  The evolution of an inhomogeneous universe composed entirely of matter is
followed from an early, nearly uniform state until the time when the
inhomogeneities have begun to grow large. The particular distribution of matter
studied in this article is chosen to have a periodic variation in only one of
the directions, which is simple enough to allow the behavior of the metric to
be solved analytically, well beyond a linear approximation based on the initial
smallness of the fluctuation. This example provides an illustration of a
universe where the inhomogeneities can affect its average expansion rate; and
its simplicity allows a condition to be derived that tells when their presence
should begin to become important. Since the averages of the non-uniform parts
of the metric and the matter density grow faster than their uniform parts, the
average expansion rate accelerates with the advent of the era governed by the
inhomogeneities.
