COMPLEMENTARY APPROACHES TO COSMOLOGY - RELATING THEORY AND OBSERVATIONS

The usual way of proceeding to determine the structure and evolution o f our observable Universe is to assume that on the largest scales it i s Friedman-Lemaitre-Robertson-Walker (FLRW), i.e. isotropic and spatia lly homogeneous, and then to determine the parameters specifying the F LRW model from observations. However, in the light of observational ev idence of deviation from a FLRW geometry on almost all scales, cosmolo gy must gradually clarify, and if necessary modify, the geometric assu mptions upon which this procedure rests, along with other presuppositi ons, such as the fluid-character of the mass-energy content. The basic requirement is to balance the two major approaches to cosmology: the FLRW model-building approach, starting from broad explanatory premises , and a descriptive approach, starting on a strictly observational bas is. Each gives us different insights and information about the nature of the Universe. Lacking a model that integrates both satisfactorily, we need to develop each approach as far as it will go, repeatedly comp aring the two and seeing what each can teach the other. As a particula r example, inter alia observational cosmology eventually needs to spec ify the length-scale at which the Universe becomes FLRW or 'almost FLR W'. This can only be done by pursuing both approaches. Here we review briefly the successes of the standard approach and also its limitation s, and indicate how we should go beyond it to develop a fully satisfac tory understanding. We describe briefly one particular integrated appr oach as an example of what can be done, assess its feasibility, and re late it as an essential complement to the standard FLRW programme.