  The ionizing background of cosmic hydrogen is an important probe of the
sources and absorbers of ionizing radiation in the post-reionization universe.
Previous studies show that the ionization rate should be very sensitive to
changes in the source population: as the emissivity rises, absorbers shrink in
size, increasing the ionizing mean free path and, hence, the ionizing
background. By contrast, observations of the ionizing background find a very
flat evolution from z~2-5, before falling precipitously at z~6. We resolve this
puzzling discrepancy by pointing out that, at z~2-5, optically thick absorbers
are associated with the same collapsed halos that host ionizing sources. Thus,
an increasing abundance of galaxies is compensated for by a corresponding
increase in the absorber population, which moderates the instability in the
ionizing background. However, by z~5-6, gas outside of halos dominates the
absorption, the coupling between sources and absorbers is lost, and the
ionizing background evolves rapidly. Our halo based model reproduces
observations of the ionizing background, its flatness and sudden decline, as
well as the redshift evolution of the ionizing mean free path. Our work
suggests that, through much of their history, both star formation and
photoelectric opacity in the universe track halo growth.
