Black Hole Radiation and S-matrix

The existence of an S-matrix below the thresholdof black hole formation would be enough to exhibit,through its analytic structure, eventual thresholds forthe creation of new objects and to describe, through analytic continuation, the physicsabove them in a unitary framework. In the context of atwo-dimensional exactly soluble model, the semiclassicaldynamics of quantum black holes is obtained by analytically continuing the description of theregime where no black hole is formed. The resultingspectrum of outgoing radiation departs from the onepredicted by the Hawking model by the time the outgoing modes arise from the horizon with Planck-orderfrequencies. The theory predicts an unconventionalscenario for the evolution: black holes only radiate outan energy of Planck mass order, stabilizing after a transitory period. A similar picture isobtained in 3 + 1 dimensions with sphericalsymmetry.