  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.
