4. Where do energetic particles come from?
Enrico Fermi proposed a theory on how particles could be accelerated by collisions with magnetic fields in the galaxy. Here are his notes showing the calculations he did for this seminal paper. Notice the date, Dec 4 1948. I did a quick check and found it was a Saturday! He did all these calculations over the weekend! Although we now know that acceleration occurs due to collision with supernovae shocks, but the central principle still holds. Here is a new paper in Science where this acceleration has been recently observed.
A great way to start