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

Theoretical and observational constraints on the HI intensity power spectrum