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Gravitational waves

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According to General Relativity theory, any mass or energy modifies the geometry of spacetime, which means that they change the distances and the time measured with a rule and a clock. If the mass has an asymetric accelerated motion, the spacetime modification can propagate like a wave on a water surface : this is a gravitational wave. More details can be found on the Virgo experiment web site.

When a massive star explodes as a supernova, when a black hole is created, when a neutron star rotates and emits periodically an intense electromagnetic radiation or when two neutron stars inspirals around each other, gravitational waves can be emitted. They are so tinily absorbed by the matter that we can detect them on Earth even when they come from sources several million light-years away.

A first indirect evidence of the existence of gravitational waves was obtained thanks to the binary neutron stars PSR1913+16, discovered in 1974 by Russell Hulse and Joseph Taylor. Using the Doppler effect, Joel Weisberg and Joseph Taylor measured, over more than 30 years, the increase of the orbital speed of the two stars and showed that the orbital period decreases in in full agreement with the hypothesis of gravitational waves emission.

Since September 14th 2015, the interferometric detectors of the LIGO and Virgo collaborations have provided the first direct evidence of the existence of the gravitational waves, by detecting several times those waves emitted by binary black hole or neutron star coalescences.

Especially, on August 17th 2017, the LIGO and Virgo interferometers have started a new multi-messengers astronomy by detecting gravitational waves produced by the coalescence of two neutron stars. A detection which has beenfollowed by those done in the electromagnetic wave domain by several observatories and satellites.