We have studied the evolution of an accretion disk around a supermassive black hole (main components of the quasar paradigm), on long timescales due to thermal-viscous instabilities. The luminosity of a quasar varies during its life time due to disk instabilities. Depending on the assumed disk model, the large amplitude variations (delta Log L~4) can be seen in a light curve of one source on timescales of 10^3-10^6 years. We assumed that quasars are subject to such variability. The light curve of a single source convolved with the mass distribution gives the quasar luminosity function (LF) at a given redshift.

The spectral energy distribution for different points on the light curve is presented. The high states (outbursts of activity) are the most luminous and contribute to the UV and X-ray background. The disks in the low states are less luminous and contribute mainly to the IR background. We discuss the contribution to the background from the population of sources given by the LF.

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