Detecting the power spectrum turnover with HI intensity mapping
  A goal for pathfinder intensity mapping (IM) surveys will be detecting
features in the neutral hydrogen (HI) power spectrum, which serve as conclusive
evidence of cosmological signals. Observing such features at the expected
scales in HI IM auto-correlations, where contribution from systematics is
uncertain, will provide a more convincing cosmological detection. We
demonstrate how the turnover, i.e. the peak of the power spectrum at
ultra-large scales, can be detected with HI IM. We find that a MeerKAT
4,000$\,{\rm deg}^2$ survey using the UHF-band is capable of a $3.1\sigma$
detection of the turnover, relative to a null model power spectrum with no
turnover. This should exceed what is capable by current galaxy surveys in
optical and near-infrared. The detection significance falls to ${\sim}1\sigma$
in MeerKAT's L-band but can reach ${\sim}13\sigma$ with the SKAO, which should
easily surpass the constraints from future Stage-IV-like spectroscopic galaxy
surveys. We also propose a new model-independent methodology for constraining
the precise turnover scale ($k_0$) and our tests on UHF-band simulated data
achieved a precision of 10%. This improved to 2.4% when using the full SKAO. We
demonstrate how the results are robust to foreground contamination by using
transfer functions, even when an incorrect cosmology has been assumed in their
construction. Given that the turnover is related to the horizon scale at
matter-radiation equality, a sufficiently precise constraint of $k_0$ presents
the possibility for a novel probe of cosmology. We therefore present a
potential methodology for constructing a standard-ruler-based distance
measurement, independent of the sound horizon, using the turnover location in
the HI power spectrum.
