Cosmic Black-Body Radiation and Galaxy Formation

The possibility is examined that galaxies may have formed from an initial spectrum of primordial fiuc- tuations of small amplitude (although not necessarily of small scale) With a cosmological interpretation of the 3° K microwave background radiation, it is found that fluctuations containing more than 1O~ Mo, but less than a certain critical size, would be optically thick at an epoch subsequent to the primordial fireball (i.e, T < 1010 ° K), and would be damped out by radiative diffusion on a time scale short com- pared to the expansion time This critical size corresponds to the mass of a typical galaxy, and is of the order of 1011 Mo. The survival of primordial fluctuations to an epoch when galaxy formation may occur is found to imply an angular anisotropy in the 30 K radiation of between approximately tO" and 30", depending on the mean value taken for the present density of matter in the Universe. An upper limit is also derived on the mass of a gravitationally bound condensation by considering the destabilizing tendency of the radiation A consequence of this result is that any large-scale (»= 100 Mpc) anisotropy in the Universe would be consistent with a value for the mean density of matter at the present epoch of on'y about 10-30 g/cm3 or less