The mystery surrounding the source of the highest-energy particles known in the Universe has grown deeper. The particles, known as cosmic rays, can show up with energies a million times higher than the biggest particle accelerators on Earth can produce. Astrophysicists believed that only two sources could make them: supermassive black holes in active galaxies, or so-called gamma ray bursts. A study in Nature has now all but ruled out gamma ray bursts as the cause.
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