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Abstract

The neutrino fast flavor instability (FFI) can change neutrino flavor on time
scales of nanoseconds and length scales of centimeters. It is expected to be
ubiquitous in core-collapse supernovae and neutron star mergers, potentially
modifying the neutrino signal we see, how matter is ejected from these
explosions, and the types of heavy elements that form in the ejecta and enrich
the universe. There has been a great deal of recent interest in understanding
the role the FFI plays in supernovae and mergers, but the short length and time
scales and the strong nonlinearity have prevented the FFI from being included
consistently in these models. We review the theoretical nature of the FFI
starting with the quantum kinetic equations, where the instability exists in
neutron star mergers and supernovae, and how the instability behaves after
saturation in simplified simulations. We review the proposed methods to test
for instability in moment-based calculations where the full distribution is not
available and describe the numerical methods used to simulate the instability
directly. Finally, we close by outlining the trajectory toward realistic,
self-consistent models that will allow a more complete understanding of the
impact of the FFI in supernovae and mergers.