Use of GRBs for Cosmology

GRB afterglows are bright enough to be used as pathfinders into the very early universe, independent of whether or not the GRB and/or afterglow phenomenon is fully understood.

In contrast to stationary sources at high redshift, GRB afterglows do not appear substantially fainter at increasing z. Relativistic time dilation implies that the obser vations of GRBs at the same time At after the GRB event in the observers frame (on Earth) will be observed at different times in the source frame, e.g., at earlier times for more distant GRB. At this earlier time the GRB is intrinsically brighter, thus partly compensating the larger distance.

While it seems unlikely that GRBs will soon be used to derive a Hubble-diagram and to constrain cosmological parameters, there are a few other implications of high-z GRB studies for cosmology:

- Since long-duration GRBs are related to the death of massive stars, it is likely that high-z GRBs exist. Theoretical predictions range between few up to 50% of all GRBs being at z > 5 [15,50,85]. The polarization data of the Wilkinson Microwave Anisotropy Probe (WMAP) indicate a high electron scattering optical depth, hinting at the possibility that the first stars formed as early as z ~ 20 [48].

- WMAP data also suggest that the onset of reionization happened at z = 11 — 20 [48]. Because WMAP only provides an integral constraint on the reionization history of the universe, it has led to the speculation that reionization was either an extended process or happened more than once. Since the intrinsic luminosity as well as the number density of quasars are expected to fade rapidly beyond z ~ 6, only GRBs are suitable to be used as bright beacons to illuminate the end of the dark age [2,53,62], and potentially allow to probe the reionization history of the early Universe [44].

- Extensive monitoring of afterglows would help to constrain their local environment, and could allow to tell whether GRB afterglows are decelerated by the IGM with an increasingly higher density at higher redshift, or by a stratified constant density medium in a bubble cleared by the progenitor star [35].

- Studying the distribution and absorption line properties of GRB host galaxies would shed light onto the cosmological structure formation and star forming history [56].

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