Fate of gravitational collapse in semiclassical gravity

While the outcome of gravitational collapse in classical general relativity is unquestionably a black hole, up to now no full and complete semiclassical description of black hole formation has been thoroughly investigated. Here we revisit the standard scenario for this process. By analyzing how semiclassical collapse proceeds we show that the very formation of a trapping horizon can be seriously questioned for a large set of, possibly realistic, scenarios. We emphasize that in principle the theoretical framework of semiclassical gravity certainly allows the formation of trapping horizons. What we are questioning here is the more subtle point of whether or not the standard black hole picture is appropriate for describing the end point of realistic collapse. Indeed if semiclassical physics were in some cases to prevent formation of the trapping horizon, then this suggests the possibility of new collapsed objects which can be much less problematic, making it unnecessary to confront the information paradox or the runaway end point problem.

Figure 1

Standard conformal diagram for a collapsing star, and its mirror-symmetric version.

Figure 2

Conformal diagram of the spacetime for a quasi-black hole. The solid line represents the surface of the collapsing object; the dotted line is at $r=2M(t)$, where $M(t)$ is the instantaneous mass of the object as measured from ${\mathcal{I}}^{+}$; dashed lines correspond to (Schwarzschild) $t=\mathrm{const}$ hypersurfaces. The period of evaporation appears short because of a distortion induced by the representation, but actually corresponds to a very long lapse of time, as one can see from the fact that the lines at $t=\mathrm{const}$ crowd around it. This diagram is compatible with current astrophysical observations of gravitationally active collapse products.