# Physics of gravitation and cosmology with high precision space astrometry

The improvement of the precision in the verification of General Relativity (GR) is of crucial importance for fundamental physics, having a cosmological and astrophysical impact on all scales. In fact, the study of the evolution of the Universe covering a timescale of 60 orders of magnitude, from the primordial to the present, depends essentially on the understanding of gravitational interaction, most reliably interpreted through General Relativity. The most used theoretical instrument to compare these theories on a local scale is the parameterized post Newtonian (PPN), in which each theory is characterised by precise assumed values from a parameter set estimated from experiments. Amongst these parameters, , (related to the curvature of space-time induced by mass) is the most studied, and also the most accessible via astrometric measurements. The estimation of this parameter allows tight constraints to be placed also on alternative formalisms that can appreciably modify the current estimate of the mass/dark energy ratio. In this sense the measurement of the parameter can be considered a powerful cosmological test using local measurements.

It is predicted that the GAIA mission can reach an accuracy of 10^{-6 }for at the 3-sigma level, for nominal performance, by measuring the temporal evolution of the positions of a billion objects all over the celestial sphere. The numerical verifications and the relevant parts of the code are under development under an Agenzia Spaziale Italiana contract, coordinated with the activity of the European consortium DPAC (Data Processing and Analysis Consortium) for the reduction of GAIA data.