The X-ray Background (XRB) is the integrated emission from all X-ray sources. Its hard spectrum has proved difficult to explain since, in the 2-10 keV band, it is flat with a power-law of energy index 0.4. This is flatter than the spectrum of any known common population of objects. For the last decade the most popular explanation has been that the XRB intensity is dominated by many absorbed sources (Setti & Woltjer 1989), with ranges of absorbing column density and redshift causing the observed spectrum to be a power-law. The absorption model has been extensively studied by Madau et al (1994), Matt & Fabian (1994), Comastri et al (1995), Celotti et al (1995), and Wilman & Fabian (1999). The most complete studies include Compton down-scattering in the estimation of the observed spectrum of the Compton-thick sources.
The absorption model is adopted here and is used in a simple way to show that black holes grow by radiatively efficient accretion and to determine a) the local mean density of black holes, b) the fraction of accretion power which has been absorbed, and c) contraints on the fraction of power in the Universe due to accretion (see also Fabian & Iwasawa 1999). After some discussion of how so much obscuring material can surround most sources, and how the nuclei might be fuelled, I then outline a model of obscuration in a forming, isothermal galaxy spheroid (Fabian 1999). The XRB is shown to be a key diagnostic of the accretion power of the Universe.