Real-time Cosmology with High Precision Spectroscopy and Astrometry
  Breakthroughs in physics and astrophysics are often driven by technological
advances, with the recent detection of gravitational waves being one such
example. This white paper focuses upon how improved astrometric and
spectroscopic measurements from a new generation of precise, accurate, and
stable astronomical instrumentation can address two of the fundamental
mysteries of our time -- dark energy and dark matter -- and probe the nature of
spacetime. Instrumentation is now on the cusp of enabling new cosmological
measurements based on redshifts (cosmic redshift drift) and extremely precise
time-series measurements of accelerations, astrophysical source positions
(astrometry), and angles (cosmic parallax). These allow tests of the
fundamental framework of the universe (the Friedmann equations of general
relativity and whether cosmic expansion is physically accelerating) and its
contents (dark energy evolution and dark matter behavior), while also anchoring
the cosmic distance scale ($H_0$).
