Gamma-Ray Bursts: New Rulers to Measure the Universe

The best measure of the Universe should be done using a standard "ruler" atany redshift. Type Ia Supernovae (SN Ia) probe the universe up to z$\sim$1.5,while the Cosmic Microwave Background (CMB) primary anisotropies concernbasically $z\sim$1000. Apparently, Gamma--Ray Bursts (GRBs) are all butstandard candles. However, their emission is collimated and thecollimation--corrected energy correlates tightly with the frequency at whichmost of the radiation of the prompt is emitted, as found by Ghirlanda et al.(2004). Through this correlation we can infer the burst energy accuratelyenough to probe the intermediate redshift ($z<10$) Universe. Using the bestknown 15 GRBs we find very encouraging results that emphasize the cosmologicalGRB role. A combined fit with SN Ia yields $\Omega_{\rm M}=0.37\pm0.10$ and$\Omega_{\Lambda}=0.87\pm 0.23$. Assuming in addition a flat Universe, theparameters are constrained to be $\Omega_{\rm M}=0.29\pm0.04$ and$\Omega_{\Lambda}=0.71\pm 0.05$. GRBs accomplish the role of "missing link"between SN Ia and CMB primary anisotropies. They can provide a new insight onthe cosmic effects of dark energy, complementary to the one supplied by CMBsecondary anisotropies through the Integrated Sachs Wolfe effect. Theunexpected Standard Candle cosmological role of GRBs motivates the mostoptimistic hopes for what can be obtained when the GRB-dedicated satellite,Swift, will be launched.Comment: 11 pages, 4 color figures, ApJ Letters (vol. 613) in pres