Introduction:
The Cosmic Neutrino Background (CNB) is a solid prediction of the Standard Models of Particle Physics and Cosmology. In the early universe, neutrinos were formed as part of the thermal bath, a hot plasma filling the universe (thermal production). As the universe expanded and cooled down to MeV temperatures, the neutrinos decoupled from the thermal bath, traveling freely through space ever since. Neutrinos are very weakly interacting particles, which makes the detection of this neutrino background very challenging and so far experimentally inaccessible. However, on the other hand, this same property implies that once we succeed in measuring the cosmic neutrino background, we can not only learn something about the properties of neutrinos but also the CNB is a window to the very early universe, back to the times of the formation of light elements in Big Bang Nucleosynthesis (BBN), when the neutrinos decoupled from the thermal bath at about two minutes after the “Big Bang”. For comparison, the cousin of the CNB, the better-known Cosmic Microwave Background (CMB), consisting of background photons instead of neutrinos, had lead to major breakthroughs in modern cosmology. This window however only leads back to temperature around 1 eV, nearly 400,000 years later than the CNB window. This paper discusses how physics beyond the standard model, in particular non-thermally generated right-handed neutrinos, can modify the CNB predictions.