  We point out that, due to the nonlinearity of the Einstein equations, a
homogeneous approximation in cosmology leads to the appearance of an additional
term in the Friedmann equation. This new term is associated with the spatial
inhomogeneities of the metric and can be expressed in terms of density
fluctuations. Although it is not constant, it decays much slower (as t^(-2/3))
than the other terms (like density) which decrease as t^(-2). The presence of
the new term leads to a correction in the scale factor that is proportional to
t^2 and may give account of the recently observed accelerating expansion of the
universe without introducing a cosmological constant.
