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Echoes from the Abyss: Tentative evidence for Planck-scale structure at black hole horizons

Jahed Abedi, Hannah Dykaar, Niayesh Afshordi

TL;DR

The paper investigates whether Planck-scale deviations near black-hole horizons can produce observable gravitational-wave echoes after mergers by modeling the horizon as a reflective near-horizon membrane and constructing a five-parameter echo template. Using Kerr QNM-based timing and LIGO data from GW150914, GW151226, and LVT151012, the authors search for repeating, damped echoes with spacing $Δt_{ m echo}$ consistent with near-horizon structure and find a tentative signal near the predicted timing, corresponding to a false-detection probability of about $1\%$ (roughly 2.5$\sigma$) when combining all three events. The result relies on an ad hoc echo template and chosen priors, so confirmation requires higher-sensitivity observations and more physically grounded templates. If confirmed, these echoes would provide empirical support for quantum-gravity-inspired horizon alternatives such as firewall or fuzzball paradigms and could impact our understanding of information retrieval in black-hole mergers.

Abstract

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of $8 M \log M$ (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the "look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at false detection probability of $1\%$ (corresponding to $2.5σ$ significance level). Future observations from interferometric detectors at higher sensitivity, along with more physical echo templates, will be able to confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in ${\it firewall}$ or ${\it fuzzball}$ paradigms.

Echoes from the Abyss: Tentative evidence for Planck-scale structure at black hole horizons

TL;DR

The paper investigates whether Planck-scale deviations near black-hole horizons can produce observable gravitational-wave echoes after mergers by modeling the horizon as a reflective near-horizon membrane and constructing a five-parameter echo template. Using Kerr QNM-based timing and LIGO data from GW150914, GW151226, and LVT151012, the authors search for repeating, damped echoes with spacing consistent with near-horizon structure and find a tentative signal near the predicted timing, corresponding to a false-detection probability of about (roughly 2.5) when combining all three events. The result relies on an ad hoc echo template and chosen priors, so confirmation requires higher-sensitivity observations and more physically grounded templates. If confirmed, these echoes would provide empirical support for quantum-gravity-inspired horizon alternatives such as firewall or fuzzball paradigms and could impact our understanding of information retrieval in black-hole mergers.

Abstract

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the "look elsewhere" effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at false detection probability of (corresponding to significance level). Future observations from interferometric detectors at higher sensitivity, along with more physical echo templates, will be able to confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in or paradigms.

Paper Structure

This paper contains 10 sections, 22 equations, 9 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Echo time-delays
  3. Data and the Echo template
  4. Results
  5. Conclusions and Discussion
  6. Estimation of the total echo energy emission
  7. Detection properties of echo template
  8. The Holiday Edition!
  9. LIGO predictions for Echo time delays
  10. Discussion and Consistency of Statistical Methodology

Figures (9)

  • Figure 1: Spacetime depiction of gravitational wave echoes from a membrane/firewall on the stretched horizon, following a black hole merger event.
  • Figure 2: LIGO original template for GW150914, along with our best fit template for the echoes.
  • Figure 3: Amplitude Spectral Densities (ASD's) of our best fit echo template (Eq. \ref{['template']}) and the main event, for GW150914. Since we have a quasi-periodic model, there are resonances in the spectrum. The ASDs are the square root of the power spectral densities, which are averages of the square of the fast Fourier transforms of the data. The noise spectra from Hanford and Livingston detectors are also shown.
  • Figure 4: Best fit (or maximum) SNR$^2$ near the expected time of merger echoes (Eq's. \ref{['nonlinear']} and \ref{['t_echo_meas']}), for the combined (top) and GW150914 (bottom) events. The significance of the peaks is quantified by the p-value of their SNR$_{\rm max}$ within the gray rectangle (see Appendix E for detail of calculation).
  • Figure 5: Average number of noise peaks higher than a particular SNR-value within a time-interval $2\% \times \overline{\Delta t}_{\rm echo}$ for combined (left) and GW150914 (right) events. The red dots show the observed SNR peak at $t_{\rm echo} = 1.0054 \Delta t_{\rm echo}$ (Fig. \ref{['SNR']}). The horizontal bar shows the correspondence between SNR values and their significance.
  • ...and 4 more figures