[Paper Review] GW190521: Search for Echoes due to Stimulated Hawking Radiation from Black Holes

Jahed Abedi, Luís Felipe Longo Micchi+1 more|arXiv (Cornell University)|Dec 31, 2021

Pulsars and Gravitational Waves Research9 citations

TL;DR

This study searches for gravitational wave echoes from stimulated Hawking radiation following the GW190521 black hole merger using two independent pipelines: a template-based Bayesian search and a model-agnostic cWB search. It reports Bayesian evidence of 8⁺⁴₋₂ for echoes at ~50 Hz, with a 6% energy excess, suggesting tentative support for quantum structure at black hole horizons, though not yet at 5σ significance.

ABSTRACT

Being arguably the most massive binary black hole merger event observed to date, GW190521 deserves special attention. The exceptionally loud ringdown of this merger makes it an ideal candidate to search for gravitational wave echoes, a proposed smoking gun for the quantum structure of black hole horizons. We perform an unprecedented multi-pronged search for echoes via two well-established and independent pipelines: a template-based search for stimulated emission of Hawking radiation, or Boltzmann echoes, and the model-agnostic coherent WaveBurst (cWB) search. Stimulated Hawking radiation from the merger is expected to lead to post-merger echoes at horizon mode frequency of $\sim50$ Hz (for quadrupolar gravitational radiation), repeating at intervals of $\sim1$ second, due to partial reflection off Planckian quantum structure of the horizon. A careful analysis using dynamic nested sampling yields a Bayesian evidence of $8^{+4}_{-2}$ (90\% confidence level) for this signal following GW190521, carrying an excess of $6^{+10}_{-5}\%$ in gravitational wave energy, relative to the main event (consistent with the predicted amplitude of Boltzmann echoes). The "look-elsewhere" effect is estimated by using General Relativity (plus Boltzmann echoes) injections in real data, around the event, giving a false (true) positive detection probability for higher Bayes factors of $1.5^{+1.2}_{-0.9}\%$ ($35\pm7\%$). Similarly, the reconstructed waveform of the first echo in cWB carries an energy excess of $13^{+16}_{-7}\%$. While the current evidence for stimulated Hawking radiation does not reach the gold standard of $5σ$ (or p-value $<3 imes10^{-7}$), our findings are in line with expectations for stimulated Hawking radiation at current detector sensitivities. The next generation of gravitational wave observatories can thus draw a definitive conclusion on the quantum nature of black hole horizons.

Motivation & Objective

• To test for post-merger gravitational wave echoes as a signature of quantum structure at black hole horizons.
• To investigate whether the exceptionally loud ringdown of GW190521, the most massive binary black hole merger observed, hosts echoes from stimulated Hawking radiation.
• To validate results using two independent search pipelines—template-based and model-agnostic—to reduce methodological bias.
• To quantify false positive rates via injections in real LIGO/Virgo data, estimating the reliability of echo detection claims.
• To provide the most robust current constraints on echo energy, morphology, and detectability for future detectors.

Proposed method

• Employed dynamic nested sampling for Bayesian evidence calculation in a template-based search for Boltzmann echoes at ~50 Hz, with 1-second recurrence intervals.
• Used the coherent WaveBurst (cWB) algorithm to detect echo candidates without assuming a specific echo template, enabling model-agnostic detection.
• Performed injections of General Relativity signals with and without echo components into real LIGO/Virgo data to estimate false positive and true positive rates.
• Compared spectrogram features across different detection thresholds and morphological assumptions (chirp vs. non-chirp) to distinguish robust echoes from noise artifacts.
• Calculated Bayes factors and p-values to quantify statistical significance, with cross-validation between PyCBC and cWB results.
• Assessed the 'look-elsewhere' effect by injecting signals before and after GW190521 to estimate detection probability under null hypothesis.

Experimental results

Research questions

• RQ1 Does the post-merger ringdown of GW190521 contain evidence for gravitational wave echoes due to stimulated Hawking radiation?
• RQ2 Can independent search pipelines—Bayesian template-based and model-agnostic cWB—converge on the same echo signal, increasing confidence in detection?
• RQ3 What is the statistical significance of the echo signal, and how does it compare to the 5σ threshold for discovery?
• RQ4 How robust are the echo candidates to changes in detection criteria, such as spectrogram morphology assumptions?
• RQ5 What is the estimated energy excess of the echo relative to the main ringdown signal, and does it align with theoretical predictions?

Key findings

• The Bayesian evidence for echoes in the PyCBC pipeline is 8⁺⁴₋₂ (90% credible interval), indicating moderate statistical support for the echo hypothesis.
• The reconstructed echo in the cWB pipeline shows a 13⁺¹⁶₋₇% energy excess relative to the main ringdown, consistent with theoretical expectations for Boltzmann echoes.
• The false positive detection probability for higher Bayes factors is estimated at 1.5⁺¹.²₋₀.₉%, while the true positive rate is 35±7% under injected signals.
• The only common echo trigger across both pipelines is at ~178.5 s and 50 Hz, which is considered robust; other features at 80 Hz are deemed non-robust and likely noise.
• The signal morphology in spectrograms differs under chirp vs. non-chirp assumptions, and only the 50 Hz feature at ~178.5 s survives both detection criteria.
• Despite strong consistency across methods and extensive injection tests, the evidence falls short of the 5σ standard, indicating that definitive confirmation awaits next-generation detectors.