THE PROBLEMS OF LARGE-SCALE STRUCTURE

More than seven decades have elapsed since Friedmann proposed his mathematical models that describe the expanding Universe. As we saw in Chapter 9, these models lead to the conclusion that the Universe was created some 10–15 billion years ago in a big explosion (the so-called big bang) after which it has been expanding but more and more slowly because of brakes applied by gravity. This model also tells us that the Universe was very hot to begin with, and dominated by radiation, but with expansion it has cooled down and the temperature of the radiation background today is 2.73 kelvin (see Figure 9–10) as measured by the COBE satellite and other groundbased detectors. And one other set of relics of the hot era, namely the light nuclei like deuterium, helium, etc., are found in the right amount all over the Universe. Thus, we concluded the last chapter with a fair degree of confidence in the big–bang scenario.

However, over the last quarter of a century astronomical observations have become more sophisticated and the views of the largescale structure of the Universe they present go well beyond the simplified assumptions of a ‘homogeneous and isotropic Universe’. We shall see, for example, in Figure 10–1 how galaxies are distributed over the sky in depth. The dots in the figure represent galaxies and their distribution is clearly not smooth, as a homogeneous Universe would have us believe.