Gravitational waves

The group Gravitational waves is part of the scientific collaboration LIGO / Virgo. It contributes to data collection, analysis and mirror production, in support of the IP2I’s LMA platform. The objective of LIGO / Virgo is to detect and characterize gravitational waves whose recent observation has opened the field to a new astronomy, formerly restricted to the detection of electromagnetic radiation only.

Thanks to gravitational waves, it is now possible to watch and listen to the Universe from several independent sources! Gravitational waves (link to the public gd tab) allow us to collect additional information about the Universe that is complementary to the historical messenger that is 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) coming from the coalescence of two neutron stars. This discovery marked the birth of multimessenger astronomy.

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

The Gravitational waves group of IP2I is involved in the data collection and analysis of LIGO / Virgo interferometers, as well as in the monitoring of the quality of the detections. The group is also involved in the development and production of mirrors by the LMA.

• Data analysis
• Mirror production, Research & Development
• Detector characterization

Data analysis

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

We do this using the analysis code of Multi-Band Template Analysis, in collaboration with our colleagues at the Annecy Laboratory of Particle Physics (LAPP) and the University of Urbino in Italy.

The analysis of the data from the third observation run O3 LIGO / Virgo is in progress and there is also a publication cataloguing the gravitational wave signals observed from the coalescences of compact objects during the first two observation runs.

Mirror production, Research & Development

All the mirrors used in the LIGO and Virgo interferometers (as well as those of KAGRA) were made by the Advanced Materials Laboratory (LMA), platform of the IP2I. The LMA is a world leader in the manufacture of optics for gravitational wave detectors. It is very active in the research and development of mirrors for new generation detectors.

Visit the LMA page for more details.

Characterization of detectors

We also take part in taking data with the Virgo detector, as well as understanding and monitoring the quality of these data (on site or offline). Members of the group participate in Virgo operations and are active in the characterization of specific background noise sources, used to establish data quality criteria to be used in analyses. Periodically, the group’s researchers are responsible for monitoring data quality to confirm or invalidate, in real time, possible alerts for gravitational wave signals.

• 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 and educational material, you can visit the websites Virgo and LIGO.

News from Virgo and LIGO

The second part of the third observation run, O3b continues until April 2020.

Check the public list of alerts for gravitational wave signals!

After the end of the third observation run, in April 2020, significant improvements are planned for the LIGO and Virgo detectors, leading to increased sensitivity. A fourth observation run should start in 2021, where the LIGO and Virgo detectors will also be joined by the interferometer KAGRAalready operational in Japan (which could even join the data collection by the end of O3). A fifth series of observations, planned for the mid 2020s, will also see the participation of the LIGO-India interferometer, an Indo-U.S. project.

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).

Page data analysis by the general public: This page which allows the general public to analyze real data.