  In a previous paper we have argued that primordial black holes can arise from
the formation and subsequent gravitational collapse of bound states of stable
supermassive elementary particles (gravitinos) during the early radiation era.
Here we offer a comprehensive picture, describing the evolution and growth of
the resulting mini-black holes through both the radiation and matter dominated
phases until the onset of inhomogeneities, by means of an exact metric solving
Einstein's equations. We show that, thanks to a special enhancement effect
producing an effective horizon above the actual event horizon, this process can
explain the observed mass values of the earliest giant black holes.
