Supermassive gravitinos and giant primordial black holes

We argue that the stable (color-singlet) supermassive gravitinos proposed in our previous work can serve as seeds for giant primordial black holes. These seeds are hypothesized to start out as tightly bound states of fractionally charged gravitinos in the radiation-dominated era, whose formation is supported by the universally attractive combination of gravitational and electric forces between the gravitinos and antigravitinos (reflecting their "almost BPS-like" nature). When lumps of such bound states coalesce and undergo gravitational collapse, the resulting mini black holes can escape Hawking evaporation if the radiation temperature exceeds the Hawking temperature. Subsequently the black holes evolve according to an exact solution of Einstein's equations, to emerge as macroscopic black holes in the transition to the matter-dominated era, with masses on the order of the solar mass or larger. The presence of these seeds at such an early time provides ample time for further accretion of matter and radiation, and would imply the existence of black holes of almost any size in the Universe, up to the observed maximum.