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It is shown that 21 cm intensity mapping could be used in the near term tomake cosmologically useful measurements. Large scale structure could bedetected using existing radio telescopes, or using prototypes for dedicatedredshift survey telescopes. This would provide a measure of the mean neutralhydrogen density, using redshift space distortions to break the degeneracy withthe linear bias. We find that with only 200 hours of observing time on theGreen Bank Telescope, the neutral hydrogen density could be measured to 25%precision at redshift 0.54<z<1.09. This compares favourably to currentmeasurements, uses independent techniques, and would settle the controversyover an important parameter which impacts galaxy formation studies. Inaddition, a 4000 hour survey would allow for the detection of baryon acousticoscillations, giving a cosmological distance measure at 3.5% precision. Theseobservation time requirements could be greatly reduced with the construction ofmultiple pixel receivers. Similar results are possible using prototypes fordedicated cylindrical telescopes on month time scales, or SKA pathfinderaperture arrays on day time scales. Such measurements promise to improve ourunderstanding of these quantities while beating a path for future generationsof hydrogen surveys.

Near term measurements with 21 cm intensity mapping: neutral hydrogen fraction and BAO at z<2