Applicability of the linearly perturbed FRW metric and Newtonian
  cosmology
  It has been argued that the effect of cosmological structure formation on the
average expansion rate is negligible, because the linear approximation to the
metric remains applicable in the regime of non-linear density perturbations. We
discuss why the arguments based on the linear theory are not valid. We
emphasise the difference between Newtonian gravity and the weak field, small
velocity limit of general relativity in the cosmological setting.
