Astronomy Colloquium

  The Lyman alpha forest remains one of the most robust predictions of cosmological hydrodynamic simulations.  Lyman alpha absorption lines have been used for decades to trace cosmic structures that -- only recently, with the exquisite sensitivity achievable with modern instruments -- are beginning to be detected in emission as well. I will discuss the predicted morphology of the Lyman alpha forest and the possibility of revealing this structure with Lyman alpha imaging surveys.  The emission signal is dependent on the ionizing background radiation which, at high redshift, is well-understood and constrained.  However, I will show that at low redshift there is a huge mismatch between our expectations and observations.  I describe a factor of 5 discrepancy between the value of the photoionization rate required to match cosmological models of the z = 0 intergalactic medium to  observations of the Lyman alpha forest and the value predicted by state-of-the-art models that account for the emissivity of stars and quasars over time.  Examining potential resolutions to this problem, I will demonstrate that solving it requires, at minimum, a major revision of our thinking about the low redshift universe.