Homogeneous Cosmological Models

Abstract

Building cosmological models is a very old human activity. Clearly, it seems that any society has its own representation of both the history and the ge­ography of the world. Therefore, Cosmology is one of the oldest human concern. On the other hand, despite the fact that scientific approaches to the question has existed from time to time, the developpment of “Physical Cosmology” is very young. So young that its scientific status is sometime questionned. However, since the discovery of the cosmic microwave back­ground the field has deeply changed and has became a mature science. This does not mean that prejudices are not yet present in the field. Any science has prejudices at some level, but it is important to identify these preju­dices, which are then called hypothesis or principles and has then to be tested. In some theories a basic principle is assumed, even if does lead to violations of physical laws. For instance, the perfect cosmological principle stated that the universe was the same everywhere and at every time. It can be shown that this requires the violation of a basic principle of physics: energy conservation, although the level at which this violation is required is small enough not to have been tested by any experiment. The tired light theory is an other example of a theory in which a new physical process working essentially on the universe scale is advocated in order to satisfy a prejudice (in both case the prejudice is that one would like to avoid any­thing looking like a “beginning” of the universe, i.e. anything like the big bang).