  Neutrino evolution, of great importance in environments such as neutron star
mergers (NSMs) because of their impact on explosive nucleosynthesis, is still
poorly understood due to the high complexity and variety of possible flavor
conversion mechanisms. In this study, we focus on so-called "fast flavor
oscillations", which can occur on timescales of nanoseconds and are connected
to the existence of a crossing between the angular distributions of electron
(anti)neutrinos. Based on the neutrino radiation field drawn from a three
dimensional neutron star merger simulation, we use an extension of the
two-moment formalism of neutrino quantum kinetics, and perform a linear
stability analysis to determine the characteristics of fast flavor
instabilities across the simulation. We compare the results to local
(centimeter-scale) three-dimensional two-flavor simulations using either a
moment method or a particle-in-cell architecture. We get generally good
agreement in the instability growth rate and typical instability lengthscale,
although the imperfections of the closure used in moment methods remain to be
better understood.
