Ultrahigh-Energy Cosmic Rays

Ultrahigh-energy cosmic rays (UHECRs) are charged particles with energies between ∼10 18 eV and ∼3 × 10 20 eV ∼ 50J. They exhibit fundamental physics at energies inaccessible to terrestrial accelerators; challenge experimental physics; and connect strongly to astronomical observations through electromagnetic, neutrino, and even gravitational wave channels. Much theoretical and observational progress has occurred in the 60 years since the discovery of UHECRs to determine their nature and identify their sources: ▪ The highest-energy UHECRs appear to be heavy nuclei with rigidity extending up to ∼10 EV. ▪ A significant (6.9σ) dipole anisotropy has been measured, but our poor understanding of Galactic magnetic fields makes it hard to interpret. ▪ The UHECR luminosity density is ∼10 44 erg Mpc −3 year −1 , which constrains explanations of their origin. ▪ The most promising acceleration mechanisms involve diffusive shock acceleration and unipolar induction. ▪ The most promising sources include intergalactic accretion shocks and relativistic jets from stellar-mass or supermassive black holes. We explore the prospects for using the highest-energy events, combined with multimessenger astronomy, to help us solve the riddle of UHECRs.