  Using the 21 cm line, observed all-sky and across the redshift range from 0
to 5, the large scale structure of the Universe can be mapped in three
dimensions. This can be accomplished by studying specific intensity with
resolution ~ 10 Mpc, rather than via the usual galaxy redshift survey. The data
set can be analyzed to determine Baryon Acoustic Oscillation wavelengths, in
order to address the question: 'What is the nature of Dark Energy?' In
addition, the study of Large Scale Structure across this range addresses the
questions: 'How does Gravity effect very large objects?' and 'What is the
composition our Universe?' The same data set can be used to search for and
catalog time variable and transient radio sources.
