Abstract
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$\,deg^2$ 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 that capable from current galaxy surveys in
optical and near-infrared. The detection significance falls to $\sim1\sigma$
in MeerKAT's L-band but can reach $\sim13\sigma$ with the SKAO, which should
easily surpass the constraint capable 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.
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