Rare Cosmic Rays Are from Far Away: GZK Limit Confirmed
University of UtahFinal results from the University of Utah's High Resolution Fly's Eye cosmic ray observatory show that the most energetic particles in the universe rarely reach Earth at full strength because they come from great distances, so most of them collide with radiation left over from the birth of the universe. The findings confirm a 42-year-old prediction known as the Greisen-Zatsepin-Kuzmin (GZK) "cutoff."