What can gamma ray bursts teach us about dark energy?

It has been suggested that Gamma Ray Bursts (GRB) may enable the expansion rate of our Universe to be measured out to very high redshifts (z ≳ 5) just as type Ia supernovae have done at z ∼ 1-1.5. We explore this possibility here, and find that GRB have the potential to detect dark energy, but they are unlikely to be competitive with future supernovae missions, such as SNAP, in measuring the properties of the dark energy. This negative conclusion results partly from including the effects of gravitational lensing, which are more pronounced for GRB than for the closer supernovae. The exception to this conclusion is if there is appreciable dark energy at early times, in which case the information from GRBs will provide an excellent complement to the z ∼ 1 information from supernovae. In particular, models with early (z ∼ 2-4) dark energy density comparable to the matter density can be excluded from upcoming GRB data.