  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.
