Forecasts for Dark Energy Measurements with Future HI Surveys
  We use two independent methods to forecast the dark energy measurements
achievable by combining future galaxy redshift surveys based on the radio HI
emission line with Cosmic Microwave Background (CMB) data from the {\sl Planck}
satellite. In the first method we focus on the `standard ruler' provided by the
baryon acoustic oscillation (BAO) length scale. In the second method we utilize
additional information encoded in the galaxy power spectrum including galaxy
bias from velocity-space distortions and the growth of cosmic structure. We
find that a radio synthesis array with about 10 per cent of the collecting area
of the Square Kilometre Array (SKA), equipped with a wide ($10-100 ~ {\rm
deg}^2$) field-of-view, would have the capacity to perform a $20{,}000 ~ {\rm
deg}^2$ redshift survey to a maximum redshift $z_{\rm max} \sim 0.8$ and
thereby produce dark energy measurements that are competitive with surveys
likely to be undertaken by optical telescopes around 2015. There would then be
powerful arguments for adding collecting area to such a `Phase-1' SKA because
of the square-law scaling of survey speed with telescope sensitivity for HI
surveys, compared to the linear scaling for optical redshift surveys. The full
SKA telescope should, by performing a $20{,}000 ~ {\rm deg}^2$ HI redshift
survey to $z_{\rm max} \sim 2$ around 2020, yield an accurate measurement of
cosmological parameters independent of CMB datasets. Combining CMB ({\sl
Planck}) and galaxy power spectrum (SKA) measurements will drive errors in the
dark energy equation-of-state parameter $w$ well below the 1 per cent level.
The major systematic uncertainty in these forecasts is the lack of direct
information about the mass function of high-redshift HI-emitting galaxies.
