  A quantum field has been coupled to a space-time with accelerating expansion.
Dynamical modes are destabilised successively at shorter material wavelengths
as they metamorphose from oscillators to repellers. Due to degeneracy of energy
levels, the number of unstable modes increases at an accelerating rate,
sufficient to account for a significant proportion of cosmic energy. For the
subsystem spanned by a finite basis of unstable runaway modes, the quantum
Hamiltonian is unbounded below. There is no Bogoliubov transformation by which
that subsystem Hamiltonian can be expressed as a linear combination of number
operators. For the remaining subsystem spanned by an infinite number of
oscillator modes, there is an appropriate vacuum state in a Fock-Cook
representation of the field algebra. The massless quantum vector field of
electromagnetism is considered when it is minimally or more generally coupled
to an expanding space-time. For a significant class of models, including
minimal coupling models and the exponential de Sitter universe coupled to the
Ricci curvature tensor, the field equations are equivalent to the Proca
equation with time-dependent mass.
