An Etymological Dictionary of Astronomy and Astrophysics

The question of the origin and nature of the → ultra high energy cosmic rays.
According to the → GZK cutoff, the UHECRs should be nearby. They are expected to be exceptional, therefore visible by some astrophysical counterpart. However, there is nothing visible (within a few tens of → Mpc) in the direction of all the UHECR detected up to now.

See also: → UHECR; → puzzle.