Squeezed gravitons from superradiant axion fields around rotating black holes

TL;DR

This paper introduces a new astrophysical mechanism where superradiant axion clouds around rotating black holes generate squeezed graviton states, with effects dominated by general relativity over Chern-Simons anomalies, leading to potentially observable quantum gravitational phenomena.

Contribution

It proposes a novel source of squeezed gravitons from axion clouds around Kerr black holes, highlighting the dominance of GR effects over anomaly contributions in graviton squeezing.

Findings

Superradiant axion clouds can produce squeezed graviton states.

GR effects dominate over Chern-Simons anomalies in squeezing.

Entanglement structure depends on the origin of the effects.

Abstract

We propose, in (3+1)-dimensional spacetimes, a novel astrophysical source of squeezed graviton states, due to superradiant axionic clouds surrounding rotating (Kerr-type) black holes (BH). The microscopic origin of these axions is diverse, ranging from the Kalb-Ramond (model-independent) axions and compactification axions in string theory, to \cm contorted geometries exemplified by a totally antisymmetric component of torsion in Einstein-Cartan theory. The axion fields couple to chiral gauge and gravitational Chern-Simons (CS) anomaly terms in the effective gravitational actions. In the presence of a Kerr BH background, such axions lead, upon acquiring a mass, to superradiance and the production of pairs of entangled gravitons in a squeezed state. The specific microscopic origin of the axions is not important, provided they are massive. This multimode squeezed-graviton state is examined through a Takagi-like decomposition, used in quantum optics. In the effective action it is shown that squeezing effects associated with conventional general relativity (GR) dominate, by many orders of magnitude, the corresponding effects due to the CS gravitational anomaly terms. For a sufficiently long lifetime of the axionic cloud of the BH, we find that significant squeezing (quantified through the average number of gravitons with respect to the appropriate vacuum) can be produced from the GR effects. It is also demonstrated explicitly that the structure of the entangled states (when the latter are expressed in a left-right polarization basis) depends highly on whether the GR or the anomalous CS effects produce the entanglement.