A dynamical model of an evaporating charged black hole and quantum instability of Cauchy horizons

The Hawking effect is investigated on a classical model spacetime (the Vaidya-Bonnor linear spacetime), which represents a dynamically evaporating charged black hole. Under the two-dimensional approximation, the expectation value is calculated of the stress-energy tensor of a massless scalar field with respect to the generalised Unruh vacuum. The obtained vacuum expectation value diverges on the Cauchy and anti-Cauchy horizons of the model spacetime. This suggests that these horizons are quantum theoretically unstable.