Determine the physical relevance of the future Unruh state

Determine the physical relevance of the future Unruh state for a massive charged scalar field on a Reissner–Nordström black hole, whose construction is possible but whose physical significance remains unresolved.

Background

The paper constructs the past Unruh state, which is intended to model Hawking radiation from a black hole formed by gravitational collapse. A future Unruh state can also be constructed by exchanging the roles of the relevant horizon regions and modes, but the paper does not analyze it because its physical interpretation is uncertain.

This question is included because a complete characterization of nonequilibrium quantum states on charged black-hole backgrounds requires understanding whether the future Unruh state represents a physically meaningful configuration or merely a formal time-reversed construction.