Quantum Gravity signatures in late time Universe

Abstract

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 spilling over into the succeeding inflationary era which gradually fade away as the universe transits into the subsequent radiation and matter driven expansion. In general scenarios as well, the imprints of possible quantum character of gravitons are typically so feeble that they are expected to remain buried under overwhelming noise from other stronger processes. In this work, we demonstrate that quantized gravitational 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 quantum correlators of the gravitons {\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, which otherwise is way too feeble. The transitions between spherical harmonics states of a hydrogen atom guided by gravitons in a particular phase of the late time era of the universe, provides an example of such a process. Thus the newly formed atoms post LSS are expected to get excited from the vacuum fluctuations of gravitons quite efficiently in this era and also get de-excited to convert the vacuum fluctuations to an additional channel of energy influx into the evolution history. Such unavoidable processes provide an avenue of quantum graviton mediated process becoming important in the late time era, which has many interesting implications.