Quantum cosmological gravitational waves?

TL;DR

This paper reviews the theoretical challenges and prospects of demonstrating the quantum nature of gravity through the quantization of cosmological gravitational waves predicted by general relativity.

Contribution

It provides a comprehensive overview of the theoretical issues involved in establishing gravitational waves as evidence of quantum gravity.

Findings

Tensor modes are gauge invariant and can be quantized independently.

Quantization of these modes could serve as proof of quantum gravity.

The review discusses the theoretical hurdles in confirming the quantum nature of gravitational waves.

Abstract

General relativity and its cosmological solution predicts the existence of tensor modes of perturbations evolving on top of our Friedman-Lema\^itre-Robertson-Walker expanding Universe. Being gauge invariant and not necessarily coupled to other quantum sources, they can be seen as representing pure gravity. Unambiguously showing they are indeed to be quantised would thus provide an unquestionable proof of the quantum nature of gravitation. This review will present a summary of the various theoretical issues that could lead to this conclusion.