How to avoid the ambiguity in applying the Copernican principle for cosmic topology: Take the observational approach

It is often stated that homogeneity and isotropy of the Universe are assumptions of the almost Friedmann-Lemaitre (FL) model (the hot big bang model), inspired from the Copernican Principle. However, only local homogeneity and isotropy are required by the model: multiply connected almost FL models are locally homogeneous and isotropic, but they can be globally anisotropic and/or globally inhomogeneous. Toy models are used here to show how global anisotropy and/or global inhomogeneity of an almost FL model could be shown directly in observations. This approach may avoid having to make any assumptions regarding global anisotropy and inhomogeneity. (C) 2002 Published by Elsevier Science Ltd on behalf of COSPAR.