The big problems

The history recounted in the preceding four chapters represents quite extraordinary progress in understanding the astrophysical origins and evolution of our Universe. The contrast between the apparently insuperable problems of determining precise values of cosmological parameters up till the 1990s and the era of precision cosmology of the early years of the twenty-first century is startling.

Yet, despite the undoubted success of the concordance model, it raises as many problems as it solves. The picture is incomplete in the sense that, within the context of the standard world models, the initial conditions listed in Tables 15.2 and 15.3 have to be put in by hand in order to create the Universe as we observe it today. How did these initial conditions arise? As the quality of the observations improved, a number of fundamental issues for astrophysical cosmology became apparent. The resolution of these problems will undoubtedly provide insight into the laws of physics under physical conditions which at the moment can only be studied by cosmological observations.

The horizon problem

This problem, clearly recognised by Robert Dicke in 1961, can be restated, ‘Why is the Universe so isotropic?’ (Dicke, 1961). At earlier cosmological epochs, the particle horizon r ~ ct encompassed less and less mass and so the scale over which particles could be causally connected became smaller and smaller.