  Intensity mapping of the neutral hydrogen brightness temperature promises to
provide a three-dimensional view of the universe on very large scales.
Nonlinear effects are typically thought to alter only the small-scale power,
but we show how they may bias the extraction of cosmological information
contained in the power spectrum on ultra-large scales. For linear perturbations
to remain valid on large scales, we need to renormalize perturbations at higher
order. In the case of intensity mapping, the second-order contribution to
clustering from weak lensing dominates the nonlinear contribution at high
redshift. Renormalization modifies the mean brightness temperature and
therefore the evolution bias. It also introduces a term that mimics white
noise. These effects may influence forecasting analysis on ultra-large scales.
