  The simplest cosmology --- the Friedmann-Robertson-Walker-Lema\^{i}tre (FRW)
model --- describes a spatially homogeneous and isotropic universe where the
scale factor is the only dynamical parameter. Here we consider how quantized
electromagnetic fields become entangled with the scale factor in a toy version
of the FRW model. A system consisting of a photon, source, and detector is
described in such a universe, and we find that the detection of a redshifted
photon by the detector system constrains possible scale factor superpositions.
Thus, measuring the redshift of the photon is equivalent to a weak measurement
of the underlying cosmology. We also consider a potential optomechanical
analogy system that would enable experimental exploration of these concepts.
The analogy focuses on the effects of photon redshift measurement as a quantum
back-action on metric variables, where the position of a movable mirror plays
the role of the scale factor. By working in the rotating frame, an effective
Hubble equation can be simulated with a simple free moving mirror.
