Predicted by Albert Einstein in his 1916 theory of general relativity, gravitational waves were first directly observed on Earth in 2015. They are the distant echoes of the most violent celestial events. But while they travel through the Universe at the speed of light, they are very difficult to observe, as the deformations they generate are infinitesimal, and the best terrestrial installations are limited in the range of observable frequencies by ambient noise.
LISA stands for Laser Interferometer Space Antenna, and is the 3rd "L-class" mission in ESA’s European Cosmic Vision program. Scheduled for launch in 2035, its objective is to detect gravitational waves.
The mission will consist of three satellites that will accompany the Earth on its orbital trajectory around the Sun. Positioned in its wake, they will follow the Earth at a distance of 50 million kilometers. Arranged in an equilateral triangle measuring 2.5 million kilometers on each side, they will form a gigantic optical interferometer oriented at 60° to the plane of the Earth’s orbit.
The units will be linked 2 by 2 by identical, synchronized laser signals, to measure displacements between free-falling test masses inside each instrument. Placed in an environment isolated from external disturbances (solar wind, parasitic forces, etc.), these test masses will witness the tiniest perturbations in space-time that encounter them. The expected sensitivity is 10 picometers, in a range of low gravitational frequencies not yet explored and impossible to measure with the LIGO and VIRGO laboratories, operating from the ground.
Observatoire de Paris - PSL is involved in a number of key areas :
◾ Data pre-processing:
SYRTE is responsible for developing part of the mission’s data analysis chain: data pre-processing.
When the instrumental data from the three satellites in the LISA constellation are received, the gravitational wave signals will be completely buried and invisible in the instrumental noise. The aim of data pre-processing - the most upstream element of the analysis chain - is to correct the experimental measurements for a large number of disturbing effects.
In a second step, a clever combination of these on-board data allows us to infer time series in which instrumental noise is greatly reduced.
Gravitational wave signals thus become clearly visible and identifiable in these pre-processed products. This is a critical and indispensable step in extracting the scientific information required.
◾ Instrumentation:
A contribution is also made at instrumental level, with activities on the IDS (Interferometric Detection System) and OTS (Optical Test System) devices, whose scientific managers are members of APC and SYRTE respectively.
This involves ground testing of payloads, the most advanced and complex technology on the mission.
In particular
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- APC supplies the prototype interferometric test benches, and supports the industrial development of the final test benches.
- SYRTE supplies the prototype laser systems required for these ground tests (frequency-stabilized lasers on iodine), and supports the industrial development of the definitive IDS and OTS laser systems.
◾ Scientific exploitation:
SYRTE and LUTH are also involved in a number of scientific exploitation projects for the LISA mission:
SYRTE’s Theory and Metrology team is taking up one of the major challenges of the LISA mission, which is the detection of a superposition of stochastic gravitational wave backgrounds, homogeneous and isotropic, originating from physical processes that occurred in the first instants of our Universe (phase transitions, cosmic strings, inflation, etc.). The challenge is to :
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- to understand the spectral forms of these signals, which are relatively poorly understood.
- to separate two types of noise - one cosmological, the other instrumental - without knowing the spectral shape of either.
![<multi>[fr] Les systèmes binaires de trous noirs supermassifs situés au cœur des galaxies produisent des ondulations dans l'espace-temps appelées ondes gravitationnelles. Les ondes gravitationnelles émises par toutes les binaires de trous noirs supermassifs de l'univers se combinent pour former un bruit de fond stochastique.[en] Supermassive binary black hole binary systems at the cores of galaxies produce ripples in spacetime called gravitational waves. Gravitational waves from all of the supermassive black hole binaaries in the universe combine to form a stochastic background.</multi>](IMG/jpg/gravwaves_2560_hq.jpg)
The team is also developing new waveforms to characterize magnetism and tidal dissipation in galactic binary systems. The idea is to use these systems to prepare new tests of Lorentz invariance, or to model binary systems with extreme mass ratios, known by the acronym EMRI (Extreme Mass Ratio Inspiral), near the Galactic Center. A few years ago, a collaboration was launched between LUTH, LESIA and SYRTE to study EMRIs around the supermassive black hole at the center of our galaxy, Sagittarius A*.
The Relativity and Compact Objects team at LUTH is carrying out a wide range of work on modeling astrophysical sources and testing gravitational theories, a crucial step in extracting signals from LISA data. These include :
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- the study of extreme mass ratio binary systems (EMRIs) and participation in the international effort to develop a self-force formalism for calculating the gravitational waveforms generated by EMRIs, which are among the main sources expected for LISA;
- fundamental physics modeling of the various forms of gravitational waves;
- participation in several LISA collaboration working groups, and contribution to the drafting of a scientific preparation white paper for the mission launch.