The Innovation Platform spoke with Viola Sordini, researcher at IP2I Lyon (CNRS) and deputy spokesperson of the Virgo Collaboration within the LIGO-Virgo-KAGRA (LVK) network, about the recently released Gravitational-Wave Transient Catalogue-4.0 (GWTC-4) and its significance for understanding gravitational waves and fundamental physics.
The recent publication of the Gravitational-Wave Transient Catalogue-4.0 presents 128 new events detected by the LIGO-Virgo-KAGRA collaboration from May 2023 to January 2024, marking a significant milestone in our understanding of the Universe. This extensive compilation provides insights into diverse binary systems, enhances our understanding of black hole formation and cosmic evolution and offers an additional method to validate the theory of general relativity.
The Innovation Platform spoke with Viola Sordini, researcher at IP2I Lyon (CNRS) and deputy spokesperson of the Virgo Collaboration, to discuss the findings from the catalogue and the implications of advancing our understanding of gravitational waves for fundamental physics and the Universe.
Can you provide a brief summary of gravitational waves and explain why understanding them can help us better understand the Universe?
The short answer is that gravitational waves (GWs) help us understand the Universe in two ways: (1) because of the information they carry as messengers, and (2) simply because they exist!
Understanding the Universe – the laws that govern it and its evolution – is one of the oldest human aspirations.
While we can learn a lot from controlled experiments in laboratories on Earth, part of our knowledge necessarily comes from observing events that we have no hope of reproducing in the lab. For example, we understand particle physics extremely well because it can be tested experimentally, but this describes only about 5% of the
energy content of the Universe – the so-called ordinary matter.
The Universe is vast, and all sorts of astrophysical events take place within it. As observers, we are like detectives: we collect clues and traces of these events and try to reconstruct what happened.
Historically, these clues have come mainly from electromagnetic radiation–light across all wavelengths, from radio to infrared, visible, X-rays and gamma rays. We can think of light as a messenger that brings us precious information. Other messengers also exist, such as neutrinos and cosmic rays, and the scientific community has developed a wide range of experimental infrastructures to detect them and extract as much information as possible.
These messengers take time to reach us, so we constantly receive information from processes that occurred at different stages of the Universe’s history. Their signal can also be altered during propagation, which must be taken into account.
With their first direct detection in 2015, gravitational waves were clearly established as a new type of cosmic messenger. Like light, many astrophysical systems can emit GWs across a wide range of frequencies, and different frequencies probe different physical phenomena, requiring different experimental strategies.
The signals observed today by the LIGO–Virgo–KAGRA network mainly come from binary systems of compact objects, such as neutron stars or black holes. In these systems, the two objects orbit each other faster and faster, losing energy through gravitational waves and spiralling inward until they merge into a single compact object, most often a more massive black hole. These systems were essentially invisible to us before and have been revealed for the first time through gravitational waves.
As of April 2026, the LIGO-Virgo-KAGRA network has reported a few hundred such events. They have helped us understand the Universe in many ways: they inform us about the nature and formation of black holes, the physics of neutron stars and the production of heavy elements, and they even allow us to measure how fast the Universe is expanding.
Gravitational waves are also important simply because they exist. Their existence is a striking confirmation of Einstein’s general relativity, our current theory of gravity, which describes it in a fundamentally different way from the other interactions. Unlike the other interactions, which are described by quantum field theories in terms of particle exchange, general relativity describes gravity as the geometry of four-dimensional spacetime.
In this theory, spacetime is not rigid: it can be curved by the presence of mass and energy. Accelerating masses can radiate energy in the form of ripples in this curvature – gravitational waves – that propagate across the Universe.
Better understanding gravity is a powerful way of exploring the Universe. Indeed, it is through gravitational effects that we have obtained observational evidence for some of the most intriguing phenomena in modern physics, such as dark matter and dark energy.
What are the key findings from the Gravitational-Wave Transient Catalogue-4.0 (GWTC-4)? Can you explain the significance of specific events such as GW231123 and GW231028?
The first outstanding feature of GWTC-4 is the accumulation of statistics: the LIGO-Virgo-KAGRA collaboration has now reported more than 200 confirmed gravitational-wave signals, emitted by binary systems of black holes and/or neutron stars. It is worth noting that the GWTC-4 dataset includes only gravitational-wave signals collected during the first third of the fourth LIGO-Virgo-KAGRA observing run. Additional data from subsequent observations are scheduled for release on May 26 and December 16, 2026, and are expected to approximately double the number of confirmed detections. All these observations feed into subsequent analyses – within the LVK and beyond – and help test general relativity, measure the expansion rate of the Universe, improve our understanding of black hole populations, and search for effects such as gravitational lensing.
