  We forecast constraints on neutral hydrogen (HI) and cosmological parameters
using near-term intensity mapping surveys with instruments such as BINGO,
MeerKAT, and the SKA, and Stage III and IV optical galaxy surveys. If
foregrounds and systematic effects can be controlled - a problem which becomes
much easier in cross-correlation - these surveys will provide exquisite
measurements of the HI density and bias, as well as measurements of the growth
of structure, the angular diameter distance, and the Hubble rate, over a wide
range of redshift. We also investigate the possibility of detecting the late
time ISW effect using the Planck satellite and forthcoming intensity mapping
surveys, finding that a large sky survey with Phase 1 of the SKA can achieve a
near optimal detection.
