Giving Cosmic Redshift Drift a Whirl
  Redshift drift provides a direct kinematic measurement of cosmic acceleration
but it occurs with a characteristic time scale of a Hubble time. Thus redshift
observations with a challenging precision of $10^{-9}$ require a 10 year time
span to obtain a signal-to-noise of 1. We discuss theoretical and experimental
approaches to address this challenge, potentially requiring less observer time
and having greater immunity to common systematics. On the theoretical side we
explore allowing the universe, rather than the observer, to provide long time
spans; speculative methods include radial baryon acoustic oscillations, cosmic
pulsars, and strongly lensed quasars. On the experimental side, we explore
beating down the redshift precision using differential interferometric
techniques, including externally dispersed interferometers and spatial
heterodyne spectroscopy. Low-redshift emission line galaxies are identified as
having high cosmology leverage and systematics control, with an 8 hour exposure
on a 10-meter telescope (1000 hours of exposure on a 40-meter telescope)
potentially capable of measuring the redshift of a galaxy to a precision of
$10^{-8}$ (few $\times 10^{-10}$). Low-redshift redshift drift also has very
strong complementarity with cosmic microwave background measurements, with the
combination achieving a dark energy figure of merit of nearly 300 (1400) for 5%
(1%) precision on drift.
