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

TL;DR

The paper proposes a novel quantum-gravity phenomenology where superradiant axion clouds around rotating black holes generate spin-polarisation entangled squeezed gravitons through both GR and gravitational Chern-Simons interactions, with multimode entanglement enhancing detectability. By formulating the squeezing in a multimode framework and estimating the corresponding kernels, it derives upper bounds on the squeezing parameter and shows that plausible cloud lifetimes can yield large photon-like graviton populations, potentially accessible to future interferometers. It further discusses indirect cosmological consequences via gCS condensates, which can drive running-vacuum-model type inflation and affect early-Universe GW spectra, PBH production, and cosmological tensions. Observationally, the work connects direct squeezed-graviton detection prospects with LIGO/Virgo bounds and suggests cross-correlation strategies to identify nonclassical GW correlations, offering a pathway to test quantum aspects of gravity in both astrophysical and cosmological contexts.

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.

Figures (5)

• Figure 1: Superradiant axionic cloud around a rotating black hole (dark blob). The non-linear interactions of axions (dashed lines) with gravitons (wavy lines) producing quantum-entangled squeezed graviton pairs are indicated explicitly in a weak-gravity effective field theory framework, we restrict our attention for our purposes here. Picture taken from Dorlis:2025zzz.
• Figure 2: The growth rate stemming from the superradiant instability of the dominant "$2p$-state" in the axion cloud of the rotating black hole of figure \ref{['cloud']}, for different values of the BH spin ratio $\alpha/(G \mathcal{M})$. Picture taken from Dorlis:2025amf.
• Figure 3: Angular and polarisation correlations for the GR interaction $S^{(2)}$ in \ref{['S12']}. The $2p$-state results in an asymmetry between the LL and RR pairs. The above figures correspond to $a_\mu=0.1$. Picture taken from Ref. Dorlis:2025zzz.
• Figure 4: Angular and polarisation correlations for the anomalous gravitational CS interaction $S^{(2)}_{CS}$ in \ref{['S12']}. Only pairs of opposite polarisations are produced, with Maximal entanglement occurring between the L and R polarisations. The plot corresponds to $a_\mu=0.1$. Picture taken from Dorlis:2025zzz.
• Figure 5: The evolution of the Hubble parameter during the short period from the end of the axion-dominated stiff era to the inflationary phase, in the stringy RVM cosmology of Basilakos:2019acjMavromatos:2020kzjDorlis:2024yqw. The Hubble rate drops by almost four orders of magnitude during that transition. Figure taken from Mavromatos:2025mmo.