  Rotating or charged classical black holes in isolation possess a special
surface in their interior, the Cauchy horizon, beyond which the evolution of
spacetime (based on the equations of General Relativity) ceases to be
deterministic. In this work, we study the effect of a quantum massless scalar
field on the Cauchy horizon inside a rotating (Kerr) black hole that is
evaporating via the emission of Hawking radiation (corresponding to the field
being in the Unruh state). We calculate the flux components (in Eddington
coordinates) of the renormalized stress-energy tensor of the field on the
Cauchy horizon, as functions of the black hole spin and of the polar angle. We
find that these flux components are generically non-vanishing. Furthermore, we
find that the flux components change sign as these parameters vary. The signs
of the fluxes are important, as they provide an indication of whether the
Cauchy horizon expands or crushes (when backreaction is taken into account).
Regardless of these signs, our results imply that the flux components
generically diverge on the Cauchy horizon when expressed in coordinates which
are regular there. This is the first time that irregularity of the Cauchy
horizon under a semiclassical effect is conclusively shown for
(four-dimensional) spinning black holes.
