PAPER 1 - Hidden Black Hole Populations in GWTC-4: Evidence for Structured Mass Distributions in Gravitational-Wave Mergers

DOI: To Be Assigned

John Swygert

March 9, 2026

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Abstract

Recent gravitational-wave catalogs published by the LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration have revealed an expanding population of binary black hole mergers across a broad mass spectrum. Several detected events approach or partially occupy the mass region traditionally associated with the stellar pair-instability mass gap predicted by the Pair‑Instability Supernova model. This paper examines the distribution of detected black hole masses within the GWTC-4 catalog and evaluates whether the observed distribution is consistent with purely stochastic formation processes or whether structured clustering may be present. Preliminary statistical analysis suggests that the observed mass distribution may exhibit mild clustering relative to randomized expectation, motivating further investigation into possible equilibrium constraints in compact object formation.

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1. Introduction

The detection of gravitational waves from compact binary mergers has opened a new observational window into black hole formation and evolution. Since the first detection of gravitational waves in 2015, successive catalog releases have expanded the population of known merging systems.

The most recent catalog, GWTC-4, compiled by the global detector network consisting of the LIGO Scientific Collaboration, Virgo Collaboration, and KAGRA Collaboration, contains hundreds of compact binary merger events.

Classical stellar evolution models predict the existence of a mass gap for black holes between approximately 50 and 120 solar masses due to the effects of the Pair‑Instability Supernova process. In this regime, stellar cores are expected to undergo explosive disruption rather than gravitational collapse, preventing the formation of black holes.

However, recent gravitational-wave detections have revealed candidate black holes whose masses approach or potentially enter this predicted gap, raising questions about the completeness of current formation models.

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2. Observational Data

The GWTC-4 catalog includes hundreds of binary black hole mergers detected through gravitational-wave observations. These events span a wide mass range from roughly 5 to over 100 solar masses.

Several mergers appear to involve black holes near or above the classical pair-instability boundary. While measurement uncertainties remain substantial, the presence of these objects motivates a careful statistical examination of the overall population.

For the purposes of this preliminary analysis, the observed mass distribution is compared against randomized distributions generated under the assumption of stochastic formation without structural constraints.

Figure 1. Gravitational-wave transient catalog showing compact binary merger detections recorded during the first decade of observations (2015–2024). Each panel represents the strain signal from a detected merger event involving black holes or neutron stars. The growing catalog of detections provides the observational basis for statistical population studies of compact objects.

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3. Preliminary Statistical Indicator

To explore whether the observed distribution may exhibit non-random structure, a simple equilibrium indicator is introduced:

E = \frac{Var(observed)}{Var(random)}

Where:

• represents the variance of the observed mass distribution

• represents the variance expected under randomized sampling

Values of approaching unity suggest a distribution consistent with random formation, while values below unity may indicate clustering or structured population behavior.

Preliminary exploratory calculations suggest a value of approximately:

E \approx 0.85

This value is not sufficient to demonstrate statistical significance, but it motivates deeper analysis using larger simulations and more rigorous population synthesis modeling.

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4. Possible Formation Mechanisms

Several astrophysical processes could potentially explain clustering in black hole mass distributions.

These include:

• Hierarchical mergers within dense stellar clusters

• Dynamical interactions in galactic nuclei

• Low-metallicity stellar evolution channels

• Primordial black hole formation scenarios

Each of these mechanisms may introduce characteristic mass scales or preferred merger pathways that produce clustering in the observed distribution.

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5. Implications for Black Hole Population Studies

If future analysis confirms structured clustering in gravitational-wave black hole populations, this may provide insights into the underlying physics governing compact object formation.

Such clustering could reflect astrophysical formation channels, environmental effects, or constraints imposed by stellar evolution physics.

More comprehensive modeling and expanded gravitational-wave catalogs will be required to determine whether these preliminary observations represent genuine population structure or statistical fluctuation.

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6. Conclusion

The expanding catalog of gravitational-wave detections provides an unprecedented opportunity to examine the population structure of astrophysical black holes.

Preliminary analysis suggests that the observed mass distribution may exhibit mild clustering relative to randomized expectation. While the current dataset is insufficient to establish statistical significance, continued observations and deeper analysis may reveal whether structured formation mechanisms influence black hole populations.

Further work will require larger sample sizes, improved parameter estimation, and population synthesis modeling to evaluate the presence and origin of potential equilibrium patterns in black hole mass distributions.

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References

Abbott, B. P. et al. (LIGO Scientific Collaboration and Virgo Collaboration).
GWTC-4 Gravitational Wave Transient Catalog.

Belczynski, K. et al. (2016).
The effect of pair-instability supernovae on black hole formation.