Demystifier said:
They don't assume that it forms spontaneously. They assume that it appears there through the process of quantum tunneling. The probability of tunneling is computed via the standard WKB formula.
Maybe I did not understand something in the paper.
Parikh and Wilczek write above equation (5):
The imaginary part of the action for an s-wave outgoing positive energy particle which crosses the horizon outwards from into out can be expressed as
The imaginary part of the action, I am S, tells the probability that the particle tunnels through the potential wall. That is the WKB approximation.
I understood the paper like this: a virtual photon representing the outgoing spherical wave appears spontaneously behind the horizon.
After tunneling through the horizon the photon has stolen some energy from the black hole, and it leaves as a real particle.
Let us compare this to how a real photon is born in ordinary quantum physics. There is an electric charge in an accelerating motion. Classically, there would be electromagnetic radiation. If quantum mechanics allows the system of charges to fall into a lower energy state, then a quantum of energy, a photon, may leave.
The emission of a photon from a black hole does not have a classical counterpart. That is one of the reasons why I do not see the assumptions of Parikh and Wilczek as obvious.
EDIT:
Can we draw a Feynman diagram style description of the process? We may assume that in a collision experiment, a large number of particles create a bound state, a black hole. Then a virtual photon steals energy from this bound state and becomes real. Unfortunately, Feynman diagrams do not allow bound states.