  Observations are inconsistent with a homogeneous and isotropic universe with
ordinary matter and gravity. The universe is far from exact homogeneity and
isotropy at late times, and the effect of the non-linear structures has to be
quantified before concluding that new physics is needed. We explain how
structure formation can lead to accelerated expansion, and discuss a
semi-realistic model where the timescale of the change in the expansion rate
emerges from the physics of structure formation.
