PRERECOMBINATION CLOUD COLLAPSE AND BARYONIC DARK-MATTER

A simple spherical model of dense baryon clouds in the hot big bang-'' strongly nonlinear primordial isocurvature baryon fluctuations'' is re viewed and used to describe the dependence of cloud behavior on the mo del parameters, baryon mass, and initial overdensity. Gravitational co llapse of clouds before and during recombination is considered includi ng radiation diffusion and trapping, remnant type and mass, and effect s on linear large-scale fluctuation modes. Sufficiently dense. clouds collapse early into black holes with a minimum mass of approximately 1 M., which behave dynamically like collisionless cold dark matter. Clo uds below a critical overdensity, however, delay collapse until recomb ination, remaining until then dynamically coupled to the radiation lik e ordinary diffuse baryons, and possibly producing remnants of other k inds and lower mass. The mean density in either type of baryonic remna nt is unconstrained by observed element abundances. However, mixed or unmixed spatial variations in abundance may survive in the diffuse bar yon and produce observable departures from standard predictions.