Gamma-ray bursts at high redshift

Gamma-ray bursts are much brighter than supernovae, and could therefore possibly probe the Universe to high redshift. The presently established GRB redshifts range from 0.83 to 5, and quite possibly even beyond that. Since most proposed mechanisms for GRB link them closely to deaths of massive stars, it is a reasonable ansatz to assume that their rate density in the past was proportional to the star formation rate. Work with Bloom, Bagla, and Natarajan (1997), as well as by other groups, does indeed show that the GRB flux distribution calculated from this assumption agrees well with the data. This means that GRB are bright lamps which illuminate an era of the Universe that may predate the quasar age somewhat, so they can become useful beacons illuminating the early Universe. It also seems that GRB can become well calibrated lamps, like supernovae. The theory that attributes the afterglows to relativistic blast waves emitting synchrotron radiation has been quite successfully tested, and in the case of GRB 970508 so well measured that the physical parameters of the burst are known. If this could be repeated regularly, GRB can become as precise probes of cosmic geometry at redshifts 2-5 as ordinary supernovae are at redshifts <~1.