The origin of the diffuse background has been an important question in X-ray astronomy starting from the earliest measurements (Giacconi, et al. 1962). When it was recognized that the X-ray background above 2 keV was isotropic and hence extragalactic (c.f. Schwartz 1970, 1979), it became evident that understanding its origin would have cosmological implications (c.f. Hoyle 1963, Rees 1973). Two general explanations representing opposite points of view have received the most attention. One is that the background is composed of faint unresolved objects which are of the same classes as, but perhaps in earlier phases than, the objects which can be detected directly and identified. If this explanation is correct, then the luminosities and/or proper densities of the objects must be larger at earlier times in the universe, and the magnitude and graininess of the background can be used to place important constraints upon the evolution of the objects. The other is that the background is truly diffuse. The most probable explanation of this type is a hot plasma that pervades the intergalactic medium. If it accounted for only 10% of the background, the mass that could be attributed to such a plasma would represent more matter in the universe than has been detected so far by all other means. Such a large mass would be important dynamically, and in determining the deceleration parameter, although it would not “close” the universe. Furthermore, the density of such a plasma would place constraints upon determining the epoch of galaxy formation.
Fluctuations in the X-ray background and the large-scale structure of the Universe