Neutral hydrogen as a cosmological probe

The distribution of neutral hydrogen (HI) will be mapped over unprecedented volumes of Universe thanks to the intensity mapping technique with several experiments. Unveiling the cosmological information over a wide scale range requires the understanding of the spatial distribution of HI, a biased tracer of dark matter. Most current approaches of the modeling of clustering are constructed to predict a linearly biased power spectrum on large scales. By doing so they ignore the coupling between small and large scale modes during the build-up of the large scale structure : density fluctuations in denser environments are enhanced as compared to those in less dense ones. Using a perturbative approach I will show how non-linearities have a significant contribution on large scales and how they modify the expected signature of baryonic acoustic oscillations as well as redshift space distortions. In addition, the level of non-linearities depends on how HI populates dark matter halos implying that astrophysics cannot be ignored. I will also show that the observed lack of clustering in the cross-correlation HI x galaxies is due to non-linearities.