Testing Quantum Gravity with Gravitational Waves from the ringdown of binary Black Holes coalescences: A New Frontier in Fundamental Physics

TL;DR

This paper investigates whether gravitational waves from the ringdown phase of binary black hole mergers can reveal quantum gravity signatures, focusing on the conjectured quantization of black hole area and mass (BHAQ) and its compatibility with observed quasi-normal modes. It reviews the classical QNM framework and then develops quantum-modified ringdown models, including degenerate microstate scenarios (ST, LQG) and semiclassical treatments via the Lee–Friedrichs formalism, predicting discrete but potentially slightly shifted QNM frequencies and coherent multi-quantum emission. It also analyzes possible quantum gravity effects on the near-horizon geometry and GW propagation, assessing their observational impact through EFT corrections, echoes, dispersion, decoherence, and birefringence. The paper concludes that next-generation detectors like the Einstein Telescope and Cosmic Explorer could test BH area quantization and distinguish quantum-modified ringdown spectra under favorable source masses and distances, while most propagation-based effects remain subdominant.

Abstract

The observation of gravitational waves emitted during the merging phase of compact binary coalescing objects has opened a new field of investigation in fundamental physics. It is now possible to test the predictions of General Relativity with unprecedented precision in the strong gravitational field regime. These initial observations therefore call for further research, as the detection of gravitational waves emitted by coalescing black holes may allow the investigation of the properties of spacetime near the event horizon, also providing valuable information on the structure of these objects. This also opens the possibility of testing predictions from quantum gravity models regarding the presumed quantized structure of black holes, related to the quantization of their surface and, consequently, their entropy. In the future, the considerable amount of data obtained by the LIGO-Virgo-KAGRA collaboration will be followed by observations from next-generation interferometers such as the Einstein Telescope or the Cosmic Explorer. It is therefore of great interest to explore the potential of gravitational wave observations for investigating aspects of quantum gravity, which we will address considering the special case of the ringdown emission following the coalescence of binary black hole systems.

Figures (6)

• Figure 1: Comparison between classical QNMs and quantum-modified QNMs, obtained by either rejecting or modifying the frequencies in accordance with the quantum prediction. The plot uses the first 12 modes.
• Figure 2: Comparison between the spectra calculated as Fourier transform of the expected signals for the classical QNMs and quantum-modified QNMs, obtained by either rejecting or modifying the frequencies in accordance with the quantum prediction. The plot uses the first 12 modes.
• Figure 3: Comparison of the Teukolsky potential with quantum gravity modfications.
• Figure 4: Sensitivity curve expected for the ET detector - Low Frequency detector - High Frequency detector - resulting sensitivity curve as sum of the prevoius plots ET_sensitivity.
• Figure 5: Comparison of the expected frequencies with the projected sensitivity curve of the ET. The frequencies are calculated for coalescing binary systems with equal masses.