The extragalactic background light (EBL) is an indicator of the total luminosity of the universe. It provides unique information on the evolution of cosmic structures at all epochs, as the cumulative emission from galactic systems and AGNs is expected to be recorded in this background. Figure 1 shows the optical EBL from known galaxies together with the recent COBE results. The value derived by integrating the galaxy counts  down to very faint magnitude levels [because of the flattening at faint magnitudes of the N(m) differential counts most of the contribution to the optical EBL comes from relatively bright galaxies] implies a lower limit to the EBL intensity in the 0.3-2.2 µm interval of Iopt 12 n W m-2 sr-1 (1). When combined with the FIRAS and DIRBE measurements (IFIR 16 n W m-2 sr-1 in the 125-5000 µm range), this gives an observed EBL intensity in excess of 28 n W m-2 sr-1. The correction factor needed to account for the residual emission in the 2.2 to 125 µm region is probably 2 . We shall see below how a population of dusty AGNs could make a significant contribution to the FIR background. In the rest of this talk I will adopt a conservative reference value for the total EBL intensity associated with star formation activity over the entire history of the universe of IEBL = 40 I40 n W m-2 sr-1.
Figure 1. Spectrum of the extragalactic background light as derived from a compilation of ground-based and space-based galaxy counts in the U, B, V, I, and K-bands (filled dots), together with the FIRAS 125-5000 µm (solid and dashed lines) and DIRBE 140 and 240 µm (filled squares) detections. The empty squares show the DIRBE points after correction for WIM dust emission .