Boltzmann Equation

With one neutrino flavor still being relativistic today, the resonance of the scattering between incoming DSNB neutrino and cosmic neutrino background can be significantly widened. The distortion of the DSNB spectrum can thus be used to probe a much wider parameter space of neutrino self-interaction. We show that a mediator at eV scale can be probed with the coupling to neutrino as low as $10^{-8}$.

We investigate the cosmological imprints of neutrino-DM-mediator interaction in a model-independent way, including the extra contribution of $N_{\rm eff}$ and the distortion of C$\nu$B. Lyman-$\alpha$ constraint is also discussed. To do this, we implemented a universal approach to solve the time evolution of dark matter in both freeze-in and freeze-out case.