Annu. Rev. Astron. Astrophys. 1992. 30: 653-703
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The observations described above show clearly that, from angular scales of arc seconds to tens of degrees, observations of the microwave background radiation have reached sensitivity levels at which the presence of confusing signals in the foreground are significant and therefore must be subtracted before significant improvements in sensitivity can be achieved. The major known sources of these confusing signals are discrete extragalactic radio sources, hot clusters of galaxies in which the Sunyaev Zel'dovich effect is appreciable, Galactic synchrotron radiation, Galactic free-free emission, and Galactic thermal emission from dust. Of these, the Sunyaev Zel'dovich Effect has important implications for cosmology, and is not simply a confusing foreground signal to be eliminated from microwave background radiation observations. We discuss each of these confusing signals separately below and we also discuss the means which can be employed in subtracting these unwanted foreground effects to reveal the intrinsic distribution. Since each of these subjects is highly developed, our discussion is focussed upon those aspects of these confusing signals which have particular relevance to microwave background radiation observations, and is not intended as a complete review of any of these topics.

The effects discussed above are known to exist, but there is another important possible source of confusing ``foreground'' signal: the scattering, both elastic and inelastic, from free electrons, and the emission from dust which could occur if there is no recombination, if the intergalactic medium is reionized at z >> 1, or if there is an early generation of stars which produces appreciable ultraviolet radiation and dust. This is in a very different category to the effects listed above, since, although it would render difficult the determination of the primordial fluctuations at z approx 1500, it would provide a powerful means of studying epochs for which 1500 >> z >> 1.