  Modern astrophysics, especially at GeV energy scales and above is a typical
example where several disciplines meet: The location and distribution of the
sources is the domain of astronomy. At distances corresponding to significant
redshift cosmological aspects such as the expansion history come into play.
Finally, the emission mechanisms and subsequent propagation of produced high
energy particles is at least partly the domain of particle physics, in
particular if new phenomena beyond the Standard Model are probed that require
base lines and/or energies unattained in the laboratory. In this contribution
we focus on three examples: Highest energy cosmic rays, tests of the Lorentz
symmetry and the search for new light photon-like states in the spectra of
active galaxies.
