Cosmic backreaction and Gauss’s law

Cosmic backreaction refers to the general question of whether a homogeneous and isotropic cosmological model is able to predict the correct expansion dynamics of our Universe. In this talk I will focus on a particular aspect of backreaction, related to the validity of the continuous approximation: does a system of point masses expand the same way as a fluid does? I’ll show that it essentially does in Newtonian cosmology, and that Gauss's law is a key ingredient for this result. However, things are expected to me much different for modified theories of gravitation.