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Proton discovery may cast doubt on dark-matter theories.
Hot on the heels of speculation that cosmic rays may have revealed the signature of elusive dark matter in space, new observations could challenge that idea and reinforce an alternative explanation.
A seven-year-long experiment at the Milagro cosmic-ray detector near Los Alamos, New Mexico, has revealed 'bright patches' of high-energy cosmic rays in the sky – something incompatible with a dark-matter source.
Cosmic rays are charged particles, mostly protons and electrons, that are produced in space and generally have a characteristic energy spectrum — the higher their energy, the rarer they are.
But last week, researchers working on the Advanced Thin Ionization Calorimeter (ATIC) experiment, which uses detectors borne by a high-altitude balloon to measure cosmic-ray electrons above the Antarctic, reported an unexpected bump in this energy spectrum, corresponding to a surfeit of electrons with energies between 300 and 800 gigaelectronvolts.
Hints at such an anomaly have been seen before. A satellite observatory — Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) — detected an excess of cosmic-ray positrons, the anti-particles of electrons, at similar energies. And a Japanese detector called the Balloon-borne Electron Telescope with Scintillating fibers (BETS) also found a small excess of cosmic-ray electrons at high energy.
These cosmic rays may be the decay products of hypothetical particles of dark matter, thought to make up about 85% of all matter in the Universe. Astronomers have invoked dark matter's gravitational effects to explain why rotating galaxies don't fall apart as they whirl through space. But as the name implies, dark matter can't be seen directly and its identity remains obscure.