Is GW190521 a gravitational wave echo of wormhole remnant from another universe?

This paper proposes that the GW190521 event could be a gravitational wave echo from a wormhole remnant in another universe, but concludes that Bayesian model selection strongly favors the standard binary black hole merger hypothesis over this exotic alternative.

The Gist

The Big Mystery: GW190521

Imagine the LIGO and Virgo detectors as giant, ultra-sensitive ears listening to the universe. Usually, when they hear two black holes collide, the sound is like a "chirp." It starts low and quiet, gets higher and louder as the black holes spin faster, and then ends with a "ring" as they merge. This is the standard "Inspiral-Merger-Ringdown" song of the cosmos.

But then, there was GW190521.

This event was weird. It didn't have the slow, building-up "chirp." It was just a sudden, short thump—like someone hitting a drum once and stopping immediately. It lasted only about 0.1 seconds. Because it lacked the usual "chirp," scientists were puzzled: What made this sound?

The New Idea: A Cosmic Doorbell

In this paper, the authors propose a wild new theory to explain that sudden "thump."

The Standard Theory: Two black holes merged in our universe, creating a massive black hole.
The New Theory: The black holes merged in a completely different universe, and what we heard was an "echo" coming through a cosmic door.

The Analogy: The Two-Room House

Imagine our universe is Room A, and there is another universe, Room B, right next to it.

• Normally, these rooms are separate.
• But, a Wormhole is like a secret tunnel (or a throat) connecting the two rooms.
• In Room B, two black holes smashed together. This created a loud "ringing" sound (gravitational waves).
• Usually, that sound stays in Room B. But because of the wormhole, some of that sound traveled through the tunnel, popped out into Room A (our universe), and hit our detectors.

Because the sound traveled through a tunnel and came from a different place, it didn't have the "chirp" of the collision happening right here. It just arrived as a sudden, isolated burst of sound. The authors call this the "Echo-for-Wormhole" model.

How They Tested It

The scientists didn't just guess; they did the math.

1. Building a Sound Template: They created a computer model of what this "wormhole echo" would sound like. They imagined it as a single, short pulse of sound, similar to a sine wave that fades out quickly.
2. The Sound Check (Signal-to-Noise): They asked, "If this echo theory were true, how loud would the signal be compared to the background static of the universe?"
   • Result: The "wormhole echo" model sounded almost as loud and clear as the standard "black hole merger" model. It fit the data reasonably well.
3. The Tug-of-War (Bayesian Comparison): This is the most important part. They used a statistical method to ask: "Given the data we have, which story is more likely to be true?"
   • Story A: It's a standard black hole merger in our universe.
   • Story B: It's an echo from a wormhole in another universe.

The Verdict

The math came back with a verdict: Story A (Standard Black Hole) is still the winner.

• The "Black Hole Merger" model was about 18 times more likely to be correct than the "Wormhole Echo" model.
• The authors admit their wormhole model was a bit simplified (they ignored things like the spin of the black holes to keep the math manageable).
• However, the fact that their wormhole model fit the data at all is exciting. It proves that this weird, short "thump" could theoretically be a message from another universe, even if the standard explanation is currently the favorite.

Why Does This Matter?

Even though they didn't prove the wormhole theory, this paper is like a detective saying, "The suspect in the standard uniform is the most likely culprit, but we can't rule out the guy in the disguise just yet."

It opens the door for future research. If we find more of these short, "chirp-less" events in the future, we might need to look closer at the "wormhole" theory. It reminds us that the universe might have secret tunnels connecting places we can't see, and sometimes, we might just hear a faint echo from the other side.

Technical Summary

1. Problem Statement

The LIGO–Virgo–KAGRA (LVK) collaboration detected the gravitational wave (GW) event GW190521, which presents unique characteristics challenging the standard binary black hole (BBH) coalescence paradigm:

• Short Duration: The signal lasts only $\sim 0.1$ seconds.
• Missing Inspiral: Unlike typical BBH mergers, GW190521 lacks a clearly identifiable inspiral phase.
• Mass Discrepancy: The inferred component masses ($85^{+21}_{-14} M_\odot$ and $66^{+17}_{-18} M_\odot$) fall within the "pair-instability supernova mass gap," making their formation via standard stellar evolution difficult.

While LVK interpreted this as a standard BBH merger, the authors propose an alternative hypothesis: GW190521 is not a merger in our universe, but a single, isolated gravitational wave echo originating from a wormhole remnant formed by a BBH merger in a different universe, connected to ours via a throat.

2. Methodology

A. Theoretical Model: Echo-for-Wormhole

The authors construct a proof-of-principle model based on a Schwarzschild-like Morris–Thorne wormhole:

• Geometry: Two universes are connected by a throat at radius $r_0 > r_S$ (Schwarzschild radius).
• Mechanism: A BBH merger occurs in the "other" universe. The resulting ringdown signal propagates toward the wormhole throat, enters it, and penetrates the photon sphere barrier on the "our universe" side.
• Signal Characteristics:
  • The signal is modeled as a scattering problem with mirror barriers (photon spheres) on both sides of the throat.
  • While a full wormhole would produce a train of echoes, the authors hypothesize that only the first echo is detectable. Subsequent echoes may be undetectable because the wormhole rapidly pinches off/collapses into a black hole, or their amplitudes fall below detector sensitivity.
  • The waveform is modeled as a Gaussian-like wave packet (specifically a sine-Gaussian template) representing the first echo pulse, governed by the wormhole's quasinormal modes (QNMs).

