The Photon Underproduction Crisis
  We examine the statistics of the low-redshift Lyman-alpha forest from
smoothed particle hydrodynamic simulations in light of recent improvements in
the estimated evolution of the cosmic ultraviolet background (UVB) and recent
observations from the Cosmic Origins Spectrograph (COS). We find that the value
of the metagalactic photoionization rate required by our simulations to match
the observed properties of the low-redshift Lyman-alpha forest is a factor of 5
larger than the value predicted by state-of-the art models for the evolution of
this quantity. This mismatch results in the mean flux decrement of the
Lyman-alpha forest being underpredicted by at least a factor of 2 (a 10-sigma
discrepancy with observations) and a column density distribution of Lyman-alpha
forest absorbers systematically and significantly elevated compared to
observations over nearly two decades in column density. We examine potential
resolutions to this mismatch and find that either conventional sources of
ionizing photons (galaxies and quasars) must be significantly elevated relative
to current observational estimates or our theoretical understanding of the
low-redshift universe is in need of substantial revision.
