Detecting the power spectrum turnover with H i intensity mapping

Abstract

ABSTRACT A goal for pathfinder intensity mapping (IM) surveys will be detecting features in the neutral hydrogen (${{\rm H}\, \small {\rm I}}$) power spectrum, which serve as conclusive evidence of cosmological signals. Observing such features at the expected scales in ${{\rm H}\, \small {\rm I}}$ IM autocorrelations, 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 ${{\rm H}\, \small {\rm I}}$ IM. We find that a MeerKAT 4000$\, \text{deg}^2$ survey using the UHF band is capable of a 3.1σ 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 ∼1σ in MeerKAT’s L band but can reach ∼13σ with the Square Kilometre Array Observatory (SKAO), which should easily surpass the constraintsno from future Stage-IV-like spectroscopic galaxy surveys. We also propose a new model-independent methodology for constraining the precise turnover scale (k0) and our tests on UHF-band simulated data achieved a precision of 10 per cent. This improved to 2.4 per cent 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 k0 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 ${{\rm H}\, \small {\rm I}}$ power spectrum.