Neutrino flavor instabilities: onset and development

Parallel talk presented at the XXI International Workshop on Neutrino Telescopes - Padova 29 September - 3 October 2025 (https://agenda.infn.it/event/44606/) Abstract: At high enough neutrino densities, the phenomenology of flavor conversion may be governed by neutrino-neutrino coherent forward scattering. In contrast to neutral current scattering on charged leptons, this process can be flavor-changing. The coherent nature of the interaction then implies that a large amount of neutrinos can undergo flavor conversion simultaneously. This collective effect is prone to runaway modes, which can change the flavor content of a neutrino ensemble on very short time scales. Capturing these instabilities is crucial in order to accurately model the large-scale dynamics of compact astrophysical objects. In this talk, I will describe the appearance and evolution of collective flavor instabilities in simple models of neutrino-dense media and provide simple recipes to predict the final flavor configuration. Funded by the European Research Council - European Union.