Extragalactic Background Light, MACHOs, and the Cosmic Stellar Baryon   Budget
  The optical/far-IR extragalactic background light (EBL) from both resolved and unresolved extragalactic sources is an indicator of the total luminosity of cosmic structures, as the cumulative emission from young and evolved galactic systems, as well as from active galactic nuclei (AGNs), is recorded in this radiation. This is a brief review of some of the implications of the observed brightness of the night sky for the stellar mass density and average metallicity of the universe today, and of the possible contribution of MACHO progenitors and QSOs to the EBL. Assuming a Salpeter initial mass function with a cutoff below 0.6 M_sun, a lower limit of Omega_{g+s}h^2>0.0015 I_{60} can be derived to the visible (recycled gas + stars) mass density required to generate an EBL at a level of I_{EBL}=60 I_{60} nW/m^2/sr: our latest, best-guess estimate is Omega_{g+s}h^2 ~ 0.0023 I_{60}, which implies a mean metallicity at the present-epoch of y_Z Omega_{g+s}/Omega_b ~ 0.2 Z_sun. If massive dark halos around spiral galaxies are partially composed of faint, old white dwarfs, i.e. if a non-negligible fraction (a few percent) of the nucleosynthetic baryons is locked in the remnants (MACHOs) of intermediate-mass stars forming at very high redshifts, then the bright early phases of such halos should contribute significantly to the observed EBL. Assuming a standard black hole accretion model for quasar activity and using recent observations of the quasar population and new synthesis models for the cosmic X-ray background, we estimate a present mass density of QSO remnants of rho_{BH} ~ 3e5 M_sun/Mpc^3 for a 10% efficiency of accreted mass-to-radiation conversion. The quasar contribution to the brightness of the night sky is I_QSO ~ 2 nW/m^2/sr. 
Piero Madau, Francesco Haardt, and Lucia Pozzetti