Superradiant Axionic Black-Hole Clouds as Seeds for Graviton Squeezing

TL;DR

This paper demonstrates that superradiant axionic black-hole clouds can generate highly entangled graviton pairs in a squeezed state, with the effect primarily driven by general relativity interactions and potentially detectable with future gravitational wave observatories.

Contribution

It introduces a novel mechanism for graviton squeezing via axionic clouds around black holes, highlighting the dominance of GR effects over Chern-Simons contributions.

Findings

GR-induced graviton squeezing exceeds CS contributions by orders of magnitude.

Long-lived axionic clouds produce significant graviton squeezing effects.

Current LIGO data constrains axion-cloud lifetimes and the associated graviton squeezing phenomena.

Abstract

It is shown that both, standard general relativity (GR) and Chern-Simons (CS) gravity, the latter containing chiral gravitational anomaly terms, seed the production of pairs of entangled gravitons in a multi-mode squeezed state. This involves the interaction of gravitons with the axionic cloud surrounding a superradiant Kerr (rotating) black hole background. The order of magnitude of the squeezing effect, specifically the number of graviton excitations in the squeezed vacuum, is estimated in the non-relativistic limit, relevant for the superradiance process. It is found analytically that the squeezing from the GR process of annihilation of two axions into two gravitons, dominates, by many orders of magnitude, that coming from the axion decay into two gravitons, induced by the higher-derivative CS term. It is also shown that significant squeezing effects are produced in the case of long-lived axionic clouds, whose lifetimes are much longer than the timescale for which superradiance is effective. A brief discussion on current exclusion (for the first time) of very-long axion-cloud lifetimes, through comparison of our results with current LIGO data, as well as potential detection of such effects in future interferometers is also given