Origin of Galactic and ultra-high-energy cosmic rays

6 problems

• Why do Auger and Telescope Array spectra differ above $3 \times 10^{19}\,\mathrm{eV}$?

  Contested

  The two largest cosmic-ray observatories, one in each hemisphere, disagree on how many particles arrive at the very highest energies.

• At what energy do extragalactic cosmic rays take over from Galactic ones?

  Contested

  Below some energy cosmic rays come from our own galaxy and above it from other galaxies; whether the handover happens near $10^{17}\,\mathrm{eV}$ or near $5 \times 10^{18}\,\mathrm{eV}$ (the ankle, where the spectrum flattens) is unsettled.

• What produces the cosmic-ray knee near 3 PeV?

  Partially resolved

  The number of cosmic rays falls off faster above about $3 \times 10^{15}\,\mathrm{eV}$, a feature called the knee, which may mark the highest energy the Milky Way's particle accelerators reach.

• Why do air showers contain more muons than simulations predict?

  Open

  Cosmic rays hitting the atmosphere make cascades of particles, and measured cascades carry tens of percent more muons (heavier cousins of the electron) than computer models of particle collisions predict, which spoils estimates of what the cosmic rays are made of.

• Is the cosmic-ray flux cutoff above $4 \times 10^{19}\,\mathrm{eV}$ a GZK effect?

  Contested

  Very few cosmic rays arrive above about $4 \times 10^{19}$ eV, either because they lose energy on the leftover light of the Big Bang (the GZK effect) or because their sources cannot push them higher.

• Which sources accelerate cosmic rays above $10^{19}\,\mathrm{eV}$?

  Open

  The most energetic cosmic rays come from outside our galaxy, but magnetic fields bend their paths, so their sources have not been identified.