  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 inhomogeneous Universe. One aspect of this issue concerns the
validity of the continuous approximation: does a system of point masses expand
the same way as a fluid does? This article shows that it is not exactly the
case in Newtonian gravity, although the associated corrections vanish in an
infinite Universe. It turns out that Gauss's law is a key ingredient for such
corrections to vanish. Backreaction, therefore, generically arises in
alternative theories of gravitation, which threatens the trustworthiness of
their cosmological tests. This phenomenon is illustrated with a toy model of
massive gravity.
