Method for Direct Measurement of Cosmic Acceleration by 21-cm Absorption
  Systems
  So far there is only indirect evidence that the Universe is undergoing an
accelerated expansion. The evidence for cosmic acceleration is based on the
observation of different objects at different distances, and requires invoking
the Copernican cosmological principle, and Einstein's equations of motion. We
examine the direct observability using recession velocity drifts (Sandage-Loeb
effect) of 21cm hydrogen absorption systems in upcoming radio surveys. This
measures the change in velocity of the {\it same} objects separate by a time
interval and is a model-independent measure of acceleration. We forecast that
for a CHIME-like survey with a decade time span, we can detect the acceleration
of a $\Lambda$CDM Universe with $5\sigma$ confidence. This acceleration test
requires modest data analysis and storage changes from the normal processing,
and cannot be recovered retroactively.
