Sub-Microarcsecond Astrometry and New Horizons in Relativistic Gravitational Physics

Attaining the limit of sub-microarcsecond optical resolution will completely revolutionize fundamental astrometry by merging it with relativistic gravitational physics. Beyond the sub-microarcsecond threshold, one will meet in the sky a new population of physical phenomena caused by primordial gravitational waves from the early universe and/or di erent localized astronomical sources, space-time topological defects, moving gravitational lenses, time variability of gravitational elds of the solar system and binary stars, and many others. Adequate physical interpretation of these yet undetectable sub-microarcsecond phenomena cannot be achieved on the ground of the \standard" post-Newtonian approach (PNA), which is valid only in the near-zone of astronomical objects having a time-dependent gravitational eld. We describe a new, post-Minkowskian relativistic approach for modeling astrometric observations having sub-microarcsecond precision and brie y discuss the lightpropagation e ects caused by gravitational waves and other phenomena related to time-dependent gravitational elds. The domain of applicability of the PNA in relativistic space astrometry is outlined explicitly. 1. Theoretical principles of relativistic astrometry For a long time the basic theoretical principles of general relativistic astrometry in the solar system were based on using the post-Newtonian approximate solution of the Einstein equations (So el, 1989; Brumberg, 1991; Will, 1993). The metric tensor of the post-Newtonian solution is an instantaneous function of coordinate time t. It depends on the eld point, x, and the coordinates, xa(t), and velocities, va(t), of the gravitating bodies, and is valid only inside the near zone of the solar system because it involves expansion of retarded eld integrals with respect to the small parameter va=c (Fock, 1959). This expansion restricts the domain of validity for which the propagation of light rays can be considered from the mathematical point of view in a self-consistent manner by the boundary of the near zone. Finding a solution of the equations of light propagation in the near zone of the solar system, for instance, can be achieved