Small-Scale Variations in the Cosmic Microwave Background

A discussion is given of some processes which produce fluctuations in the 3 °K radiation on a small angular scale $$(\leqslant 1^{\circ}).$$ Apart from fluctuations caused by a primordial spectrum of spatial inhomogeneities in the matter and radiation densities, effects of scattering on the gravitational fields of clusters of galaxies as well as scattering by gravitational radiation of extremely large coherence or wave-lengths are considered. While scattering by static gravitational field perturbations may be neglected in most cases, scattering by gravitational radiation with large wave-lengths may produce variations on an observable level. The measurements by Conklin & Bracewell provide upper limits on the equivalent mass density of a gravitational radiation background for coherence lengths $$L \gt 1$$ Mpc. The fluctuations may be strongly affected by electron scattering in ionized intergalactic matter.