The LIGO-Virgo-KAGRA Collaboration's Gravitational-Wave Transient Catalog (GWTC-5.0), now containing nearly 400 detections of neutron star and black hole mergers, confirms that black hole binaries form through multiple distinct pathways rather than a single process.
The expanded dataset, combined with other recent astrophysical observations, has provided new insights into black hole masses, possible dark matter interactions, and the origins of a record-breaking high-energy neutrino. Key findings include evidence for a pair-instability mass gap around 45 solar masses, the identification of potential primordial black holes, and the formulation of models predicting gravitational wave signals from mergers in dark matter environments.
Gravitational Wave Catalog: GWTC-5.0
The LVK Collaboration has published the Gravitational-Wave Transient Catalog (GWTC-5.0), which contains nearly 400 detections of cosmic collisions, primarily binary black hole mergers. These observations were made by the twin LIGO detectors and the Virgo detector.
Multiple Formation Pathways
Analysis of the catalog indicates that black hole binaries originate from distinct sub-populations, forming through different cosmic pathways. Researchers identified three primary formation mechanisms:
- Gas cloud collapse: Black holes formed from massive stars born in collapsing gas clouds.
- Dynamical capture in clusters: Black holes formed in dense star clusters where they "wander into each other."
- Hierarchical mergers: Black holes that are products of previous black hole mergers.
Sharan Banagiri (Monash University, OzGrav) stated that the data provide a clear indication that binary black hole mergers form through multiple pathways. Eric Thrane (Monash University, OzGrav) noted that the field is transitioning from individual event discovery to statistical population profiling.
Spin and Mass Distributions
Analysis shows that some black holes exhibit rapid spin, splitting into two mass ranges: approximately 10–20 solar masses and above ~45 solar masses. Heavier black holes (exceeding 45 solar masses) are more likely to merge with lower-mass black holes. Rapidly spinning black holes are hypothesized to be products of hierarchical mergers from previous black hole pairs.
Notable Individual Events
- GW241127: A binary black hole system with very different masses and wobbling orbits due to tilted spins.
- GW240615: The best-localized gravitational-wave event on the sky to date.
Black Hole Mass Gap and Pair-Instability Supernovae
An Australian-led study, published in Nature, identified evidence of a rare type of exploding star and a "forbidden gap" in black-hole masses.
Observations
When examining data from the LIGO–Virgo–KAGRA Collaboration, researchers identified that black holes exceeding approximately 45 times the mass of the Sun are unexpectedly rare. This 'forbidden range' indicates a mechanism that prevents stars of corresponding masses from collapsing into black holes.
Proposed Explanation: Pair-Instability Supernovae
The observed mass gap supports the theory of pair-instability supernovae. In extremely massive stars, intense heat can cause light particles to transform into electron-positron pairs. This process reduces the internal pressure supporting the star, leading to a rapid collapse. Instead of forming a black hole, this collapse triggers a runaway thermonuclear explosion that obliterates the entire star, leaving no remnant. The absence of black holes in the identified mass range serves as indirect evidence for these stellar explosions.
Research Leadership
The project was led by Hui Tong, a PhD candidate from Monash University's School of Physics and Astronomy and the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav). Major collaborators include Professor Maya Fishbach from the University of Toronto and CITA, and Professor Eric Thrane from Monash and OzGrav. Hui Tong stated that the study found a mass range where stars do not appear to form black holes, and that black holes within this range originate from merging smaller black holes, not from direct stellar collapse.
Black Holes in the "Forbidden Range"
A small number of black holes have been detected within this otherwise absent mass range. One primary hypothesis is that these black holes form through successive mergers of smaller black holes, gradually accumulating mass beyond what is possible from a single stellar collapse.
Two Distinct Black Hole Populations from Stellar Collapse and Hierarchical Mergers
A study published in Nature Astronomy analyzed gravitational wave data from the LIGO-Virgo-KAGRA collaboration's GWTC-4 catalog, which includes 153 black hole merger detections. The researchers identified two distinct populations: a lower-mass population consistent with ordinary stellar collapse and a higher-mass population with spin characteristics indicative of hierarchical mergers in dense star clusters.
The high-mass black holes, above approximately 45 solar masses, exhibit more rapid spins with random orientations, matching predictions for repeated mergers in cluster environments. The study provides evidence for a pair-instability mass gap around 45 solar masses. Co-author Dr. Isobel Romero-Shaw stated that the separation of the high-mass population was surprising. The research was led by Fabio Antonini from Cardiff University.
Potential Primordial Black Hole Detection
A gravitational-wave signal, designated S251112cm, was detected on November 12, 2025, by the LIGO observatories. The signal indicated a collision where one object had a mass less than a single solar mass, which is not typically observed in black holes formed from stellar collapse.
