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The Milky Way’s newfound high-energy glow hints at the secrets of cosmic rays

The Milky Way glows with a gamma ray haze, with energies vastly exceeding anything physicists can produce on Earth, according to a new paper. Gamma rays detected in the study, to be published in Physical Review Letters, came from throughout the galaxy’s disk, and reached nearly a quadrillion (1015) electron volts, known as a petaelectron volt or PeV.

These diffuse gamma rays hint at the existence of powerful cosmic particle accelerators within the Milky Way. Physicists believe such accelerators are the source of mysterious, highly energetic cosmic rays, charged particles that careen through the galaxy, sometimes crash-landing on Earth. When cosmic rays — which mainly consist of protons — slam into interstellar debris, they can produce gamma rays, a form of high-energy light.

Certain galactic environments could rev up cosmic ray particles to more than a PeV, scientists suspect. In comparison, the Large Hadron Collider, the premier particle accelerator crafted by humans, accelerates protons to 6.5 trillion electron volts. But physicists haven’t definitively identified any natural cosmic accelerators capable of reaching a PeV, known as PeVatrons. One possibility is that supernova remnants, the remains of exploded stars, host shock waves that can accelerate cosmic rays to such energies

If PeVatrons exist, the cosmic rays they emit would permeate the galaxy, producing a diffuse glow of gamma rays of extreme energies. That’s just what researchers with the Tibet AS-gamma experiment have found. “It’s nice to see things fitting together,” says physicist David Hanna of McGill University in Montreal, who was not involved with the study.

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