Back-Reaction in Relativistic Cosmology
  We introduce the concept of back-reaction in relativistic cosmological
modeling. Roughly speaking, this can be thought of as the difference between
the large-scale behaviour of an inhomogeneous cosmological solution of
Einstein's equations, and a homogeneous and isotropic solution that is a
best-fit to either the average of observables or dynamics in the inhomogeneous
solution. This is sometimes paraphrased as `the effect that structure has of
the large-scale evolution of the universe'. Various different approaches have
been taken in the literature in order to try and understand back-reaction in
cosmology. We provide a brief and critical summary of some of them,
highlighting recent progress that has been made in each case.
