Black holes from a Higgs-like field in the radiation era
Light spectator fields during inflation can acquire superhorizon fluctuations that cross a potential barrier between positive and negative regions of their potential. Motivated by the Standard Model Higgs instability, in this work we study the subsequent evolution of patches where this occurs in the radiation era after inflation ends for a Higgs-like spectator field. We utilise fully nonlinear, spherically symmetric numerical relativity. Across the black hole forming configurations in our investigation we find a robust two-stage evolution. First, the central negative potential region reverses its expansion, becomes kinetic dominated, and forms a primordial black hole that hides the runaway core. The positive potential barrier that survives outside this first horizon then determines one of two late-time branches. In the subcritical branch the original apparent horizon grows smoothly and engulfs the remaining scalar structure. In the supercritical branch, however, the potential energy of the barrier dominates the local evolution. The result is a transient wormhole throat, a bifurcating trapping horizon, and an inflating child universe branch. In both branches the parent radiation dominated universe is ultimately left with an ordinary primordial black hole whose subsequent growth is governed by radiation accretion.