4.1 Introduction
The standard cosmological analyses implicitly make several assumptions, none of which are seriously
challenged by current data. Nevertheless, Euclid offers the possibility of testing some of these basic
hypotheses. Examples of the standard assumptions are that photon number is conserved, that the
Copernican principle holds (i.e., we are not at a special place in the universe) and that the universe is
homogeneous and isotropic, at least on large enough scales. These are the pillars on which standard
cosmology is built, so it is important to take the opportunity offered by Euclid observations to test these
basic hypotheses.