Gravitational waves (GW) correspond to changes of the metric properties of the space that propagate through the universe at the speed of light. Nowadays, their direct detection has become an important challenge in physics, as it will allow to thoroughly test the Einstein's General Relativity and open a new window on the Universe with implications in astrophysics and cosmology.

• Given the importance of the scientific and technological developments, the challenge of their experimental detection is tackled by a worldwide network of detectors. It comes in three complementary approaches in terms of spectral coverage: ground based interferometers (Virgo, LIGO, etc..) from 10 Hz - 10 kHz ; in space, with eLISA/NGO , which covers from a few mHz to a fraction of hertz; and the pulsar timing arrays, covering the very low frequencies (~ nHz). A first generation of detectors, operating a few years ago, validated the technological and instrumental choices, demonstrating high precision metrology. From 2016, it will give way to a second generation with a sensitivity ten times better, probing a volume of universe a thousand times larger and leading (probably) to the first direct detection of gravitational waves. 
  

Among the sources of gravitational waves, violent astrophysical events (mergers, gravitational collapse) involving compact objects such as neutron stars and black holes, may be accompanied by electromagnetic radiation and particles (neutrinos, in particular) at high energy. The observation of an electromagnetic counterpart (optical, X-ray, gamma, radio...) and neutrinos (low or high energy), could then be a crucial ingredient to identify the astrophysical origin of the GW signals.

• This prospect justifies a "multi-messengers approach", which consists in correlating the GW detectors with other observational methods. Such a strategy requires cooperation with optical telescopes designed for quick follow-ups on a "robotic" opportunistic mode, and with the projects of wide field transient surveys in the radio frequency band (LOFAR), or optical/near-infrared (LSST). At higher energy, a good synergy also exists with the search for gamma-ray bursts, via Fermi or Swift, and the Sino-French mission SVOM. In the neutrino spectrum, work has been initiated with ANTARES and and can be extended naturally to the future European KM3 neutrino detector.

CNRS is heavily involved in ground based detectors with Virgo. The first generation has demonstrated the feasibility of these experiments through scientific runs at the expected sensitivity and, for lack of detections, has established interesting astrophysical limits on sources. It also gave the opportunity to operate the various detectors as a network and to take the first steps toward a multi-messenger astronomy involving gravitational waves. The second generation, due to come online in a few years with a 10-fold increase in sensitivity (Advanced Virgo, Advanced LIGO...), offers rich prospects and the promise of true gravitational astronomy. In the longer term, the third generation European project, the Einstein Telescope (ET), will allow us to probe the Universe in depth. ET is one of the "Magnificent Seven", the projects recommended by the ASPERA network for the future developments of Astroparticle Physics in Europe.

For reasons of efficiency, gravitational wave science is a global effort. Extracting information on GW sources requires several interferometers operating simultaneously on different sites. The scientific communities in the United States (LIGO), Germany - United Kingdom (GEO600) and Italy - France and the Netherlands (Virgo) share technologies, R&D and theoretical advances, as well as data analysis methods. The European project ET will help improve this cooperation around the world.