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O rbiting W ide-angle L ight-collectors (OWL)
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Cosmic rays are sub-atomic particles that have been accelerated up to close to the speed of light. Below about 1015 eV (1,000,000,000,000,000 electron volts) cosmic rays are believed to have been accelerated in supernovae remnants in the galaxy. But supernovae cannot accelerate particles more than this, and there is no other known source of higher energy particles in the galaxy. So it is believed that above 1015 eV, the particles are extragalactic. Indeed, there is a change in the shape of the energy spectra of cosmic rays (known as the knee) at about this energy.
Above a few times 1019 eV, the number of cosmic rays was expected to drop off sharply, because they can interact with the omnipresent 2.7 K microwave background (which has been blue shifted up to gamma ray energies for these cosmic rays). Observations show an initial drop off (known as the ankle), but then the spectrum becomes more shallow, meaning there are a lot more particles at these energies than was expected. These are subatomic particles with more kinetic energy than a major league fastball!
Is there a new, unknown source of ultra-high energy particles? Is there new physics? Where are these particles coming from? These are the important questions that OWL hopes to answer.