Physical
GZK Paradox (Greisen-Zatsepin-Kuzmin)
Ultra-high-energy cosmic rays exist beyond theoretical limits
Overview
The observation of cosmic rays with energies that shouldn't be possible given interactions with cosmic microwave background.
Theory predicts ultra-high-energy protons should interact with cosmic microwave background radiation, limiting their energy and travel distance (GZK cutoff). Yet we observe cosmic rays beyond this limit. Where do they come from?