SEMIINFINITE THROAT AS THE END-STATE GEOMETRY OF 2-DIMENSIONAL BLACK-HOLE EVAPORATION

We study a modified two-dimensional dilaton gravity theory which is exactly solvable in the semiclassical approximation including back reaction. Infalling matter in an initially static radiationless spacetime forms a black hole if its energy is above a certain threshold. The black hole singularity is initially hidden behind a timelike apparent horizon. As the black hole evaporates by emitting Hawking radiation, the singularity meets the shrinking horizon in finite retarded time to become naked. A boundary condition exists at the naked singularity which preserves energy conservation, stability, and continuity of the metric and results in a unique end state for all evaporating black holes. The end-state geometry is static and asymptotically flat at its right spatial infinity, while its left spatial infinity is a semi-infinite throat extending into the strong coupling region. This end-state geometry is the ground state in our model.