  This dissertation examines the impact of quantum gravity on electromagnetism
and its backreaction, using perturbative general relativity as an effective
field theory. Our analysis involves quantum-correcting Maxwell's equations to
obtain a gauge-independent, real, and causal effective field equation that
describes quantum gravitational effects on electromagnetism. Additionally, we
present a perturbative mechanism through which quantum gravity induces a
dimension six coupling between a massive scalar and electromagnetism. To
investigate the effects of electromagnetism on the gravitational sector, we
derive an exact, dimensionally regulated, Fourier mode sum for the Lorentz
gauge propagator of a massive photon on an arbitrary cosmological background
supported by a scalar inflaton. This allows us to calculate the effective
potential induced by photons. Finally, we use a similar Fourier mode sum for a
time-dependent mass to study the effective force on the inflaton 0-mode and its
impact on reheating.
