High-Energy Cosmic Rays
For decades, astronomers have known that the Earth is consistently struck by high-energy cosmic rays — charged particles that are usually the nuclei of elements — that originate from a source in space outside our solar system. These cosmic rays possess the highest possible energies observed in nature, even higher than what man-made particle accelerators can reproduce, and now, a team of scientists thinks they might have solved the mystery of their origin.
In a study published Science, the researchers, known collectively as the Pierre Auger Collaboration, suggest that these cosmic signals may originate from outside of the Milky Way. Their conclusions were drawn using recordings from the Pierre Auger Observatory in Argentina, the largest cosmic ray observatory currently in existence, and other data.
Although cosmic rays with energy greater than two joules rarely reach Earth, when they do, their interaction with nuclei in the planet's atmosphere creates a shower of electrons, photons, and muons, making them detectable by researchers. These showers of more than 10 billion particles spread out across diameters measuring several kilometers.
When one of the particles within this shower hits one of the Pierre Auger Observatory's 1,600 detectors, which are spread out over an area of 3,000 square kilometers (1,158 square miles), researchers can determine its originating direction. In the new research, the Auger collaboration studied the arrival directions of more than 300,000 cosmic particles and discovered that the arrival rates of the cosmic rays vary and aren't uniformly spread in all directions. The rate is actually higher for certain directions.
According to the team, this anisotropy indicates an extragalactic origin for the cosmic particles, as many are coming from an area where the distribution of galaxies is fairly high. However, because the direction points toward a broad area of the sky, the specific sources remain undetermined.
Our Celestial Origins
We still have much to learn about cosmic rays, and the Pierre Auger Collaboration expects to supplement their findings when upgrades to the Auger Observatory are completed in 2018. Still, this new discovery is worthwhile. Any new knowledge about these particles can help us better understand matter from outside the solar system and, as this research suggests, from outside the Milky Way.
"We are now considerably closer to solving the mystery of where and how these extraordinary particles are created, a question of great interest to astrophysicists," University of Wuppertal professor Karl-Heinz Kampert, a spokesperson for the Auger collaboration, said in a press release.
Figuring out the mechanisms behind these cosmic rays could help explain how galaxies form and what in their composition accounts for the creation of such high-energy particles. Furthermore, since these cosmic rays are made of particles that are also found on Earth, they could also provide important clues into the fundamental questions about our origins — perhaps even the origins of the universe itself.
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