  Principles in the form of heuristic guidelines or generally accepted dogma
play an important role in the development of physical theories. In particular,
philosophical considerations and principles figure prominently in the work of
Albert Einstein. As mentioned in the talk by Jiri Bicak at this conference
Einstein formulated the equivalence principle, an essential step on the road to
general relativity, during his time in Prague 1911-1912. In this talk, I would
like to discuss some aspects of cosmological models. As cosmology is an area of
physics where "principles" such as the "cosmological principle" or the
"Copernican principle" play a prominent role in motivating the class of models
which form part of the current standard model, I will start by comparing the
role of the equivalence principle to that of the principles used in cosmology.
I will then briefly describe the standard model of cosmology to give a
perspective on some mathematical problems and conjectures on cosmological
models, which are discussed in the later part of this paper.
