Mapping the Universe in HD
  Hydrogen deuteride (HD) is prevalent in a wide variety of astrophysical
environments, and measuring its large-scale distribution at different epochs
can in principle provide information about the properties of these
environments. In this paper, we explore the prospects for accessing this
distribution using line intensity mapping of emission from the lowest
rotational transition in HD, focusing on observations of the epoch of
reionization ($z\sim6-10$) and earlier. We find the signal from the epoch of
reionization to be strongest most promising, through cross-correlations within
existing [CII] intensity mapping surveys. While the signal we predict is out of
reach for current-generation projects, planned future improvements should be
able to detect reionization-era HD without any additional observations, and
would help to constrain the properties of the star-forming galaxies thought to
play a key role in reionization. We also investigate several avenues for
measuring HD during "cosmic dawn" ($z\sim10-30$), a period in which HD could
provide one of the only complementary observables to 21$\,$cm intensity maps.
We conclude that existing and planned facilities are poorly matched to the
specifications desirable for a significant detection, though such a measurement
may be achievable with sustained future effort. Finally, we explain why HD
intensity mapping of the intergalactic medium during the cosmic dark ages
($z\gtrsim 30$) appears to be out of reach of any conceivable experiment.
