Gamma-ray bursts and cosmic evolution

Gamma-ray bursts are brief flashes of high-energy radiation, occurring a few times per day; at random locations on the sky. They apparently do not repeat, so one must monitor the full sky at all times to catch these events. Fortunately, they are very bright and thus easy to detect. Their light output varies on very short time scales (similar to ms), suggesting compact objects as their sources. Recent breakthroughs with X-ray detectors led to the discovery of X-ray, optical, and radio afterglows, which in turn led to very accurate positions. From afterglow observations we learned that GRBs are cosmological, located at red-shifts competetive with those of quasars. Their large apparent brightnesses and distances makes them the most energetic explosions in the universe, with energies of order M(circle dot)c(2). Their host galaxies are faint, but actively star forming at rates typical for galaxies in the early universe. The current paradigm associates GRBs with the formation of black holes in either massive, rotating stars, or with the merger of compact binary star systems. In either case, GRBs trace the cosmic: star formation rate, and might thus turn out to be useful pointers to the earliest generation of stars. Their high photon energies and short; variability timescale also makes GRBs a valuable probe of theoretical models of Quantum Gravity. GRBs have been used to show that the speed of light is color blind at the Deltac/c similar to 10(-20) level, which begins to constrain our current models of the foam-like structure of space-time.