Theoretical and observational constraints on the HI intensity power
  spectrum
  Mapping of the neutral hydrogen (HI) 21-cm intensity fluctuations across
redshifts promises a novel and powerful probe of cosmology. The neutral
hydrogen gas mass density, $\Omega_{\rm HI}$ and bias parameter, $b_{\rm HI}$
are key astrophysical inputs to the HI intensity fluctuation power spectrum. We
compile the latest theoretical and observational constraints on $\Omega_{\rm
HI}$ and $b_{\rm HI}$ at various redshifts in the post-reionization universe.
Constraints are incorporated from galaxy surveys, HI intensity mapping
experiments, damped Lyman-$\alpha$ system observations, theoretical
prescriptions for assigning HI to dark matter halos, and the results of
numerical simulations. Using a minimum variance interpolation scheme, we obtain
the predicted uncertainties on the HI intensity fluctuation power spectrum
across redshifts 0-3.5 for three different confidence scenarios. We provide a
convenient tabular form for the interpolated values of $\Omega_{\rm HI}$,
$b_{\rm HI}$ and the HI power spectrum amplitude and their uncertainties. We
discuss the consequences for the measurement of the power spectrum by current
and future intensity mapping experiments.
