Description
Baryon acoustic oscillations (BAO) in the early Universe have provided us with a theoretically well understood standard ruler for measuring cosmological distances. The ruler has been imaged on the sky using correlations between low redshift galaxies (SDSS) and acoustic peaks in the CMB anisotropy spectrum (WMAP).
Future projects aim to produce a ``Double Hubble diagram'' giving the ruler's apparent transverse and radial sizes vs. redshift. This will allow searches for small time drifts in the density of the dark energy driving the acceleration of the expansion of the Universe. I will discuss two projects that hope to do this by using atomic hydrogen as an imaging medium: BOSS-II using Lyman-alpha absorption of background quasars and HSHS using hyperfine 21cm emission.