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 to left-handed active ones at relatively late times can
carry information from reheating by propagating freely over the thermal
history. For not too small mixing angles, suitable right-handed neutrino masses
are around ${\cal O}$(MeV-GeV). We identify the typical spectra and argue that
they provide information on the ratio of the inflaton mass to the reheating
temperature. This primordial neutrino signal can be strong enough that it can
be detected in IceCube. More speculatively, for a reheating temperature and
inflaton mass satisfying $T_R={\cal O}(1-100)\, {\rm MeV}$, and $m_\phi={\cal
O}(10^{16-19})\,$GeV they may even explain the observed PeV events. Also more
general relativistic dark particles can play the role of such messengers,
potentially not only allowing for the PeV events but also alleviating the
$H_0$-tension .
