The universe is full of neutral hydrogen that shines in the 21-cm spin-flip transition. Mapping the large scale structure of the universe in 3D using this line is in principle an extremely cheap and simple, but has in practice proven to be devilishly difficult. I will argue that this technique nevertheless offers the most promising way to complete the programmatic goal of measuring the linear scales of the universe deep into the pre-acceleration era after the current crop of optical experiments complete observing. It will give us expansion history measurements to z=6 using Baryon Acoustic Oscillations, several new screens to study weak gravitational lensing, unprecedented constraints on primordial non-Gaussianity and even allow direct measurements of real-time expansion of the universe. I will discuss the scientific, technological and programmatic issues that we need to overcome to make such an experiment a reality. I present some recent advances in our thinking on both how to build an optimal experiment and how to extract most information from it.
Host: Ben Saliwanchik