New angle on cosmic rays

Cosmic rays are nuclei that have been accelerated to relativistic velocities by astrophysical sources, arriving at Earth after traversing the space between us and the source. As electrically charged particles, they are deflected by magnetic fields, which scramble their directions in space (1). Finding deviations from the highly isotropic angular distribution of high-energy cosmic rays in the sky has long been a prime goal of cosmic-ray researchers. Marginal detections have been reported in the past that failed to hold up. On page 1266 of this issue, The Pierre Auger Collaboration (2) report a strong detection of a pronounced anisotropy in the arrival directions of cosmic rays with energies (E) of ≥8 EeV (8 × 10¹⁸ electron volts), indicating that they are of extragalactic origin.