Thoughts on the Cosmological Principle Dominik

Wolfgang Kummer was a great teacher and mentor to me. Although Wolfgang never published any research on cosmology himself, he had a great interest in that field and supported me in my decision to get engaged with cosmological issues. Thus, I decided to describe current ideas and some of my own thoughts on one of the corner stones of modern cosmology — the cosmological principle. This principle says that the Universe is spatially homogeneous and isotropic. It predicts, among other phenomena, the cosmic redshift of light, the Hubble law and the black body shape of the cosmic background radiation spectrum. Nevertheless, the existence of structure in the Universe violates the (exact) cosmological principle. A more precise formulation of the cosmological principle must allow for the formation of structure and must therefore incorporate probability distributions. Below, I discuss how to formulate a new version of the cosmological principle, how to test it, and how to possibly justify it by fundamental physics. But let me, before doing so, describe in short some of my memories of Wolfgang.