  We consider the emergence of large-scale cosmological expansion in
scalar-tensor theories of gravity. This is achieved by modelling sub-horizon
regions of space-time as weak-field expansions around Minkowski space, and then
subsequently joining many such regions together to create a statistically
homogeneous and isotropic cosmology. We find that when the scalar field can be
treated perturbatively, the cosmological behaviour that emerges is well
modelled by the Friedmann solutions of the theory. When non-perturbative
screening mechanisms occur this result no longer holds, and in the case of
scalar fields subject to the chameleon mechanism we find significant deviations
from the expected Friedmann behaviour. In particular, the screened mass no
longer contributes to the Klein-Gordon equation, suppressing deviations from
general relativistic behaviour.
