Humans can push physics to the absolute extreme within the precisely controlled conditions of a lab — the coldest place in the known universe, for instance, isn’t in some distant nebula but was instead created in Massachusetts. But, sometimes, the universe has us beat. Take cosmic rays, the rarest of which are so unimaginably energetic that no particle accelerator could hope to recreate them.
While we know where many cosmic rays come from — supernovas, mostly — those ultra-high-energy rays are so rare and so unusual that we don’t actually know where they originate. Now, a team of about 400 astronomers from all over the world have put their heads together and confirmed where those rays don’t come from. They’re not from anywhere in our galaxy. Beyond that? As ever, nobody is quite sure.
As described in Wednesday’s issue of the journal Science, the researchers used Argentina’s Pierre Auger Observatory, the largest cosmic ray observatory ever built, to track the sources of the particles. What they were trying to find out is whether the particles appeared to be coming from any specific direction.
Over a 12-year period, they spotted thousands of these highly energetic particles, and there was a definite imbalance in where they were coming from. That alone suggests these cosmic rays don’t originate inside the Milky Way, as then it would be expected the particles’ directions would be distributed more or less evenly.
But the situation gets more intriguing when you consider the imbalance pointed toward a particular part of the sky where there’s an unusually high concentration of galaxies. This might suggest that the particles are produced in those distant galaxies and are then sent rocketing out across intergalactic space, traveling millions of light-years before finally reaching Earth for astronomers to observe.
What’s happening in those galaxies to produce those rays, however, is still not clear at all. One theory is that some of those galaxies have explosively violent black holes at their center, and those massive black holes accelerate the particles to incredible energies.
Understanding what’s going on with these cosmic rays could reveal some of the universe’s deepest secrets — after all, if the astronomers are right in their findings, every one of these particles we discover is a traveler from a galaxy far from our own. Unraveling the mystery of these particles’ origins could reveal deep secrets about black holes, the formation of galaxies, and even the Big Bang itself.