Most cosmological models studied today are based on the assumption of
homogeneity and isotropy. Observationally one can find evidence that supports
these assumptions on very large scales, the strongest being the almost isotropy
of the Cosmic Microwave Background radiation after assigning the whole dipole
to our proper motion relative to this background. However, on small and on
intermediate scales up to several hundreds of Mpcs, there are strong deviations
from homogeneity and isotropy. Here the problem arises how to relate the
observations with the homogeneous and isotropic models. The usual proposal for
solving this problem is to assume that Friedmann-Lemaitre models describe the
mean observables. Such mean values may be identified with spatial averages. For
Newtonian fluid dynamics the averaging procedure has been discussed in detail
in Buchert and Ehlers (1997), leading to an additional backreaction term in the
Friedmann equation. We use the Eulerian linear approximation and the
`Zel'dovich approximation' to estimate the effect of the backreaction term on
the expansion. Our results indicate that even for domains matching the
background density in the mean, the evolution of the scale factor strongly
deviates from the Friedmann solution, critically depending on the velocity
field inside.Comment: 4 pages LaTeX, 2 figures, a4wide.sty include
