  The theory of quantum fields propagating on an isotropic cosmological quantum
spacetime is reexamined by generalizing the scalar test field to an
electromagnetic (EM) vector field. For any given polarization of the EM field
on the classical background, the Hamiltonian can be written in the form of the
Hamiltonian of a set of decoupled harmonic oscillators, each corresponding to a
single mode of the field. In transition from the classical to quantum spacetime
background, following the technical procedure given by Ashtekar {\em et al.}
[Phys. Rev. D 79, 064030 (2009)], a quantum theory of the test EM field on an
effective (dressed) spacetime emerges. The nature of this emerging dressed
geometry is independent of the chosen polarization, but it may depend on the
energy of the corresponding field mode. Specifically, when the backreaction of
the field on the quantum geometry is negligible (i.e., a test field
approximation is assumed), all field modes probe the same effective background
independent of the mode's energy. However, when the backreaction of the field
modes on the quantum geometry is significant, by employing a Born-Oppenheimer
approximation, it is shown that a rainbow (i.e., a mode-dependent) metric
emerges. The emergence of this mode-dependent background in the Planck regime
may have a significant effect on the creation of quantum particles. The
production amount on the dressed background is computed and is compared with
the familiar results on the classical geometry.
