  In the cosmological settings, Quantum Gravity effects are typically
understood to be limited towards very early phase of the universe, namely in
the pre-inflationary era, with limited signatures remaining present in the
succeeding inflationary era. These signatures also gradually fade away as the
universe grows and exits the inflationary era. In the subsequent radiation and
matter dominated era quantum gravity is expected to play no significant role.
Classicalized primordial perturbations such as the scalar perturbations and the
gravitational waves are expected to leave an imprint on CMB anisotropy and its
polarization respectively but quantized gravitational waves are not expected to
lead to any appreciable observable effects. Apart from cosmology, in other
avenues as well, the imprints of quantum character of gravitational waves are
typically so subdued that any possible signature gets buried under a huge pile
of the noise or effects from other much stronger processes. In this work, we
demonstrate that quantum gravity perturbations cause strong observable effects
in cosmological settings post the Last Scattering Surface (LSS) more
prominently than any other classical or quantum processes. This
counter-intuitive effect is facilitated by the fact that the correlators of the
gravitational waves {\it grow divergently large in the matter dominated era}
unlike any other background fields, leading to an abrupt rise in the processes
mediated by correlators of quantum gravitons. The transitions between spherical
harmonics states of newly formed hydrogen atom at the LSS, when the universe
resides in strong matter dominated era, provides an example of such a process.
We further establish that the late time epoch just before the kicking in of the
dark energy provides one of the cleanest avenues to study such quantum gravity
effects.
