The homogeneity of our universe is among the founding principles of cosmology—yet are we really sure that it is so? Until now, the main argument supporting the idea that it is homogeneous was based on a coherence test that implicitly used the homogeneity of the universe for its demonstration.
Astrophysicists wanted to test this hypothesis more rigorously by identifying quasars—sources chosen for their intense luminosity and for the fact that this luminosity can be detected at great distances. They were able to record the positions and "redshifts" (related to their distances) of the quasars observed by the SDSS III international collaboration up to distances of the order of 20 billion light-years. Their conclusion was that quasars are homogeneously distributed in space.
This does not necessarily imply that the total density of matter in the universe is homogeneous; it only points at a so-called "spatial isotropy" of the total density of matter, meaning that at each distance from us, the density is the same regardless of the intended direction. To go further, the physicists invoke the Copernican principle, stating that an isotropic universe, such as the one we observe today, is also an isotropic universe for an observer located one billion light-years away from us. As a consequence, they were able to conclude, mathematically, that the universe is not only isotropic—it is also homogeneous.
Contact: Jean-Marc Le Goff