Gravitational waves

The IP2I Gravitational Waves group is part of the Virgo scientific collaboration and the LIGO–Virgo–KAGRA (LVK) network. It contributes to data acquisition, data analysis, and mirror production, supporting the LMA platform at IP2I. The goal of the LIGO-Virgo-KAGRA network is to detect and characterize gravitational waves, whose recent observation has opened the door to a new form of astronomy, previously limited to the detection of electromagnetic radiation alone.

In 2025, ten years after the first direct detection of a gravitational wave signal by the LIGO detectors, the LVK network has completed its fourth joint observation period. The review of this first decade is a clear success, with around 400 public alerts for gravitational wave signals, released less than a minute after the signal reached Earth, the majority corresponding to black hole coalescences in binary systems. LVK network data is only partially public and already includes more than de 200 confirmed detections. A significant portion of the collected data is still under analysis and will gradually be made available to the scientific community by the end of 2026. Many discoveries and analyses in fundamental physics are still forthcoming, making this first decade of gravitational wave astronomy a field that remains highly dynamic.

Thanks to gravitational waves, it is now possible to both observe and “listen” to the Universe from multiple independent sources! Gravitational waves provide complementary information about the Universe in addition to the historic messenger of electromagnetic radiation (visible light, X-rays, radio waves, microwaves, etc.). This was the case in 2017 during the first observation of a gravitational wave (GW170817) from the merger of two neutron stars. This discovery marked the birth of multi-messenger astronomy with gravitational waves.

Virgo, KAGRA, and the two LIGO detectors are Michelson interferometers: they combine two laser beams traveling along two perpendicular arms several kilometers long to produce an interference pattern that can be analyzed. Gravitational waves are vibrations of space-time: as they pass, the relative lengths of the interferometer arms are affected very slightly (10⁻¹⁹ m) but enough to produce a measurable effect on the interference pattern.

The Gravitational Waves group at IP2I is involved in data acquisition and analysis from the LIGO , Virgo and KAGRA interferometers (LVK) as well as in monitoring the quality of detections. The group is also engaged in the European third-generation gravitational wave detection project, the Einstein Telescope (ET). Through the LMA platform, IP2I also participates in the development and production of mirrors for gravitational wave detectors.

This page provides a concise overview of the group’s interests and activities, along with a (non-exhaustive) list of links to relevant publications. The page was last updated in February 2026.

For further questions, please contact Viola Sordini.

LVK – Detection of Compact Object Coalescence Signals

At IP2I, we analyze data from the LIGO-Virgo-KAGRA interferometers to search for gravitational wave signals from the coalescence of compact objects such as black holes or neutron stars.

We use the Multi-Band Template Analysis code, one of the tools employed within the LVK network, which applies matched filtering techniques based on a large library of precomputed waveforms. At IP2I, we focus particularly on offline analyses for the Gravitational Waves Transient Catalogs, flagship publications of the LVK network.

We also study gravitational wave signals from coalescences of compact objects lighter than the Sun, which cannot be produced by known stellar evolution processes but are predicted, for example, in primordial black holes models. While no signal has yet been detected in this category, these searches have interesting implications for dark matter models.

Selected recent associated publications (non-exhaustive):

• The MBTA pipeline for detecting compact binary coalescences in the fourth LIGO-Virgo-KAGRA observing run
• GWTC-4.0: Updating the Gravitational-Wave Transient Catalog with Observations from the First Part of the Fourth LIGO-Virgo-KAGRA Observing Run
• Search for subsolar-mass black hole binaries in the second part of Advanced LIGO and Virgo’s third observing run

LVK – Cosmology with Gravitational Waves and Black Hole Populations

Après une détection confirmée, les données de LIGO, Virgo et KAGRA sont analysées plus en détail par des processus d’inférence bayésienne, pour déterminer les propriétés des objets astrophysiques qui ont généré le signal observé. L’équipe de l’IP2I s’intéresse à ces analyses. Dans le passé, cet intérêt a porté sur les études de matière dense. Plus récemment, le groupe se concentre sur la compréhension des populations de trous noirs et la cosmologie.

After a confirmed detection, LIGO, Virgo, and KAGRA data are analyzed in more detail using Bayesian inference to determine the properties of the astrophysical objects that generated the observed signal. The IP2I team is involved in these analyses. Historically, the focus was on dense matter studies; more recently, the group has concentrated on understanding black hole populations and cosmology.

Gravitational waves from compact object coalescences provide information about the source’s luminosity distance. Combined with an estimate of the source’s cosmological redshift, they can be used to measure the Universe’s expansion history, in particular the Hubble constant (H₀). Direct measurement of H₀ via gravitational waves is especially relevant today due to the significant tension between values derived from Type Ia supernovae (associated with Cepheids) and those predicted by the ΛCDM model calibrated with high-redshift cosmic microwave background data.

Since the number of gravitational wave observations with independently known redshifts is still relatively limited, alternative methods have been developed. The IP2I team is particularly involved in the population method using the ICAROGW, which aims to determine H₀ and the mass distribution of observed black holes jointly from many binary black hole coalescence observations.