Most of the observed signals are consistent with binary black hole systems. Two events, however, are interpreted as mergers between a black hole and a neutron star. In one of them, GW230529, the black hole is among the lowest-mass black holes observed by the LVK, raising questions about its exact nature and formation. Some observations from the remainder of the fourth observing run—not included in GWTC-4—have already attracted special attention and are publicly available. These include GW250114, the loudest LIGO-Virgo-KAGRA observation to date, which has enabled unprecedented tests of black hole properties. Additionally, the pair of events GW241011 and GW241110 provide valuable insights into black hole formation and the dynamics of binary systems.
The event GW231123 was generated by the merger of two black holes of approximately 100 and 140 times the mass of the Sun, making it the most massive binary system observed by the LVK so far. Black holes in this mass range are difficult to form through standard stellar evolution alone and may instead be the result of previous mergers. This scenario is supported by the fact that the black holes are inferred to be rapidly spinning, with the heavier one among the fastest-spinning black holes observed to date. Because of its high mass, this system also allowed the LVK detectors to clearly observe not only the inspiral, but also the merger and the ringdown – the relaxation phase of the remnant black hole – providing a particularly clean test of general relativity.
Although slightly lighter, the binary black hole system responsible for GW231028 also has a high total mass of about 150 times the mass of the Sun. It is notable for exhibiting one of the strongest and most confidently measured spin alignments, with the spins of the black holes largely aligned with the orbital angular momentum of the system.
How do these observations enhance or change our understanding of the Universe and existing theories?
With GWTC-4, the LIGO-Virgo-KAGRA collaboration roughly doubles the number of observed gravitational-wave signals.
These new observations do not drastically change the conclusions of previous analyses, but they significantly strengthen them by increasing the available statistics. In particular, they allow us to better characterise the masses and spins of stellar-origin black holes, and to improve our understanding of how such objects form and end up in binary systems.
The data also provide further evidence for black holes that are too massive to be easily explained by standard stellar evolution. These so-called intermediate-mass black holes may instead be the result of successive mergers (hierarchical formation). In addition to GW231123, several events in the catalogue have a remnant mass above 100 solar masses with high probability, supporting this scenario. In addition, the growing set of spin measurements – both their magnitudes and orientations – provides important clues about the formation channels of these systems, helping to distinguish binaries formed in isolation from those assembled dynamically in dense environments.
At the other end of the mass spectrum, the event GW230529 carries the imprint of a merger between a neutron star and a compact object with a mass in the so-called ‘lower mass gap’, a region expected to be depleted of compact objects formed through standard stellar evolution. The heavier object in this system is therefore either the lowest-mass black hole or the highest-mass neutron star observed by the LVK so far.
More generally, the catalogue includes another observation of a neutron star–black hole binary. These systems are particularly interesting because neutron stars contain matter at extreme densities, and the gravitational-wave signal can, in principle, provide information about their internal structure. Although the current observations do not yet allow for precise constraints, the detection of such systems is very promising.
The interpretation of gravitational-wave signals from compact binaries relies on accurate waveform models, computed within general relativity. The data can then be used to test how well the theory describes the observations. The new events in GWTC-4 enable more stringent tests of general relativity and confirm its excellent agreement with the data. They also place strong constraints on alternative theories predicting possible deviations, in some cases providing the most stringent bounds to date.
Finally, GWTC-4 observations are also used to search for signatures of gravitational lensing of gravitational waves by massive objects along their path. No evidence has been found so far, which translates into constraints on the rate of such phenomena.
The findings from GWTC-4 play a role in measuring the expansion rate of the Universe. Can you discuss how this is achieved and the benefits of using gravitational waves to measure expansion?
A GW signal from a compact binary coalescence carries the information of the luminosity distance of the source.
The luminosity distance tells us how far away a source is, inferred from how its signal weakens as it travels to us.
With light, the luminosity distance is usually inferred indirectly using sources of known brightness, while gravitational waves provide a more direct measurement.
Any signal reaching Earth has been affected by the expansion of the Universe, and a quantity called redshift measures how much this expansion has stretched the signal during its propagation, shifting it to lower frequencies.
Because the Universe is expanding, more distant sources appear more redshifted, and the precise relation between distance and redshift depends on how fast the Universe expands.
A GW signal, if combined with a measurement of the redshift of its source, can help determine the current rate of expansion of the Universe, described by a quantity named H0.
The golden way to measure the redshift of the source of a GW signal from a compact binary merger is to observe an electromagnetic counterpart from the same event, which allows us to identify and localise its host galaxy. The redshift of the galaxy can then be measured from its electromagnetic spectrum and combined with the luminosity distance obtained from the GW observation.
Unfortunately, none of the observations in GWTC-4 had an electromagnetic counterpart. Such counterparts are only expected for mergers involving neutron stars–most prominently for binary neutron star mergers–which are relatively rare. Alternative methods are therefore needed to estimate the redshift of the sources.
