Hawking radiation from a collapsing dust sphere and its back reaction at the event horizon -Weak value approach-

To see the back reaction of the Hawking radiation in a dynamical spacetime of the spherical gravitational collapse, we explicitly calculate the weak value of the energy-momentum tensor of the massless scalar field. The background geometry of a collapsing dust sphere is specified by using the Painleve-Gullstrand coordinates, in which the time coordinate coincides with the proper time of a free-falling observer and the metric tensor is regular at the event horizon. The result is that in the remote future the weak value diverges at the event horizon. We argue that since the semi-classical approximation of the Einstein equation in the sense of the weak value breaks down there, the future geometry of the spacetime cannot be the Schwarzschild geometry.