  Density perturbations and their dynamic evolution from early to late times
can be used for an improved understanding of interesting physical phenomena
both in cosmology and in the context of heavy-ion collisions. We discuss the
spectrum and bispectrum of these perturbations around a longitudinally
expanding fireball after a heavy-ion collision. The time-evolution equations
couple the spectrum and bispectrum to each other, as well as to higher-order
correlation functions through nonlinear terms. A non-trivial bispectrum is thus
always generated, even if absent initially. For initial conditions
corresponding to a model of independent sources, we discuss the linear and
nonlinear evolution is detail. We show that, if the initial conditions are
sufficiently smooth for fluid dynamics to be applicable, the nonlinear effects
are relatively small.
