Super-horizon primordial black holes

Abstract

We discuss a new class of solutions to the Einstein equations which describe a primordial black hole (PBH) in a flat Friedmann background. Such solutions arise if a Schwarzschild black hole is patched onto a Friedmann background via a transition region. They are possible providing the black hole event horizon is larger than the cosmological apparent horizon. Such solutions have a number of strange features. In particular, one has to define the black hole and cosmological horizons carefully and one then finds that the mass contained within the black hole event horizon decreases when it is larger than the Friedmann cosmological apparent horizon, although its area always increases. These solutions involve two distinct future null infinities and are interpreted as the conversion of a white hole into a black hole. Although such solutions may not form from gravitational collapse in the same way as standard PBHs, there is nothing unphysical about them, since all energy and causality conditions are satisfied. Their conformal diagram is a natural amalgamation of the Kruskal diagram for the extended Schwarzschild solution and the conformal diagram for a black hole in a flat Friedmann background. In this paper, such solutions are obtained numerically for a spherically symmetric universe containing a massless scalar field, but it is likely that they exist for more general matter fields and less symmetric systems.

Nonexistence of self-similar solutions containing a black hole in a universe with a stiff fluid or scalar field or quintessence

“…It also suggests that accretion onto black holes is suppressed by the cosmological expansion in a scalar field system. This is consistent with numerical simulations, which show that PBHs with different initial sizes eventually become much smaller than the particle horizon [29,30]. Note that, for the massless case, it has already been shown analytically that the accretion rate is exactly zero for a black hole whose size is exactly that of the cosmological apparent horizon [29,30].…”

Gravitational collapse of a massless scalar field in a periodic box

“…However, as is reported in Ref. [26] for a spherically symmetric system, such decreasing area of a horizon can be realized by transition from a past outer trapping horizon(POTH) to a future outer trapping horizon(FOTH) defined in Ref. [27].…”

Some results on the evolution of primordial black holes

“…1) c) PBHs growing substantially by accreting quintessence fields [10] are fine-tuned in Newtonian gravity [11], and vanish entirely in a fully relativistic treatment [12].…”