Physics and astrophysics with gamma-ray telescopes
  In the past few years gamma-ray astronomy has entered a golden age. A modern
suite of telescopes is now scanning the sky over both hemispheres and over six
orders of magnitude in energy. At $\sim$TeV energies, only a handful of sources
were known a decade ago, but the current generation of ground-based imaging
atmospheric Cherenkov telescopes (H.E.S.S., MAGIC, and VERITAS) has increased
this number to nearly one hundred. With a large field of view and duty cycle,
the Tibet and Milagro air shower detectors have demonstrated the promise of the
direct particle detection technique for TeV gamma rays. At $\sim$GeV energies,
the Fermi Gamma-ray Space Telescope has increased the number of known sources
by nearly an order of magnitude in its first year of operation. New classes of
sources that were previously theorized to be gamma-ray emitters have now been
confirmed observationally. Moreover, there have been surprise discoveries of
GeV gamma-ray emission from source classes for which no theory predicted it was
possible. In addition to elucidating the processes of high-energy astrophysics,
gamma-ray telescopes are making essential contributions to fundamental physics
topics including quantum gravity, gravitational waves, and dark matter. I
summarize the current census of astrophysical gamma-ray sources, highlight some
recent discoveries relevant to fundamental physics, and describe the synergetic
connections between gamma-ray and neutrino astronomy. This is a brief overview
intended in particular for particle physicists and neutrino astronomers, based
on a presentation at the Neutrino 2010 conference in Athens, Greece. I focus in
particular on results from Fermi (which was launched soon after Neutrino 2008),
and conclude with a description of the next generation of instruments, namely
HAWC and the Cherenkov Telescope Array.
