Quantum gravity fluctuations in the timelike Raychaudhuri equation

Abstract

We consider a timelike geodesic congruence in the presence of perturbative quantum fluctuations of the spacetime metric. We calculate the change in the volume of a bundle of geodesics due to such fluctuations and thereby obtain a quantum-gravitationally modified timelike Raychaudhuri equation. Quantum gravity generically increases the convergence of congruences and the production of caustics.

The quantum optics of gravitational waves

“…Recently, considerable effort has been devoted to understanding possible signatures at LIGO-like experiments of hypothetical quantum states of gravity that have no classical analog [16][17][18][19][20] (see also [15,[21][22][23][24] for other approaches to the detection of non-classical states; compare [25] for a more critical viewpoint on these proposals). These signatures can be fruitfully investigated by using an effective field theory description of gravity and using the formal tools of quantum optics.…”

On the decoherence of primordial gravitons

“…During inflation, the vacuum of the graviton evolves into a quantum squeezed state [1][2][3][4], opening up possibilities to probe the non-classicality of gravitons via observations. There have been proposals to test the non-classicality of squeezed gravitons, including the quantum noise in gravitational-wave detectors and geodesics [5][6][7][8][9][10][11][12], 1 indirect detection through decoherence of matter [8,17] and electromagnetic field [18], the Hanbury Brown-Twiss interferometry with the related sub-Poissonian statistics [19][20][21] and the interaction with an optical cavity [22,23]. In particular, the quantum noise produced by the primordial gravitational wave can be largely enhanced by the inflationary squeezed states, providing a chance to make the non-classicality of gravitons detectable.…”