Boosted neutrinos and relativistic dark particles as messengers from reheating

Usually information from early eras such as reheating is hard to come by. In this paper we argue that, given the right circumstances, right-handed sterile neutrinos decaying left-handed active ones at relatively late times can carry by propagating freely over thermal history. For not too small mixing angles, suitable neutrino masses are around MeV–GeV. We identify typical spectra and that they provide on ratio of inflaton mass temperature. This primordial signal be strong enough it detected in IceCube. More speculatively, for a temperature satisfying TR=(1–100) MeV, m?=(1016-19) GeV may even explain observed PeV events. Also more general relativistic dark particles play role messengers, potentially only allowing events but also alleviating H0-tension.