Open questions
A century after the discovery of cosmic rays, the highest-energy particles in the universe still guard their secrets.
Where do they come from?
No individual UHECR source has been identified. Magnetic fields — Galactic and extragalactic — deflect charged particles by degrees to tens of degrees, blurring any pointing back to the sources. The observed dipole anisotropy above 8 EeV proves an extragalactic origin, and intermediate-scale excesses (the TA hotspot, correlations with catalogs of starburst galaxies and AGN) are tantalising, but none has crossed the discovery threshold. Candidate accelerators include radio galaxies and AGN jets, starburst-driven winds, gamma-ray bursts, and tidal disruption events — transient sources are particularly attractive because magnetic time delays would erase their temporal signature.
What are they made of?
Composition measurements from Xmax indicate a light (proton-dominated) composition around a few EeV, growing gradually heavier toward the highest energies — as if sources accelerate nuclei to a maximum rigidity, so heavier elements reach proportionally higher energies. But composition above ~50 EeV, exactly where astronomy would become possible with nearly-straight-propagating light nuclei, remains essentially unmeasured: the fluorescence duty cycle runs out of statistics. AugerPrime is designed to close this gap.
Is the cutoff GZK or source exhaustion?
The flux suppression near 5×10¹⁹ eV was predicted by Greisen, Zatsepin and Kuzmin (the GZK effect: protons losing energy to photo-pion production on the cosmic microwave background). But a heavier composition points to an alternative: the sources themselves may simply run out of accelerating power at these energies. Distinguishing propagation losses from source limits is a central task for next-generation observatories.
The muon puzzle
Air-shower simulations based on hadronic interaction models tuned to LHC data underpredict the number of muons observed in showers by 20–50% at the highest energies. This “muon deficit” indicates that hadronic interactions at centre-of-mass energies beyond the LHC behave differently than extrapolated — making UHECR data a probe of particle physics at otherwise inaccessible energies.
The multi-messenger connection
Wherever UHECRs are accelerated, their interactions must produce secondary neutrinos and gamma rays. The IceCube diffuse neutrino flux, the possible association of neutrinos with blazars and tidal disruption events, and the still-undetected cosmogenic (GZK) neutrinos all tie into the UHECR puzzle. Finding the sources will likely be a joint achievement of cosmic-ray, neutrino, and gamma-ray astronomy.