Effects of primordial fluctuations on relic neutrino simulations

After decoupling, relic neutrinos traverse the evolving gravitational imhomogeneities along their trajectories. Once they turn non-relativistic, this results in a significant amplification of the anisotropies in the cosmic neutrino background (C nu B). Past studies have reconstructed the phase-space distribution of relic neutrinos from the local distribution of matter (accounting for the Milky Way halo and the surrounding large-scale structures), but have neglected the C nu B anisotropies in the initial conditions of neutrino trajectories. Using our previously developed N-1-body simulation framework, we show that including these primordial fluctuations in the initial conditions can be important, as it produces similar effects on the abundance and anisotropies of the C nu B as the inclusion of large-scale structures beyond the Milky Way halo. Interpretability of data from future C nu B observatories like PTOLEMY therefore depends on correctly modelling these effects. GitHub: our jax-accelerated simulation code can be found here.