B. Waveform Template

The authors utilize a simplified sine-Gaussian template for the strain $h(t)$:
$$h(t) \sim A \cos(2\pi f_c(t - t_c) + \phi) e^{-\frac{(t-t_c)^2}{2\beta^2}}$$

• Parameters: Central frequency ($f_c$), arrival time ($t_c$), width ($\beta$), amplitude ($A$), phase ($\phi$), polarization ($\psi$), and inclination ($\iota$).
• Normalization: The amplitude is rescaled by a dimensionless factor $A_m$ relative to a reference strain at 1 Mpc to ensure numerical stability.
• Constraints: The sky location (Right Ascension and Declination) was fixed to the electromagnetic counterpart candidate ZTF19abanrhr to break degeneracies between arrival time and sky position.

C. Analysis and Comparison

• Data: Analysis performed on data from LIGO Livingston (L1), LIGO Hanford (H1), and Virgo (V1).
• Signal-to-Noise Ratio (SNR): Calculated using the PyCBC software package. The network SNR is the quadrature sum of individual detector SNRs.
• Bayesian Model Selection:
  • Compared the "Echo-for-Wormhole" model against the standard BBH model (using the IMRPhenomXPHM waveform template, which handles high mass ratios and spin precession).
  • Computed the Log Bayes Factor ($\ln \mathcal{B}_{BBH}^{Echo}$) to determine which hypothesis the data favors.
  • Priors were set to be uniform (or log-uniform for amplitude) across physically motivated ranges.

3. Key Contributions

1. Novel Hypothesis: Proposes that GW190521 is a trans-universal signal (an echo from a wormhole connecting two universes) rather than a local merger.
2. Model Construction: Develops a specific waveform template for a single, isolated echo pulse resulting from a wormhole throat transmission, distinct from standard echo searches that look for repeating pulses within a single universe.
3. Systematic Comparison: Performs a rigorous Bayesian comparison between this exotic model and the standard BBH merger model using the same dataset and detector configurations.
4. Sky Localization Consistency: Demonstrates that the inferred sky location of the echo model is consistent with the BBH model and the electromagnetic counterpart ZTF19abanrhr, validating the geometric plausibility of the signal origin.

4. Results

• Signal-to-Noise Ratio (SNR):

  • The Echo model yields a network SNR of 14.45.
  • The Standard BBH model yields a network SNR of 15.59.
  • Conclusion: The Echo model provides a fit comparable in significance to the standard BBH model.

• Parameter Estimation (Echo Model):

  • Central Frequency ($f_c$): $56.93^{+1.63}_{-1.86}$ Hz.
  • Pulse Width ($\beta$): $\approx 0.02$ s.
  • These parameters align with the narrow-band features expected for a wormhole echo.

• Bayesian Model Selection:

  • The Log Bayes Factor is calculated as: $\ln \mathcal{B}_{BBH}^{Echo} \simeq -2.9$.
  • Interpretation: A negative value indicates that the data favor the standard BBH hypothesis over the echo-for-wormhole hypothesis. However, the magnitude suggests the echo model is not ruled out entirely but is less probable given current data.

• Sky Location:

  • Both models converge on a sky location consistent with ZTF19abanrhr (RA $\approx 3.36$, Dec $\approx 0.61$ rad), confirming that the echo hypothesis does not require an implausible source location.

5. Significance and Limitations

• Significance:

  • The study provides a proof-of-principle framework for testing "trans-universal" gravitational wave signals.
  • It highlights that short-duration, burst-like GW events (like GW190521 and the recently reported GW231123) warrant systematic testing against exotic alternatives, not just standard compact binary coalescences.
  • It connects the GW190521 anomaly to broader theoretical questions regarding the black hole information paradox and the existence of horizonless compact objects.

• Limitations & Future Work:

  • Simplifications: The current model assumes a non-spinning setup. The actual remnant of GW190521 is highly spinning ($\chi_f \approx 0.72$), which would modify the effective potential and echo spectrum.
  • Template Completeness: The model only considers the first echo. A more complete treatment would include self-consistent modeling of potential subsequent echoes and the dynamics of the wormhole collapsing into a black hole.
  • Bayesian Preference: While the BBH model is currently favored, the authors suggest that future improvements to the echo waveform templates (incorporating rotation and detailed physics) could alter the Bayes factor, potentially making the exotic hypothesis more competitive.

In summary, while the data currently support the standard binary black hole interpretation of GW190521, this paper successfully demonstrates that a wormhole echo from another universe is a viable, physically consistent alternative that yields a comparable signal-to-noise ratio, urging further investigation into exotic compact objects for future short-duration GW events.