Cosmic rays are a background-free observable into the universe, but are bent in ensuing magnetic fields. They are observed at large experiments like the Pierre Auger Observatory in Argentina and Telescope Array in Utah. Astrophysical neutrinos point at their sources, but have backgrounds from cosmic ray air showers.

Protons and nuclei lose energy as they propagate through the universe. For a source emitting protons, helium nuclei, or iron nuclei at various distances from the Earth, the energy of the flux that we will see is shown here.