned the example of the Steady State theory which avoided the problem by invoking some new physics.

The cosmologist must decide which of these cosmological models best describes the Universe. In spite of the great number of candidates, some are obviously more suitable than others. The cosmologist favours those which have a hot beginning because in these there is a ready explanation of the origin of the radiation field observed at centimeter wavelengths to pervade the Universe. This radiation becomes the relic of the earlier hot phase, cooled down by the cosmic expansion. Cosmologists favour those models which at the present time look the same in all directions. The observed cosmic background radiation is isotropic to better than one part in a thousand, so this eliminates all but a relatively small number of the models. The question still remains however: which model best describes our Universe ?

Among the cosmological models remaining at this stage of the argument, some have magnetic fields, some have matter and anti¬matter initially, and others have turbulence. To progress with the question of which cosmology, the cosmologist now has to change his tactics. He considers the very simplest among these models and sees how far he can get with these towards accounting for the properties of the observed Universe. If he reaches an impasse, he is prepared to modify the simple assumptions underlying the model and try again. It is perhaps remarkable that the very simplest models are, with only minor modifications, capable of providing a satisfactory picture of the Universe. The modifications referred to arise from a wish to understand the complex question of the origin of galaxies.

The simplest models are those that are homogeneous and isotropic at all times, and which have the desirable feature of being hot at the beginning. As such a model universe expands, its density and temperature decrease in a way that can be calculated from the laws of gravity and thermodynamics. The density histories of two such models are depicted . The models differ only in that one has a present density of one hydrogen atom per 10000 liters, whereas the other has a density of one hydrogen atom per 100 liters. The latter model will, at sometime in the future, stop expanding and recollapse: the cosmologist refers to this as a CLOSED UNIVERSE. The other model expands forever, and is called an OPEN UNIVERSE.

Before following the life histories of these universes, we should ask: how far into the past of the Universe can we confidently apply the terrestrially-established laws of physics? We should certainly start to lose confidence when the densities of matter greatly exceed nuclear densities, and the temperatures exceed hundreds of billions of degrees. We have no direct experience of such conditions and our knowledge of the structure of matter is hardly adequate to allow any more than an informed guess. Nonetheless, this ignorance relegates only the first fraction of a second of the Universe to the realm of scientific speculation.

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