BLACK-HOLE AND GALAXY FORMATION IN A COLD EARLY UNIVERSE

The only hot models of the early Universe capable of producing primordial black holes would tend to produce them too prolifically to be consistent with observation. However, if the early Universe was cold (photonless), one could expect black holes to form prolifically without contravening observation. This is a self-consistent situation in the sense that the most plausible way to heat the Universe after the hadron era is through black hole accretion. The required accretion could occur providing the holes had at least a tenth of the critical density and providing the radiation was generated in the period 1010-1013 s. This is possible if the density fluctuations in the early Universe have the right form. The same fluctuations on a larger scale could also produce galaxies and clusters of galaxies: these are envisaged as forming out of the gas which collects at the centres of bound black hole clusters. This scenario has the advantages that (1) it could entail a critical density of black holes which are more uniformly distributed than galaxies (as observations would seem to demand), (2) it produces galaxies with black-hole-halos out of small initial density fluctuations, (3) it explains why the times of matter-radiation equilibrium and decoupling are comparable, and (4) it may be able to provide an initial burst of heavy element enrichment through nucleosynthesis in primordial stars. It has the disadvantage that one has to assume that primordial stars generate the observed helium abundance, which may be implausible. © 1977 Royal Astronomical Society. Provided by the NASA Astrophysics Data System.