THE COSMIC ULTRAVIOLET BACKGROUND

Despite much recent progress, a coherent description of the thermal state and ionization degree 
of the intergalactic medium (IGM) remains elusive. It is generally thought that the IGM is kept
ionized by the integrated UV emission from active nuclei and star-forming galaxies, but the relative 
contributions of these sources as a function of epoch are poorly known.The intensity and spectrum of 
the cosmic ultraviolet background remain one of the most uncertain yet critically important astrophysical 
input parameters for cosmological simulations of the IGM and for interpreting quasar absorption-line 
data and derive information on the distribution of primordial baryons  and of the 
nucleosynthetic products of star formation. 

In this talk, I will review recent observations and models of the evolving spectrum of the cosmic
ionizing  background from quasar sources and star forming galaxies. I will discuss how models need
to include features such as: (1) the sawtooth modulation from resonant line absorption in the Lyman
series of  intergalactic hydrogen and helium; (2) the X -ray emission from obscured and unobscured
active galactic nuclei (AGNs); (3) an up-to-date piecewise parameterization for the distribution in
column density of intervening absorbers; and (4) an accurate treatment of their photoionization structure,
which enters in the calculation of the helium opacity and of recombination radiation. Finally, I will
discuss how models are sensitive to a number  of poorly determined input parameters and suffer from various
degeneracies, stressing how their predictive power should be constantly tested against new observations.