  We study the dynamics of nonequilibrium instabilities in anisotropically
expanding systems. The most prominent example of such a system is the 'Glasma'
in the context of relativistic heavy-ion collision experiments, where the
expansion is a consequence of approximately boost-invariant initial conditions.
Here we consider the problem of parametric resonance in scalar $N$-component
quantum field theories with boost-invariant initial conditions, which is
similar in spirit. We find that many aspects of the dynamics can be treated
analytically by introducing a generalized conformal time. Primary
instabilities, which are described by the linearized evolution equations, are
seen to lead to a secondary regime of amplifications with strongly enhanced
growth rates due to nonlinear corrections. For the secondary instabilities we
present a power-counting scheme for weak coupling, and discuss their role for
the question of isotropization and the establishment of an equation of state.
