  The matter in our Universe comes in two flavors: dark and baryonic. Of these,
only the latter couples to photons, giving rise to the well-known baryon
acoustic oscillations and, in the process, generating supersonic relative
velocities between dark matter and baryons. These velocities---imprinted with
the acoustic scale in their genesis---impede the formation of the first stars
during cosmic dawn ($z\sim 20$), modulating the expected 21-cm signal from this
era. In a companion paper we showed, combining numerical simulations and
analytic models, that this modulation takes the form of robust velocity-induced
acoustic oscillations (VAOs), with a well-understood shape that is frozen at
recombination, and unaffected by the unknown astrophysics of star formation.
Here we propose using these VAOs as a standard ruler at cosmic dawn. We find
that three years of 21-cm power-spectrum data from the upcoming HERA
interferometer should be able to measure the Hubble expansion rate $H(z)$ at
$z=15-20$ to percent-level precision, ranging from $0.3\%$ to $11\%$ depending
on the strength of astrophysical feedback processes and foregrounds. This would
provide a new handle on the expansion rate of our Universe during an otherwise
unprobed epoch, opening a window to the mysterious cosmic-dawn era.
