Gamma-Ray Bursts as Cosmological Probes

The status and prospects for gamma-ray bursts (GRBs) as cosmological probes are reviewed. Long duration GRBs can potentially be used as an indicator of star formation rate (SFR), though GRB rate might be systematically different from SFR, by the effect of e.g. metallicity. There are several papers claiming that the cosmic GRB rate history is different from that of SFR in the sense that GRB rate is relatively higher than SFR at higher redshifts, which may be explained by the metallicity effect. However, considering the large uncertainties about the efficiency of GRB afterglow detection and redshift determination, it would be conservative to state that the observed GRB rate is roughly consistent with the star formation history. GRBs can also be used as a unique and powerful tool to reveal the reionization history. However, there is practically no progress in this direction since the first GRB-based useful constraint on reionization in 2005 (GRB 050904). The bottleneck now is the insufficient sensitivity of near infrared spectroscopy, even with 8m class telescopes. The planned 30m class telescopes will bring the next breakthruough. Finally, GRBs can potentially be used as a standard candle to study cosmology by a geometrical test. However, there are still many steps for GRBs to overcome before it produces a result that has strong impact on the cosmology community in the precision cosmology era.