An accurate low-redshift measurement of the cosmic neutral hydrogen
  density
  Using a spectral stacking technique, we measure the neutral hydrogen (HI)
properties of a sample of galaxies at $z < 0.11$ across 35 pointings of the
Westerbork Synthesis Radio Telescope (WSRT). The radio data contains 1,895
galaxies with redshifts and positions known from the Sloan Digital Sky Survey
(SDSS). We carefully quantified the effects of sample bias, aperture used to
extract spectra, sidelobes and weighting technique and use our data to provide
a new estimate for the cosmic HI mass density. We find a cosmic HI mass density
of $\Omega_{\rm HI} = (4.02 \pm 0.26)\times 10^{-4} h_{70}^{-1}$ at $\langle
z\rangle = 0.066$, consistent with measurements from blind HI surveys and other
HI stacking experiments at low redshifts. The combination of the small
interferometer beam size and the large survey volume makes our result highly
robust against systematic effects due to confusion at small scales and cosmic
variance at large scales. Splitting into three sub-samples with $\langle
z\rangle$ = 0.038, 0.067 and 0.093 shows no significant evolution of the HI gas
content at low redshift.
