Bound on Quantum Fluctuations in Gravitational Waves from LIGO-Virgo

TL;DR

This paper derives quantum fluctuation equations for gravitational waves, explores quantum states including squeezed states, and uses LIGO-Virgo data to place bounds on the quantum properties of gravitational waves.

Contribution

It provides a theoretical framework for quantum gravitational wave states and constrains their quantum squeezing using observational data.

Findings

Bound on the squeezing parameter: ζ < 41

Derived explicit quantum states for gravitational waves

Connected quantum states to observable correlations in LIGO-Virgo data

Abstract

We derive some of the central equations governing quantum fluctuations in gravitational waves, making use of general relativity as a sensible effective quantum theory at large distances. We begin with a review of classical gravitational waves in general relativity, including the energy in each mode. We then form the quantum ground state and coherent state, before then obtaining an explicit class of squeezed states. Since existing gravitational wave detections arise from merging black holes, and since the quantum nature of black holes remains puzzling, one can be open-minded to the possibility that the wave is in an interesting quantum mechanical state, such as a highly squeezed state. We compute the time and space two-point correlation functions for the quantized metric perturbations. We then constrain its amplitude with LIGO-Virgo observations. Using existing LIGO-Virgo data, we place a bound on the (exponential) squeezing parameter of the quantum gravitational wave state of $\zeta < 41$.