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 th
an 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.
