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Gravitational Waves Universe

Artist’s rendition of a black hole merger © Alex Andrix

Gravitational waves (GWs) are ripples in spacetime that travel at the speed of light. Albert Einstein first predicted them in 1916, as a consequence of the theory of General Relativity.

After many decades of theoretical and experimental challenges,  on the 14th of September, 2015 the LIGO Scientific Collaboration and the Virgo Collaboration finally observed  them for the first time, using data from the two LIGO detectors. That first gravitational wave signal ever is known as GW150914 – signals are named based on the day (YYMMDD) they were recorded.

To produce significant gravitational waves, large and compact masses in accelerated relativistic motion are needed. Astrophysical events and celestial bodies are the sources targeted by the Virgo detector. These include: merging binary systems of black holes and neutron stars; supernovae explosions; starquakes; black-hole vibrations; spinning neutron stars.
Virgo aims also to detect  the incoherent sum of numerous gravitational waves which cannot be detected individually,  and others, including unexpected GW sources.

The vast majority of GWs detected so far have been mergers of systems composed of two black holes orbiting around one another. We observe the GWs emitted in the last few cycles of the inspiral, and then their coalescence with the formation of a single, heavier, black hole.

We have also detected signals from binary systems including two neutron stars, as the famous GW170817 event that gave birth to multi-messenger astronomy, and also from mixed systems of a neutron star and a black hole.