Catastrophic fate of Schwarzschild black holes in a thermal bath

Using the Schwarzschild metric as a rudimentary toy model, we pedagogically revisit the curious prediction that the mass of a classical black hole in a constant temperature thermal bath diverges in a finite amount of time. We study in detail how this instability behaves if the temperature of the bath is allowed to vary with time and conclude that whatever the background behavior (but for a zero-measure subspace of the initial conditions), the black hole mass either diverges or vanishes in a finite time if the Hawking radiation is taken into account. The competition between both effects is subtle and not entirely governed by the hierarchy of the relevant temperatures. This instability is also shown to be reached before the background singularity in a contracting universe, which has implications for bouncing models. The results are generalized to spaces with extra dimensions, and the main conclusions are shown to remain true. The limitations of the model are reviewed, both from the point of view of the dynamical black hole horizon and from the point of view of the background space expansion. Comparisons with other approaches are suggested and possible developments are underlined.

Automated summary

Using the Schwarzschild metric as a toy model, this article revisits and explores the prediction that the mass of a classical black hole in a constant temperature thermal bath either diverges or vanishes in a finite amount of time. The study considers the behavior of this instability when the temperature of the bath varies with time and shows that the competition between the effects of temperature and Hawking radiation is subtle and not entirely determined by temperature hierarchy. The results are also extended to spaces with extra dimensions and the implications for bouncing models are discussed.