Dynamics of metastable particle and neutrinos in the primordial era

Cosmological observations like BBN and CMB offer crucial insights into the Early Universe, enabling us to trace its evolution back to lifetimes as short as 0.01 seconds and testing different extensions of the Standard model if they decay at such times. Upcoming CMB spectrum measurements will achieve unprecedented precision, which raises a requirement of a more accurate study of the dynamics of such particles and their decay products. In this talk, I demonstrate that the non-trivial dynamics of metastables in the plasma alters the impact of the new physics particles on the neutrino population, including the effective number of neutrino degrees of freedom, modifies neutrino spectral distortions, and may induce asymmetries in neutrino and antineutrino energy distributions. These modifications have important implications for observables such as Big Bang Nucleosynthesis and the Cosmic Microwave Background. In the second part of the talk, I will briefly discuss the novel approach of studying the dynamics of neutrinos, specifically their spectral distortions due to injections of high-energy neutrinos from decays of hypothetical particles.