# A Standard Ruler at Cosmic Dawn

## TL;DR

This paper proposes using velocity-induced acoustic oscillations in the 21-cm signal from cosmic dawn as a standard ruler to measure the Universe's expansion rate at high redshifts, leveraging upcoming HERA data.

## Contribution

It introduces VAOs as a robust, astrophysics-independent standard ruler for cosmic dawn, enabling precise measurements of the Hubble parameter at high redshifts.

## Key findings

- HERA can measure H(z) at z=15-20 with 0.3% to 11% precision.
- VAOs are unaffected by star formation uncertainties.
- Potential to probe the Universe's expansion during cosmic dawn.

## Abstract

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.