Black hole evaporation endpoint

4 problems

• Does an evaporating black hole tunnel into a white hole?

  Open

  Some quantum gravity models predict that the collapsed matter bounces back, turning the black hole into a white hole that releases what fell in. The predicted lifetime, and therefore any observable signal, depends on poorly controlled quantum effects.

• What remains after a black hole evaporates to the Planck scale?

  Open

  Hawking's calculation stops working when the black hole shrinks to about $2 \times 10^{-5}\,\mathrm{g}$, the Planck mass. The options include complete disappearance into radiation, a stable tiny leftover, a white hole, or a disconnected baby universe.

• Are long-lived Planck-mass remnants consistent with quantum field theory?

  Contested

  If tiny black-hole leftovers stored all the information, each would need an enormous number of internal states. Such objects might then be produced in huge numbers in ordinary processes, contradicting experiment.

• Self-consistent evaporation of a 4D black hole including back-reaction

  Open

  Most evaporation calculations assume the black hole shrinks slowly while keeping its shape. A full four-dimensional calculation in which the radiation reshapes spacetime as it goes has not been done.