/ AbstractThis 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 $Λ$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.