Testing general relativity by micro-arcsecond global astrometry

The global astrometric observations of a GAIA-like satellite were modeled within the PPN formulation of Post-Newtonian gravitation. An extensive experimental campaign based on realistic end-to-end simulations was conducted to establish the sensitivity of global astrometry to the PPN parameter gamma, which measures the amount of space curvature produced by unit rest mass. The results show that, with just a few thousands of relatively bright, photometrically stable, and astrometrically well behaved single stars, among the similar to10(9) objects that will be observed by GAIA, gamma can be estimated after I year of continuous observations with an accuracy of similar to10(-5) at the 3sigma level. Extrapolation to the full 5-year mission of these results based on the scaling properties of the adjustment procedure utilized suggests that the accuracy of similar or equal to2 x 10(-7), at the same 3sigma level, can be reached with similar to10(6) single stars, again chosen as the most astrometrically stable among the millions available in the magnitude range V = 12-13. These accuracies compare quite favorably with recent findings of scalar-tensor cosmological models, which predict for gamma a present-time deviation, \1-gamma\, from the General Relativity value between 10(-5) and 10(-7).