Uniqueness of the Fock quantization of fields with unitary dynamics in nonstationary spacetimes

Understand

The Fock quantization of fields propagating in cosmological spacetimes is not uniquely determined because of several reasons.

• Apart from the ambiguity in the choice of the quantum representation of the canonical commutation relations, there also exists certain freedom in the choice of field: one can scale it arbitrarily absorbing background functions, which are spatially homogeneous but depend on time.
• Each nontrivial scaling turns out into a different dynamics and, in general, into an inequivalent quantum field theory.
• In this work we analyze this freedom at the quantum level for a scalar field in a nonstationary, homogeneous spacetime whose spatial sections have $S^3$ topology.