Fast Flavor Transformations

The neutrino fast flavor instability (FFI) can change neutrino flavor on timescales of nanoseconds and length scales of centimeters. It is expected to beubiquitous in core-collapse supernovae and neutron star mergers, potentiallymodifying the neutrino signal we see, how matter is ejected from theseexplosions, and the types of heavy elements that form in the ejecta and enrichthe universe. There has been a great deal of recent interest in understandingthe role the FFI plays in supernovae and mergers, but the short length and timescales and the strong nonlinearity have prevented the FFI from being includedconsistently in these models. We review the theoretical nature of the FFIstarting with the quantum kinetic equations, where the instability exists inneutron star mergers and supernovae, and how the instability behaves aftersaturation in simplified simulations. We review the proposed methods to testfor instability in moment-based calculations where the full distribution is notavailable and describe the numerical methods used to simulate the instabilitydirectly. Finally, we close by outlining the trajectory toward realistic,self-consistent models that will allow a more complete understanding of theimpact of the FFI in supernovae and mergers.