Cosmic rays are the highest energy particles in the Universe, produced by the most extreme astrophysical events, including gamma ray bursts and the accretion of matter onto super massive black holes. Cosmic rays can reach energies millions of times greater than those seen in the LHC, packing the energy of a hard-thrown baseball into a single proton. In a recent Science paper, researchers identified a radio relic from colliding galaxy clusters that should be able to produce cosmic rays in abundance. The finding confirms theoretical models that indicate cluster-merging shocks can push cosmic rays to energies up to 10¹⁹ eV.

Massive clusters of galaxies collide regularly and form the large scale structure of the cosmic web which we observe in the Universe. Their collisions release massive amounts of energy (10⁶⁴ erg on timescales of 1-2 billion years), but not in the way many might expect. None of the stars collide; instead the diffuse gas in each galaxy collides, creating a shock front like that created by a jet fighter going supersonic.

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Merging galaxies leave "radio relic" of high-energy cosmic rays