  There have been a number of suggestions that the r = 0 singularity of a
spherically symmetric (uncharged) evaporating black hole can be circumvented by
a quantum transition to a white hole, which eventually releases all trapped
quantum information, consistent with overall unitary evolution of the quantum
fields. Some of these scenarios rely on loop quantum gravity to impose a
minimum area of two-spheres, but are quite vague on how to deal with black hole
evaporation, particularly its endpoint. In this paper I present a rather
complete toy model for the evolution of the geometry and the effective
stress-energy tensor derived from the geometry via the classical Einstein
equations. Modifications of the Schwarzschild geometry once the formation of
the black hole is complete are very small outside regions of high curvature,
and the curvature never becomes super-Planckian. The evolution of the white
hole is roughlythe time reverse of the formation and evaporation of the black
hole. The mass of the white hole increases as it gradually emits the negative
energy that flowed into the black hole during its evaporation until the matter
and radiaton that collapsed to form the black hole emerges and the white hole
disappears. I consider the compatibility of the model with some of the quantum
energy condition proposed in the literature, and the implications for the
interpretation of black hole entropy.
