  This white paper envisions a revolutionary post-DESI, post-LSST dark energy
program based on intensity mapping of the redshifted 21cm emission line from
neutral hydrogen at radio frequencies. The proposed intensity mapping survey
has the unique capability to quadruple the volume of the Universe surveyed by
optical programs, provide a percent-level measurement of the expansion history
to $z \sim 6$, open a window to explore physics beyond the concordance
$\Lambda$CDM model, and to significantly improve the precision on standard
cosmological parameters. In addition, characterization of dark energy and new
physics will be powerfully enhanced by cross-correlations with optical surveys
and cosmic microwave background measurements. The rich dataset obtained by the
proposed intensity mapping instrument will be simultaneously useful in
exploring the time-domain physics of fast radio transients and pulsars,
potentially in live "multi-messenger" coincidence with other observatories. The
core dark energy/inflation science advances enabled by this program are the
following: (i) Measure the expansion history of the universe over $z=0.3-6$
with a single instrument, extending the range deep into the pre-acceleration
era, providing an unexplored window for new physics; (ii) Measure the growth
rate of structure in the universe over the same redshift range; (iii) Observe,
or constrain, the presence of inflationary relics in the primordial power
spectrum, improving existing constraints by an order of magnitude; (iv)
Observe, or constrain, primordial non-Gaussianity with unprecedented precision,
improving constraints on several key numbers by an order of magnitude. Detailed
mapping of the enormous, and still largely unexplored, volume of cosmic space
will thus provide unprecedented information on fundamental questions of the
vacuum energy and early-universe physics.