Selected recent associated publications (non-exhaustive):

• ICAROGW: A python package for inference of astrophysical population properties of noisy, heterogeneous, and incomplete observations
• Study of systematics on the cosmological inference of the Hubble constant from gravitational wave standard sirens
• The spin magnitude of stellar-mass black holes evolves with the mass
• GWTC-4.0: constraints on the cosmic expansion rate and modified gravitational-wave propagation

LVK – Virgo Detector Characterization and Data Quality

We also participate in data acquisition with the Virgo detector, as well as in understanding and monitoring the quality of its data (both on-site and offline). Group members take part in Virgo operations and are active in characterizing specific background noise sources, which are used to establish data quality criteria for analyses. During the fourth LVK observation period, group members contributed to real-time monitoring of potential gravitational wave signal alerts.

The team has a particular interest in the impact of Virgo data on LVK science, especially regarding the precision of signal localization in the sky.

Selected recent associated publications (non-exhaustive):

• Virgo detector characterization and data quality: results from the O3 run
• Characterizing Low-Latency Sky Localization in Multi-Detector Gravitational-Wave Networks

LVK – Computing

IP2I is engaged in computing for the LVK network and is responsible for managing links between the Virgo collaboration, the LVK network, and the CCIN2P3 computing center in Lyon.

LVK – Data Acquisition and artificial intelligence

IP2I is involved in the Virgo data acquisition system. In this context, and in collaboration with the eDAQ service, the group is exploring applications of artificial intelligence techniques.

ET – Preparation, Collaboration and Digital Infrastructure

The IP2I team is heavily involved in the Einstein Telescope Preparatory Phase (2022–2026), particularly in the design and implementation of a computing model and the optical design of the interferometer (via LMA). IP2I is also engaged in the construction of the ET collaboration.

Selected recent associated publications (non-exhaustive):

• Computing Challenges for the Einstein Telescope project.

Mirror Production, Research & Development

The reflective coatings of all main mirrors used in the LIGO and Virgo interferometers (as well as KAGRA) have been produced by the Advanced Materials Laboratory (LMA), an IP2I platform. LMA is a world leader in manufacturing optics for gravitational wave detectors and is highly active in research and development for mirrors for next-generation detectors.

For more details, visit the LMA webpage .

• What are gravitational waves?
• Why detect gravitational waves?
• How to detect gravitational waves ?
• News from Virgo and LIGO
• The future

What are gravitational waves ?

Gravitational waves are space-time “quivers” caused by some of the most violent and energetic processes in the Universe, such as the fusion of black holes and/or neutron stars, the collapse of supernovae or the rotation of neutron stars that are not perfectly spherical. In addition, gravitational wave detectors could eventually succeed in measuring the remnants of the gravitational radiation of the primordial Universe.

Gravitational waves travel through the Universe at the speed of light, carrying valuable information about the phenomena at their source: measuring them has profound implications for astrophysics, cosmology, nuclear physics and helps to understand the nature of gravity itself.

The existence of gravitational waves was predicted by Einstein in 1916 and the first detection, by LIGO interferometers, occurred in 2015. Since then, several signals have been (are) detected, giving rise to a new way of listening to the Universe…

Why detect gravitational waves ?

Historically, scientists have relied almost exclusively on electromagnetic radiation (visible light, X-rays, radio waves, microwaves, etc.) to study the Universe. Recently, two additional messengers have come to provide additional and complementary information: neutrinos and gravitational waves.

Gravitational waves are totally independent of EM radiation and interact very weakly with matter, allowing us to obtain undistorted information about their origin and to observe events invisible to EM radiation (such as colliding black holes).

Finally, in some cases, the same event can give rise to several detectable signals, from EM radiation to neutrinos and gravitational waves: we can now watch and listen to the Universe from several independent sources!

This was the case, for the first time, during the coalescence of two neutron stars GW170817, which marked the birth of multimessenger astronomy.

Nowadays, each time a signal is detected by LIGO / Virgo, an automatic alert is generated, so that astronomers and neutrino physicists can make associated observations!

How to detect gravitational waves ?

Virgo and the two LIGO detectors are Michelson interferometers: they superimpose two light sources to obtain an interference pattern that can be analyzed. They are composed of two perpendicular arms of the same length, where two laser beams are trapped by mirrors and converge towards a photodetector, designed to be in perfect destructive interference in the absence of gravitational waves.

Gravitational waves are space-time vibrations: as they pass through, space itself stretches in one direction, while compressing in the perpendicular direction. The passage of a gravitational wave therefore causes the length of an arm of the interferometer to oscillate, inducing a measurable effect on the interference pattern.

Such length changes are very small (of the order of 10-19 meters!) and therefore very difficult to detect, over background noise: the detection of gravitational waves by LIGO and Virgo is a huge success also from a technological point of view.

For more information, you can visit the websites of  LIGO, Virgo and KAGRA.

For more ressources on gravitational waves, available data and data analysis techniques, please refer to the  Gravitational Waves Open Science Center.

News from Virgo and LIGO

Here is the summary and plans for LVK observing periods: https://observing.docs.ligo.org/plan/.

Check the public list of alerts for gravitational wave signals!

The future

In the longer term, several projects exist to continue exploring the Universe with gravitational waves, either on Earth with increased sensitivity (Einstein Telescope, Cosmic Explorer) or in space, looking at different frequencies and thus different phenomena (LISA).