Gamma-Ray Bursts: New Rulers to Measure the Universe

The best measure of the universe should be done using a standard "ruler" at any redshift. Type Ia supernovae (SN Ia) probe the universe up to z ~ 1.5, while the cosmic microwave background (CMB) primary anisotropies concern basically z ~ 1000. Apparently, gamma-ray bursts (GRBs) are all but standard candles. However, their emission is collimated, and the collimation-corrected energy correlates tightly with the frequency at which most of the radiation of the prompt is emitted, as found by Ghirlanda et al. Through this correlation we can infer the burst energy accurately enough to probe the intermediate-redshift (z < 10) universe. Using the best known 15 GRBs we find very encouraging results that emphasize the cosmological GRB role. A combined fit with SN Ia yields ΩM = 0.37 ± 0.10 and ΩΛ = 0.87 ± 0.23. Assuming in addition a flat universe, the parameters are constrained to be ΩM = 0.29 ± 0.04 and ΩΛ = 0.71 ± 0.05. GRBs accomplish the role of "missing link" between SN Ia and CMB primary anisotropies. They can provide a new insight on the cosmic effects of dark energy, complementary to the one supplied by CMB secondary anisotropies through the integrated Sachs-Wolfe effect. The unexpected standard candle cosmological role of GRBs motivates us with the most optimistic hopes for what can be obtained when the GRB-dedicated satellite, Swift, is launched.