Semiclassical energy fluxes at the inner horizon of a rotating black hole

Astrophysical black holes are known to be rotating. The simplest spacetime solution describing a classical rotating black hole (the Kerr solution) reveals a non-trivial spacetime structure, in which the geometry connects through an inner horizon to another external universe. But does such a traversable passage really exist inside a physically-realistic spinning black hole?

Answering this question, along others, requires one to understand the manner in which quantum energy fluxes affect the internal geometry of a black hole. It has been widely anticipated, yet inconclusive (till this work), that semiclassical effects would diverge at the inner horizon of a spinning black hole. Such a divergence, if indeed takes place, may drastically affect the internal black hole geometry, potentially preventing the inner horizon traversability. Clarifying this issue requires the computation of the quantum energy fluxes in black hole interiors. However, this has been a serious challenge for decades. 

Using a combination of new and old methods, we have recently managed to compute the semiclassical energy fluxes at the inner horizon of a spinning black hole, in a vacuum state corresponding to an evaporating black hole. We found that these fluxes are either positive or negative, depending on the black hole spin (and polar angle). The sign of these fluxes may be crucial to the nature of their backreaction on the geometry (as should be dictated by the semiclassical Einstein equation).

In this talk, we shall describe the basic framework of semiclassical general relativity and the regularization procedure, and then present our novel results for the semiclassical fluxes at the inner horizon of a rotating black hole, briefly mentioning possible implications for the inner horizon traversability.