Gamma-Ray Burst Hubble Diagram to z=4.5
  Gamma-ray bursts (GRBs) are tremendous explosions visible across most of the Universe, certainly out to redshifts of z=4.5 and likely out to z~10. Recently, GRBs have been found to have a roughly constant explosive energy as well as to have two luminosity indicators (the spectral lag time and the variability) that can be used to derive the burst's luminosity distance from the gamma-ray light curve alone. There currently exists enough information to calibrate luminosity distances and independent redshifts for nine bursts. From these, a GRB Hubble diagram can be constructed, where the observed shape of the curve provides a record of the expansion history of our Universe. The current nine burst diagram is sparse, yet formal limits can be placed on the mass density of a flat Universe. This first GRB Hubble diagram provides a proof of concept for a new technique in cosmology at very high redshifts. With the launch of the SWIFT satellite in 2003, we should get ~120 bursts to produce a Hubble diagram impervious to all effects of dust extinction and out to redshifts impossible to reach by any other method. 
Bradley E. Schaefer (Univ. Texas Austin)