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Review

Probing the Sources of Ultra-High-Energy Cosmic Rays—Constraints from Cosmic-Ray Measurements

1
Nationaal Instituut voor Subatomaire Fysica (NIKHEF), Science Park 105, 1098 XG Amsterdam, The Netherlands
2
Institute for Mathematics, Astrophysics and Particle Physics, Radboud University, Houtlaan 4, 6525 XZ Nijmegen, The Netherlands
Universe 2025, 11(10), 331; https://doi.org/10.3390/universe11100331
Submission received: 11 September 2025 / Revised: 24 September 2025 / Accepted: 26 September 2025 / Published: 3 October 2025

Abstract

Ultra-high-energy cosmic rays (UHECRs) are the most energetic particles known—and yet their origin is still an open question. However, with the precision and accumulated statistics of the Pierre Auger Observatory and the Telescope Array, in combination with advancements in theory and modeling—e.g., of the Galactic magnetic field—it is now possible to set solid constraints on the sources of UHECRs. The spectrum and composition measurements above the ankle can be well described by a population of extragalactic, homogeneously distributed sources emitting mostly intermediate-mass nuclei. Additionally, using the observed anisotropy in the arrival directions, namely the large-scale dipole >8 EeV, as well as smaller-scale warm spots at higher energies, even more powerful constraints on the density and distribution of sources can be placed. Yet, open questions remain—like the striking similarity of the sources that is necessary to describe the rather pure mass composition above the ankle, or the origin of the highest energy events whose tracked back directions point toward voids. The current findings and possible interpretation of UHECR data will be presented in this review.
Keywords: ultra-high-energy cosmic rays; cosmic-ray sources; arrival directions; large-scale anisotropy; intermediate-scale anisotropies; cosmic magnetic fields; Galactic magnetic field ultra-high-energy cosmic rays; cosmic-ray sources; arrival directions; large-scale anisotropy; intermediate-scale anisotropies; cosmic magnetic fields; Galactic magnetic field

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MDPI and ACS Style

Bister, T. Probing the Sources of Ultra-High-Energy Cosmic Rays—Constraints from Cosmic-Ray Measurements. Universe 2025, 11, 331. https://doi.org/10.3390/universe11100331

AMA Style

Bister T. Probing the Sources of Ultra-High-Energy Cosmic Rays—Constraints from Cosmic-Ray Measurements. Universe. 2025; 11(10):331. https://doi.org/10.3390/universe11100331

Chicago/Turabian Style

Bister, Teresa. 2025. "Probing the Sources of Ultra-High-Energy Cosmic Rays—Constraints from Cosmic-Ray Measurements" Universe 11, no. 10: 331. https://doi.org/10.3390/universe11100331

APA Style

Bister, T. (2025). Probing the Sources of Ultra-High-Energy Cosmic Rays—Constraints from Cosmic-Ray Measurements. Universe, 11(10), 331. https://doi.org/10.3390/universe11100331

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