Cross-correlation cosmography with HI intensity mapping
  The cross-correlation of a foreground density field with two different
background convergence fields can be used to measure cosmographic distance
ratios and constrain dark energy parameters. We investigate the possibility of
performing such measurements using a combination of optical galaxy surveys and
HI intensity mapping surveys, with emphasis on the performance of the planned
Square Kilometre Array (SKA). Using HI intensity mapping to probe the
foreground density tracer field and/or the background source fields has the
advantage of excellent redshift resolution and a longer lever arm achieved by
using the lensing signal from high redshift background sources. Our results
show that, for our best SKA-optical configuration of surveys, a constant
equation of state for dark energy can be constrained to $\simeq 8\%$ for a sky
coverage $f_{\rm sky}=0.5$ and assuming a $\sigma(\Omega_{\rm DE})=0.03$ prior
for the dark energy density parameter. We also show that using the CMB as the
second source plane is not competitive, even when considering a COrE-like
satellite.