Primordial Black Hole Hypothesis
A study published in the Astrophysical Journal by Nico Cappelluti and Alberto Magaraggia from the University of Miami provides a quantitative framework suggesting the signal is consistent with a merger involving a primordial black hole (PBH). Primordial black holes are theoretical black holes hypothesized to have formed during the universe's early stages, specifically during the QCD epoch, when extreme density regions collapsed directly into black holes without requiring a star's collapse, as initially proposed in 1966 by Soviet physicists Yakov Zel'dovich and Igor Novikov and further detailed by Stephen Hawking and Bernard Carr.
Caveats
The researchers noted that this single detection does not confirm the existence of primordial black holes. The signal's false alarm rate is approximately once every four years, and final parameter analysis from the LIGO collaboration was pending. Alternative explanations, such as rare stellar processes involving sub-solar neutron stars, are considered less probable. Extensive electromagnetic follow-up did not identify a kilonova counterpart.
Future Prospects
Future upgrades to LIGO and upcoming instruments, including the European Space Agency's Laser Interferometer Space Antenna (LISA) in 2035 and the proposed U.S. Cosmic Explorer, are expected to provide increased sensitivity to detect more sub-solar merger events and further investigate the primordial black hole hypothesis. If identified, primordial black holes could contribute to understanding dark matter, which is hypothesized to constitute approximately 85 percent of the universe's matter.
Dark Matter Imprints in Gravitational Waves
A study published in Physical Review Letters on May 23, 2025, developed a method to predict gravitational wave signals from black hole mergers occurring in dark matter environments, as opposed to vacuum conditions.
Methodology and Findings
The method was applied to 28 clear gravitational wave signals from the LIGO-Virgo-KAGRA observatories' first three observing runs. 27 signals were consistent with mergers in a vacuum. One signal, GW190728, showed a preference for the dark matter model, but with low statistical significance.
Background and Statements
Dark matter is hypothesized to make up over 85% of the universe's matter, interacting only via gravity. "Light scalar" dark matter particles, when near spinning black holes, may undergo superradiance, increasing in density. High-density dark matter around merging black holes could imprint on the emitted gravitational waves. Co-authors included researchers from MIT, UCLouvain, University of Amsterdam, Queen Mary University of London, and Oxford University.
Josu Aurrekoetxea (MIT postdoc) stated that the method allows searching for dark matter imprints but that current evidence is not sufficient for a detection. The authors noted that without such models, dark matter mergers might be misclassified as vacuum events. Further analysis by independent groups was recommended before any claims of detection.
High-Energy Neutrino Detection and Potential Origins
In 2023, a neutrino with an energy level of approximately 220 PeV was detected by the Cubic Kilometer Neutrino Telescope (KM3NeT), located off the coast of Sicily in the Mediterranean Sea. This energy is reported to be one billion times more energetic than an average solar neutrino and significantly exceeds the energy levels produced by the Large Hadron Collider. The detection, designated KM3-230213A, was recorded on February 13, 2023, when the KM3NeT detector was operating with 21 active detection lines, corresponding to approximately 10% of its final planned volume.
Candidate Explanations
Multiple potential origins for this neutrino have been proposed by different research teams:
Blazar Hypothesis: A study published in the Journal of Cosmology and Astroparticle Physics (JCAP) by the KM3NeT collaboration suggests the neutrino's source may be a population of blazars. Blazars are active galactic nuclei that host supermassive black holes and emit plasma jets directed toward Earth. The model accounted for the rarity of such ultra-high-energy events in existing neutrino datasets and ensured the blazar contribution did not exceed the extragalactic gamma-ray background measured by the Fermi Gamma-ray Space Telescope. The absence of an electromagnetic "counterpart" coinciding with the neutrino suggests it may originate from a diffuse background rather than a single event.
Primordial Black Hole Hypothesis: Researchers from the University of Massachusetts Amherst, in a study published in Physical Review Letters, propose that the neutrino could originate from the explosion of a "quasi-extremal primordial black hole." According to Stephen Hawking's theory of Hawking radiation, black holes lose mass over time by emitting particles. Primordial black holes, which are smaller, would emit Hawking radiation efficiently enough to explode within a relevant timescale, producing high-energy neutrinos.
Detection Discrepancy and "Dark Charge" Model
The IceCube Neutrino Observatory, located at the South Pole, has not detected a neutrino event of comparable energy, despite being designed for such observations. To address this discrepancy, the UMass Amherst team proposes that primordial black holes possessing a "dark charge"—termed quasi-extremal PBHs—could be the missing factor. This dark charge is described as a copy of the electric force, incorporating a hypothesized, very heavy particle called a "dark electron." The dark-charge model is suggested to account for the otherwise unexplained neutrino event.
It is also noted that IceCube's detection capabilities are limited to 10 PeV, which could explain its non-detection of KM3-230213A. Michael Baker, the lead researcher, stated that the increased complexity of their dark-charge model may offer a more accurate representation of reality. If the hypothesis is valid, the model may provide experimental verification of Hawking radiation, evidence for primordial black holes and new particles beyond the Standard Model, and a potential explanation for dark matter.