The past years have seen a remarkable progress in our understanding of the Universe at high (> 100 MeV) and very high (VHE; > 0.1 TeV) energies. The advanced capabilities of modern gamma-ray instruments like H.E.S.S., MAGIC, VERITAS or Fermi are offering a unique tool to explore fundamental processes at highest photon energies and to elucidate the nature of cosmic objects. For the first time, a wide variety of energetic phenomena can be studied with unprecedented sensitivity over a large range in energy, including black holes and active galaxies (AGN), gamma-ray bursts and pulsars. At ultra high energies (UHE; > 10 EeV), on the other hand, cosmic-ray research has gained a strong impetus by indications of a possible correlation between the Pierre Auger-measured UHE cosmic ray events and the nearby AGN distribution, as well as evidence for a cosmic-ray composition that seems to become heavier towards the highest measured energies. These highest energy particles must originate in extreme astrophysical environments and are allowing to probe particle physics in an otherwise inaccessible energy range. More recently, the IceCube detection of high-energy (>10 TeV) neutrinos exceeding Galactic expectations has opened up a new window on astronomy and fundamental physics.

Much of the research I have carried out in recent years aims at a scientific foundation and understanding of the observational exploration with VHE instruments, including key science drivers for, e.g., the upcoming international Cherenkov Telescope Array (CTA) project. Current and on-going research projects include: