  Nonthermal phenomena are ubiquitous in the Universe, and cosmic rays (CRs)
play various roles in different environments. When, where, and how CRs are
first generated since the Big Bang? We argue that blast waves from the first
cosmic explosions at z~20 lead to Weibel mediated nonrelativistic shocks and
CRs can be generated by the diffusive shock acceleration mechanism. We show
that protons are accelerated at least up to sub-GeV energies, and the fast
velocity component of supernova ejecta is likely to allow CRs to achieve a few
GeV in energy. We discuss other possible accelerators of the first CRs,
including accretion shocks due to the cosmological structure formation. These
CRs can play various roles in the early universe, such as the ionization and
heating of gas, the generation of magnetic fields, and feedbacks on the galaxy
formation.
