How to avoid the ambiguity in applying the Copernican Principle for cosmic topology: Take the observational approach Original Research Article

It is often stated that homogeneity and isotropy of the Universe are assumptions of the almost Friedmann-Lemaître (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.