In that case, cosmic rays must be the poor man's particle accelerator. Roughly one is seen by the array each year, and there is no indication of any limit to their energy.
Current theories say that is impossible.
If these cosmic rays are protons or atomic nuclei , as the experiments hint, they must be moving almost at the speed of light. At that clip, the cosmic microwave background, a tenuous gas of primordial radiation that fills space, looks like a thick sea. After traveling 150 million light-years, no ordinary particle could still have the observed energies. Yet astronomers have seen no plausible source within that distance. Exploding stars can propel particles up to only about 1 percent of the required energy.