Axions, Black Holes and the Detection of Gravitons: from Astrophysics to Cosmology

Abstract: We review a novel scenario for the emergence of spin-polarisation entangled squeezed graviton states from superradiant axionic clouds in the neighborhood of astrophysical rotating black holes (BHs). The entangled squeezed graviton states are produced by both, conventional General-Relativity (GR) type axion-gravity interactions, and gravitational Chern-Simons (gCS) anomalous terms coupled to axions, which are non-trivial in the presence of rotating BHs. The two kinds of terms have different-symmetry contributions to the entangled squeezed states. The squeezing parameter is estimated in a weak-quantum-gravity framework. Some phenomenology with respect to current and future interferometric detection devices is discussed. Importantly, current data from LIGO/Virgo Experiments can impose upper-bound constraints on the value of the squeezing parameter and, thus, on the lifetime of the axionic clouds. In addition to the above rather direct-detection possibility of squeezed gravitons, there is also the possibility of indirect detection of quantum gravitons in Cosmology, given that chiral quantum gravitational-wave (GW) perturbations in the primordial Universe may imply condensation of gCS terms. This, in turn, leads to inflation of running vacuum type, with in principle observable patterns in the profile of the GW produced during the post-inflationary early radiation era, as well as the potential of alleviating cosmic tensions in the current era.