Astro2020 White Paper: A Direct Measure of Cosmic Acceleration
  Nearly a century after the discovery that we live in an expanding Universe,
and two decades after the discovery of accelerating cosmic expansion, there
remains no direct detection of this acceleration via redshift drift - a change
in the cosmological expansion velocity versus time. Because cosmological
redshift drift directly determines the Hubble parameter H(z), it is arguably
the cleanest possible measurement of the expansion history, and has the
potential to constrain dark energy models (e.g. Kim et al. 2015). The challenge
is that the signal is small - the best observational constraint presently has
an uncertainty several orders of magnitude larger than the expected signal
(Darling 2012). Nonetheless, direct detection of redshift drift is becoming
feasible, with upcoming facilities such as the ESO-ELT and SKA projecting
possible detection within two to three decades. This timescale is uncomfortably
long given the potential of this cosmological test. With dedicated experiments
it should be possible to rapidly accelerate progress and detect redshift drift
with only a five-year observational baseline. Such a facility would also be
ideal for precision radial velocity measurements of exoplanets, which could be
obtained as a byproduct of the ongoing calibration measurements for the
experiment.
