The X- and gamma-ray backgrounds

The spectrum of the extragalactic X-ray and gamma-ray backgrounds, originating in a redshift of about 3.0, provides a measurement of the integrated X- and gamma-ray evolution of known sources such as clusters of galaxies, active galaxies, and quasars. It also limits possible new classes of sources, such as accreting black holes and a hot intergalactic medium. Attention is given to the contribution to the X- and gamma-ray backgrounds of known evolved and unevolved sources, as well as the character of small scale and large scale fluctuations in the X-ray background. Speculations as to the origin of the X- and gamma-ray background include such possibilities as the inverse Compton scattering of microwave background photons by intergalactic cosmic-ray electrons, accretion of the intergalactic medium onto pregalactic black holes, and matter-antimatter annihilation in a baryon-symmetric universe