We investigate the consequences of the collapse-induced radiation anticipated before formation of the event horizon. After reviewing the principles underlying semi-classical analysis of black holes we illustrate them by modelling collapse of evaporating massive thin dust shells using two families of metrics for the exterior geometry: the outgoing Vaidya metric and the retarded Schwarzschild metric. We describe how hypothetical pre-Hawking radiation modifies the equation of motion for the shell. Provided that a non-zero radiation flux is perceived by a distant observer, the shell never gets closer than a certain sub-Planckian distance from the Schwarzschild radius. This distance depends only on the shell's mass and evaporation rate. The stress-energy tensor is everywhere finite, but a comoving observer encounters firewall-like energy density and flux. We emphasize the logical connections between different assumptions within the semi-classical approach and discuss consequences of the apparent contradictions between them.
- thin shells
- pre-Hawking radiation