A standard ruler at cosmic dawn

Dark matter and baryons posses supersonic relative velocities after recombination, as only the latter suffers baryon acoustic oscillations (BAOs). These velocities - imprinted with the acoustic scale in their genesis - impede the formation of the first stars during the cosmic dawn (z=20), modulating the expected 21-cm signal from this era, and inducing new velocity-induced acoustic oscillations (VAOs) in the 21-cm power spectrum. I will explain how to use these VAOs as a standard ruler at cosmic dawn, for which we take advantage of the immunity of VAOs to the unknown astrophysics of the first stars. I will show that the upcoming HERA interferometer will be able to measure the Hubble expansion rate H(z) at z = 17 - 21 to percent-level precision using VAOs.