Gravitational waves

Gravitational waves are ripples in spacetime produced by large masses in accelerated motion during violent astrophysical phenomena, such as, for example, the merging of pairs of black holes or neutron stars. Their possible existence was postulated, for the first time, by Albert Einstein in 1916, as the result of his general theory of relativity.
However, Einstein himself did not believe that it would ever be possible to observe them. Gravitational waves are actually very weak vibrations, very difficult to detect even with extremely sensitive equipment. This has not discouraged scientists who, beginning from the 1950s, started to design and build tools and experiments in order to prove and directly measure these signals.

Artistic representation of a neutron star, very small but very heavy, which rotates on itself 25 times every second, around which a white dwarf orbits every two and a half hours. (© ESO/L. Calçada)

Aerial view of the VIRGO interferometer (© EGO-INFN)

After decades of attempts and technological progress – from the technique of resonant bars to the use of interferometer lasers, which today constitute the reference detection tools for gravitational waves – the undertaking was finally achieved in September 2015. This was thanks to the LIGO interferometer in the United States, which observed a gravitational wave signal produced by the merging of two black holes at a distance of more than one billion light years from us. The Virgo experiment researchers also contributed to this historic discovery. Virgo was founded by INFN and the French CNRS and is located in Italy, at the European Gravitational Observatory. At the time of the first Virgo detection, it had not yet started data acquisition, but the two scientific collaborations had already been working since 2007 as a single, large global collaboration.

From then, over the course of three data acquisition runs, LIGO and Virgo observed approximately 90 signals of gravitational waves, produced by the merging of pairs of stellar black holes in almost all cases. The two experiments, which were recently joined by the Japanese KAGRA observatory, will continue their activities for approximately another ten years, before handing the baton to the next generation of experiments: the Einstein Telescope, which will be built in Europe and is led by INFN in cooperation with the Dutch research institute Nikhef, and Cosmic Explorer, in the United States. Both will be equipped with a much higher sensitivity. But the research into gravitational waves is not just limited to terrestrial experiments. The launch of LISA is, in fact, planned for the next decade – a space observatory of the European Space Agency and NASA. LISA will hunt gravitational waves with lower frequency than those accessible for terrestrial observatories, which are produced by even more extreme astrophysical events, like the merging of supermassive black holes at the centre of galaxies.

Rendering of a satellite of the LISA mission (© Max Planck Institute for gravitation physics)

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