Evaluating backreaction with the peak model of structure formation

We evaluate the average expansion rate of a universe which contains a realistic evolving ensemble of non-linear structures. We use the peak model of structure formation to obtain the number density of structures, and take the individual structures to be spherical. The expansion rate increases relative to the Friedmann - Robertson - Walker value on a timescale of 10 - 100 billion years, because the universe becomes dominated by fast expanding voids. However, the increase is not rapid enough to correspond to acceleration. We discuss how to improve our treatment. We also consider various qualitative issues related to backreaction.