Accelerated expansion from structure formation

We discuss the physics of backreaction-driven accelerated expansion. We explain how large-scale smoothness does not imply that the effect of inhomogeneity and anisotropy on the expansion rate is small, using the exact equations for the behaviour of averages in an inhomogeneous and anisotropic dust universe. We demonstrate with an analytical toy model how gravitational collapse can lead to acceleration. We find the conjecture that the accelerated expansion is due to structure formation to be in agreement with the general observational picture of structures in the universe, and more quantitative work is needed to make a detailed comparison with observations.