Speaker
Description
Understanding the origin and evolution of cosmic rays (CRs) across cosmic time is a key problem in high-energy astrophysics and cosmology. Recent studies suggest that the earliest CRs may have been produced at collisionless shocks driven by the supernova explosions of the first stars at z ~ 20. In this work, we explore a possible scenario for CR production across cosmic epochs, connecting plasma processes in the pre-structure universe to particle acceleration at large-scale structure shocks. In weakly magnetized intergalactic plasmas prior to nonlinear structure formation, large-scale plasma motions naturally generate pressure anisotropy with respect to the background magnetic field. Such anisotropy can drive kinetic instabilities, including mirror and firehose modes, which significantly enhance magnetic-field amplification during cosmological magnetogenesis. As magnetic fields grow, the associated increase in gyrofrequency enhances pitch-angle scattering, potentially enabling stochastic (second-order Fermi) acceleration in turbulent plasma. We investigate the efficiency of this process by comparing the stochastic acceleration timescale with the cosmological expansion timescale and by solving a Fokker–Planck equation for the evolution of the proton distribution. Our results indicate that instability-enhanced scattering can reduce the acceleration timescale, but the resulting particle population remains primarily suprathermal prior to the onset of large-scale structure formation. These suprathermal particles may nevertheless provide seed populations for subsequent diffusive shock acceleration (DSA) at structure formation shocks. In particular, shocks associated with hierarchical structure formation, including strong accretion shocks in cluster outskirts and weaker internal shocks in the intracluster medium, can further accelerate particles and generate observable nonthermal emission in galaxy clusters. We discuss how plasma microphysics operating across different cosmic epochs collectively shapes the origin and evolution of cosmic rays in the Universe.
