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Most Powerful Cosmic Rays Come From Galaxies Far, Far Away (space.com)
A new study finds the highest-energy cosmic rays to bombard Earth come from galaxies far, far away. Space.com reports: The sun emits relatively low-energy cosmic rays. However, for more than 50 years, scientists have also detected ultra-high-energy cosmic rays, ones far beyond the capability of any particle accelerator on Earth to generate. One way to discover the origins of ultra-high-energy cosmic rays is to study their directions of travel. However, ultra-high-energy cosmic rays only rarely strike Earth's atmosphere, with one hitting any given area about the size of a soccer field about once per century, the researchers said. In order to detect ultra-high-energy cosmic rays, scientists look for the spray of electrons, photons and other particles that result when ultra-high-energy cosmic rays hit the top of Earth's atmosphere. Each of these showers contains more than 10 billion particles, which fly downward in a disk shaped like a giant plate miles wide, according to the statement. Scientists examined the sprays from ultra-high-energy cosmic rays using the largest cosmic-ray observatory yet: the Pierre Auger Observatory built in the western plains of Argentina in 2001. It consists of an array of 1,600 particle detectors deployed in a hexagonal grid over 1,160 square miles (3,000 square kilometers), an area comparable in size to Rhode Island. A connected set of telescopes is also used to see the dim fluorescent light the particles in the sprays emit at night.
The researchers analyzed data collected between 2004 and 2016. During these 12 years, the scientists detected more than 30,000 ultra-high-energy cosmic rays. If ultra-high-energy cosmic rays came from the Milky Way, one might perhaps expect them to come from all across the sky, or perhaps mostly from the direction of the supermassive black hole at the galaxy's center. However, the researchers saw that ultra-high-energy cosmic rays mostly came from a broad area of sky about 90 degrees away from the direction of the Milky Way's core.
The researchers analyzed data collected between 2004 and 2016. During these 12 years, the scientists detected more than 30,000 ultra-high-energy cosmic rays. If ultra-high-energy cosmic rays came from the Milky Way, one might perhaps expect them to come from all across the sky, or perhaps mostly from the direction of the supermassive black hole at the galaxy's center. However, the researchers saw that ultra-high-energy cosmic rays mostly came from a broad area of sky about 90 degrees away from the direction of the Milky Way's core.
So if you extrapolate this facts, don't they suggest that it stands to reason that, the further away in distance [and thus the further back in time] that we look, the higher the energies we would expect to observe.
To get to the energies of cosmic rays you have to go back to before 10^-13 s after the Big Bang. Back then the Universe was incredibly small and incredibly dense. So dense and energetic that everything, even things like neutrinos, were colliding and interacting with everything around them. This meant that everything was roughly in thermal equilibrium and had comparable energies.
By the time than the charged particles responsible for cosmic rays the energy and density of the universe would have been much, much lower since it would require photons to decouple first which happened 380,000 years after the Big Bang. The result is that there is no way that a Cosmic ray, as a charged particle, can get its energy directly from the Big Bang.
It could get it indirectly if there were some high mass, exotic and as yet undiscovered particle which was created in the Big Bang and which decays with a lifetime of billions of years or which might annihilate with itself to create these particles. This is one possible way to detect Dark Matter but it is extremely unlikely (impossible without even more new physics) that this would provide enough energy to explain high energy cosmic rays.