Inferring the Cosmological Parameters: GRBs as Standard Candles?

The fact that GRBs are detected up to very high redshifts (the current record holder, GRB 050904, is at z = 6.29) makes it tempting to try and use them as standard candles that could be used to constrain the cosmological parameters, similar to SNe Ia. The problem is that GRBs appear to be anything but standard candles: they have a very wide range of isotropic equivalent luminosities and energy outputs. Several suggestions have been made to calibrate them as better standard candles by using correlations between various properties of the prompt emission, and in some cases also the afterglow emission. While there is good motivation for such cosmological applications of GRBs, there are many practical difficulties. There is no good sample of nearby (z << 1) GRBs that can be used in order to calibrate these correlations, and furthermore the few known nearby events appear to be intrinsically very different from events at higher redshifts. One might also expect a significant evolution of the intrinsic properties of GRBs with redshift (also between intermediate and high redshifts) which can be hard to disentangle from cosmological effects. In addition, even after properly accounting for the observed correlations, the scatter in the luminosity of the standardized candles is still fairly large. We will discuss the pros and cons, as well as prospects for the future.