Primordial neutrinos and new physics: novel approach to solving neutrino Boltzmann equation

Understanding how new physics influences the dynamics of cosmic neutrinos during their decoupling is crucial in light of upcoming precise cosmological observations and the need to reconcile cosmological and laboratory probes. Existing approaches to solving the neutrino Boltzmann equation are often model-dependent, computationally inefficient, and yield contradictory results. To solve this problem, we introduce a novel method to comprehensively study neutrino dynamics. We apply this method to several case studies, resolving the discrepancy in the literature about the impact of non-thermal neutrinos on $N_{\rm eff}$ and providing important insights about the role of decaying new physics particles on MeV plasma.