Independently of the Fischer-Moncrief picture the study of
small (but finite) perturbations of Bianchi models is an avenue
for making progress in understanding expanding cosmological
models. There is a large literature on linear perturbations of
cosmological models and it would be desirable to determine what
insights the results of this work might suggest for the full
nonlinear dynamics. Just as it is interesting to know under what
circumstances homogeneous cosmological models become isotropic in
the course of expansion, it is interesting to know when more
general models become homogeneous. This does happen in the case
of small perturbations of the Milne model. On the other hand,
there is an apparent obstruction in other cases. This is the
Jeans instability [170,
43]. A linear analysis indicates that under certain circumstances (*e.g.*
perturbations of a flat Friedmann model) inhomogeneities grow
with time. As yet there are no results on this available for the
fully nonlinear case. A comparison that should be useful is that
with Landau damping in plasma physics, where rigorous results are
available [123].

The most popular matter model for spatially homogeneous cosmological models is the perfect fluid. Generalizing this to inhomogeneous models is problematic since formation of shocks or (in the case of dust) shell-crossing must be expected to occur. These signal an end to the interval of evolution of the cosmological model, which can be treated mathematically with known techniques. Criteria for the development of shocks (or their absence) should be developed, based on the techniques of classical hydrodynamics.

In the case of polarized Gowdy spacetimes there is a
description of the late-time asymptotics in the literature [87], although the proofs have unfortunately never been published.
The central object in the analysis of these spacetimes is a
function
*P*
that satisfies the equation
. The picture that emerges is that the leading asymptotics are
given by
for constants
*A*
and
*B*, this being the form taken by this function in a general Kasner
model, while the next order correction consists of waves whose
amplitude decays like
, where
*t*
is the usual Gowdy time coordinate. The entire spacetime can be
reconstructed from
*P*
by integration. It turns out that the generalized Kasner
exponents converge to (1, 0, 0) for inhomogeneous models. This
shows that if it is stated that these models are approximated by
Kasner models at late times it is necessary to be careful in what
sense the approximation is supposed to hold. Information on the
asymptotics is also available in the case of small but finite
perturbations of the Milne model and the Bianchi type III
form of flat spacetime, as discussed in Sections
5.3
and
5.4, respectively.

There are not too many results on future geodesic completeness for inhomogeneous cosmological models. A general criterion for geodesic completeness is given in [62]. It does not apply to cases like the Kasner solution but is well-suited to the case where the second fundamental form is eventually negative definite.

Theorems on Existence and Global Dynamics for the
Einstein Equations
Alan D. Rendall
http://www.livingreviews.org/lrr-2002-6
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