The Origin of Cosmic
Rays
Cosmic Rays reach the Earth from space with
energies of up to more than 1020 eV, carrying information on the
most powerful particle accelerators that Nature has been able to assemble.
Understanding where and how cosmic rays originate has required almost one
century of investigations, and, although the last word is not written yet,
recent observations and theory seem now
to fit together to provide us with a global picture of the origin of cosmic
rays of unprecedented clarity. Here we
will describe what we learned from recent observations of astrophysical sources
(such as supernova remnants and active galaxies) and we will illustrate what
these observations tell us about the physics of particle acceleration and
transport. We will also discuss the “end” of the Galactic cosmic ray spectrum,
which bridges out attention towards the so called ultra high energy cosmic rays
(UHECRs). At ~1020 eV the gyration scale of cosmic rays in cosmic
magnetic fields becomes large enough to allow us to point back to their
sources, thereby allowing us to perform “cosmic ray astronomy”, as confirmed by
the recent results obtained with the Pierre Auger Observatory. We will discuss
the implications of these observations for the understanding of UHECRs, as well
as some questions which will likely remain unanswered and will be the target of
the next generation of cosmic ray experiments.