Particles from Heaven

Cosmic rays are energetic particles that reach us from outer space, arriving from all directions. They are generally electrically charged particles, such as protons, heavy nuclei, electrons and positrons, but more broadly one includes among them also electrically neutral particles such as neutrons and neutrinos from outer space. If one subtracts those that arrive from the Sun, the rest arrive essentially isotropically, constituting a uniform background of cosmic-ray radiation, made up of particles with a finite mass. In addition to these, there is also a separate photon background, which includes the cosmic microwave background, the diffuse starlight optical-infrared background, and X-ray and gamma-ray backgrounds, all of which are also essentially isotropic, after subtraction of individual resolved sources.

A major difference between the cosmic-ray background and the photon background is that photons are massless and electrically neutral, so they travel essentially in straight lines from their sources, making it (at least at some wavelengths) easier to identify where they ultimately came from. The vast majority of cosmic rays, however, are electrically charged, and this makes it far harder to discern where they came from. This is because the interstellar and intergalactic space is woven through by random magnetic fields, and the Earth's atmosphere is permeated by an ordered magnetic field, so that as a result of propagating through these magnetic fields the cosmic ray path has little to do with the direction of whatever source they originated from [57].