  Line-intensity mapping of the 21cm line is a powerful probe of large scale
structure at z<6, tracing large-scale structure via neutral hydrogen content
that is found within galaxies. In principle, it enables cost-efficient surveys
of the matter distribution up to z~6, unlocking orders of magnitude more modes
for observational cosmology. Canada has been a traditional leader in this
field, having led the first detections of the cosmological 21cm signal via
cross-correlations with optical galaxy surveys and having constructed the
Canadian Hydrogen Intensity Mapping Experiment (CHIME). The field is now
entering a new era where data is abundant, allowing studies in how to overcome
systematics to be tackled in an empirical, head-on fashion. In the next few
years, this will produce the first detection of the 21cm auto power spectrum,
which will pave the way towards a large suite of scientific possibilities.
These potentially include precision measurements on the dark energy equation of
state and other LCDM parameters, constraints on how HI mass traces dark matter,
a detection of neutrino effects on large-scale structure, and the use of 21cm
lensing to further constrain cosmology. To turn these promising directions into
reality, we recommend a sustained program of investment in 21cm cosmology,
starting with funding for the Canadian Hydrogen Observatory and Radio transient
Detector (CHORD), followed by small-scale development efforts targeting
next-generation hardware and sustained support for theory and technical staff
support. Additionally, Canada should invest in complementary line-intensity
mapping efforts (such as with CO or [CII] lines) and maintain participation in
next-generation international efforts such as the Packed Ultra-wideband Mapping
Array (PUMA) and the Square Kilometre Array (SKA).
