Cosmic γ-ray bursts as a probe of star formation history

The cosmic γ-ray burst (GRB) formation rate, as derived from the variability-luminosity relation for long-duration GRBs, is compared with the cosmic star formation rate. If GRBs are related to the collapse of massive stars, one expects the GRB rate to be approximately proportional to the star formation rate. We found that these two rates have similar slopes at low redshift. This suggests that GRBs do indeed track the star formation rate of the Universe, which in turn implies that the formation rate of massive stars that produce GRBs is proportional to the total star formation rate. It also implies that we can use GRBs as a probe of the cosmic star formation rate at high redshift. We find that the cosmic star formation rate increases steeply with redshift at z>2.5. This is in apparent contrast to what is derived from measurements of the cosmic star formation rate at high redshift from optical observations of field galaxies, suggesting that much high-z star formation is being missed in the optical surveys, ...