Quanta Magazine recently published an in-depth article looking at the work currently being done by the Telescope Array project. The article gives an excellent introduction to cosmic ray research, covering topics from the discovery of cosmic rays though the various cosmic ray experiments and on to some of the pressing questions that physicists are studying today.
The crux of the article is the cosmic ray hotspot, an area of the northern sky from which an unusually high number of ultra-high-energy cosmic rays originate. This hotspot, which initially appeared to be too weak to be noteworthy, has continually grown in significance as more and more data has been collected by the Telescope Array. Researchers are now 99.994 percent certain that they have actually found something.
Ultra-high-energy cosmic rays are unusual in that they seem to carry more energy than a particle theoretically should be able to carry. Traveling through cosmic background radiation causes even the most energetic particles to lose energy, placing an upper limit on the energy of comic rays - the GZK Cutoff. Ultra-high-energy cosmic rays exceed this limit, suggesting that they were generated near enough to earth that they haven't lost their excess energy. This leaves astrophysicists with a mystery - what could be generating these particles? As Dave Kieda puts it, "It’s like you’ve got a gorilla in your backyard throwing bowling balls at you, but he’s invisible."
This cosmic ray hotspot could provide a clue to the origin of these particles. Located in the constellation Ursa Major, 27 percent of all ultra-high-energy cosmic rays come from this tiny portion of the sky. Although deflection by atmospheric particles obscures their exact origin, this gives astrophysicists a good place to look for the origin of these cosmic rays.
The full article in Quanta Magazine can be found here.