The scientific community has also been inventive in finding ways to exploit the hundreds of signals observed from binary black hole systems. A notable method relies on galaxy catalogues, which are collections of observed galaxies with associated redshift measurements. In addition to measuring the luminosity distance, the LVK can localise the source of a GW signal on the sky. We can then identify the galaxies that are consistent with this localisation and use their redshifts. It can be shown that, with increasing statistics, this method converges towards the correct value of H0. Another method relies on the fact that the masses we observe in our detectors are affected by the expansion of the Universe and therefore carry information about the redshift of the source. This approach aims at constraining the black hole mass distribution and the H0 parameter jointly.
By combining these methods and analysing all detections up to GWTC-4, the LIGO-Virgo-KAGRA collaboration has been able to measure H0 with increased precision compared to previous analyses. Although this measurement is not yet competitive with the most precise standard methods, it is very interesting because it is complementary and relies on completely independent assumptions.
Have you experienced, or do you anticipate, any challenges in analysing and interpreting the vast dataset derived from these gravitational wave detections?
With increasing sensitivities and ever longer observing campaigns, the LIGO-Virgo-KAGRA network is certainly facing challenges in data analysis and in the optimisation of computing resources.
One important challenge is to ensure that we can analyse the data with very low latency, in order to rapidly identify interesting signals and send alerts to the broader astronomical community for potential follow-up observations. On longer timescales, the analyses that produce the results included in the catalogue, as well as those that use these events for scientific interpretation, are also demanding in terms of resources. With such extended observing periods, low-latency and high-latency activities must proceed in parallel.
During the fourth observing period, the LVK infrastructure, analysis pipelines, and internal procedures have been put to a hard test, which our scientific collaboration has met thanks to improved methods and continuous research and development, better coordination, and a constant effort to learn from experience and improve.
In terms of interpretation, the growing number of observations is also bringing to light some special events that challenge our understanding. GW231123 is a typical example: the event was so massive, and the signal so short in the LVK sensitivity band, that our usual techniques for detecting compact binary coalescences were pushed to their limits. For this reason, although the most likely interpretation of the source of this signal is the merger of two very massive black holes, alternative scenarios cannot be completely excluded. These include, for example, gravitational lensing or more exotic possibilities such as primordial black holes, boson star mergers, or cosmic strings.
Were there any specific technical advancements that enabled the detection of new gravitational wave signals?
Certainly, the improvement in detector sensitivities has a direct impact on the success of the fourth observing period. These improvements come from a continuous effort of the observatories’ teams to better understand and reduce noise sources, as well as from upgrades to the instruments themselves, including higher laser power, light squeezing and improved mirror quality and control.
Part of this success also comes from the ever-increasing coordination between the different observatories and communities within the LIGO–Virgo–KAGRA network. The streamlining of operations, the robustness of procedures, and the stability of the detectors – ensured by our deep understanding of their behaviour – have allowed us to carry out the longest joint observing run to date, with very high duty cycles.
In addition, advances in data analysis methods have improved our ability to identify weaker and more complex signals. This includes more accurate waveform models, better noise mitigation techniques, and improved search pipelines, which together increase both the sensitivity and the reliability of detections.
How might the data from the GWTC-4 catalogue influence future gravitational wave detection and shape research priorities moving forward?
In terms of physics, GWTC-4 confirms that the gravitational-wave frequency range explored by the LVK is a true treasure trove for understanding the Universe. In this sense, the scientific motivation for this field is further strengthened. These results also support the development of next-generation ground-based detectors, which will explore the same frequency range with much higher sensitivities, allowing us to observe black hole coalescences across a large fraction of the observable Universe.
At the same time, the success of the current physics programme is very promising for future detectors exploring different but neighbouring frequency bands. This is the case for LISA, the space-based mission that will be sensitive to gravitational waves in the millihertz region. Coalescing stellar-mass black hole or neutron star binaries could be observed by LISA years before their signal enters the sensitivity band of ground-based detectors.
In addition, LISA will allow us to access a completely different population from the one observed by ground-based detectors, as it will be sensitive to intermediate and supermassive black hole binaries, with masses up to about 10^7 times the mass of the Sun, illustrating the strong complementarity between the Earth-based and space-based scientific programmes.
Beyond the scientific results, the LVK has also learned important operational lessons from the production of GWTC-4, in particular in terms of coordination and organisation. The network is now moving towards an even more integrated global scientific collaboration, called IGWN (International Gravitational-Wave Observatory Network), where different observatories and scientific communities can work together in a coordinated and sustainable way to fully exploit these increasingly rich datasets, enabling increasingly precise and diverse scientific studies.
Please note, this article will also appear in the 26th edition of our quarterly publication.
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