The Cosmic Expansion History from Line-Intensity Mapping
  Line-intensity mapping (LIM) of emission form star-forming galaxies can be
used to measure the baryon acoustic oscillation (BAO) scale as far back as the
epoch of reionization. This provides a standard cosmic ruler to constrain the
expansion rate of the Universe at redshifts which cannot be directly probed
otherwise. In light of growing tension between measurements of the current
expansion rate using the local distance ladder and those inferred from the
cosmic microwave background, extending the constraints on the expansion history
to bridge between the late and early Universe is of paramount importance. Using
a newly derived methodology to robustly extract cosmological information from
LIM, which minimizes the inherent degeneracy with unknown astrophysics, we show
that present and future experiments can gradually improve the measurement
precision of the expansion rate history, ultimately reaching percent-level
constraints on the BAO scale. Specifically, we provide detailed forecasts for
the SPHEREx satellite, which will target the H$\alpha$ and Lyman-$\alpha$
lines, and for the ground-based COMAP instrument---as well as a future stage-3
experiment---that will target the CO rotational lines. Besides weighing in on
the so-called Hubble tension, reliable LIM cosmic rulers can enable
wide-ranging tests of dark matter, dark energy and modified gravity.
