  We study the dynamics of active-sterile neutrino oscillations in the early
universe using full momentum-dependent quantum-kinetic equations. These
equations are too complicated to allow for an analytical treatment, and
numerical solution is greatly complicated due to very pronounced and narrow
structures in the momentum variable introduced by resonances. Here we introduce
a novel dynamical discretization of the momentum variable which overcomes this
problem. As a result we can follow the evolution of neutrino ensemble
accurately well into the stable growing phase. Our results confirm the
existence of a "chaotic region" of mixing parameters, for which the final sign
of the asymmetry, and hence the SBBN prediction of He(4)-abundance cannot be
accurately determined.
