Cosmology with the Very Large Telescope Interferometer using a space based astrometric reference frame

Cosmology with large interferometric telescopes is a rich and largely unexplored subject, involving three types of measurement: astrometric measurement of absolute distances and proper motions, dispersions of relative proper motions, and images. The ground based interferometers can have huge apertures, which are necessary for faint cosmological targets. But, alone, they are limited to astrometry within the isoplanatic patch, and hence to relative positions, which are of little use for parallaxes and proper motions because reference stars have unknown parallaxes and huge (500 \mu arcsec) unknown motions. We propose that space missions should measure global astrometric parallaxes and proper motions for (V > 16) reference stars within the isoplanatic patches of important cosmological and Galactic targets. Ground based interferometers can then measure absolute distances (parallaxes) and proper motions to 10 \mu arcsec, tied to these reference stars. But cosmological observations stretch the VLTI technically. To observe the few best targets, we need to be able to measure positions to < 10 \mu arcsec over a large portion of the sky. Since natural guide stars are too far apart, or too faint, laser guide stars are needed to correct the wavefronts of the individual 8-m unit telescopes, and the fringe tracking system must have an extremely high throughput to work on the brightest stars (V > 16) near to important targets. Most of the science is at 1 --2 microns, where excellent adaptive optics will be needed on the 8-m telescopes.