# Origin and Impacts of the First Cosmic Rays

## TL;DR

This paper explores the origin of the first cosmic rays, proposing that early cosmic explosions and structure formation shocks generated CRs that influenced the early universe's ionization, magnetic fields, and galaxy formation.

## Contribution

It introduces a model for the generation of the first cosmic rays through shocks from early cosmic explosions and structure formation, highlighting their potential roles in the early universe.

## Key findings

- Protons can be accelerated up to sub-GeV energies in early shocks.
- First CRs likely achieved energies of a few GeV.
- Early CRs contributed to ionization, heating, and magnetic field generation.

## Abstract

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.