Black holes are the final stage in the evolution of the most massive stars. Some black holes form a pair, orbiting around each other and gradually getting closer while losing energy in the form of gravitational waves, until a point is reached where the process suddenly accelerates. They then end up coalescing into a single black hole. Merging black holes have already been observed three times by the LIGO detectors, in 2015 and early 2017 . This time, three instruments detected the event on 14 August 2017 at 10:30 UTC, enabling vastly improved localization in the sky.
This new event confirms that pairs of black holes are relatively common, and will contribute towards the study of such objects. The two black holes, with masses 25 and 31 times that of the Sun, coalesced into a black hole of 53 solar masses, while the equivalent of 3 solar masses was converted into energy in the form of gravitational waves. The event took place at a distance of about 1.8 billion light years from Earth, which means that the gravitational waves travelled through space for 1.8 billion years before being detected by the Advanced LIGO detector located in Louisiana (USA), then 8 000th of a second later by the second LIGO detector in Washington State (USA), and finally 6 000th of a second after that by the Advanced Virgo detector near Pisa (Italy).
The contribution of a third instrument, Advanced Virgo, has made it possible to significantly improve the localization of the astrophysical events that give rise to gravitational waves. These extremely violent events can in certain cases cause the ejection of very hot matter that can be seen using optical telescopes. With only two gravitational wave detectors, the region of sky that needs to be searched covers an area equivalent to several thousand times the surface area of the full Moon. Now, thanks to Advanced Virgo, this area is approximately ten times smaller, while the accuracy of the estimated distance of the source has also been increased by a factor of two. This significantly improves the possibility of observing signals with other instruments.
For this event, combining the arrival times with the amplitude of the observed signals made it possible to establish that the region of the sky containing the source had a size of 60 square degrees, which was searched by 25 groups of astronomers. As for previous events, no optical signals were detected.
Another feature of the Virgo detector is its orientation, which differs from that of the two LIGO instruments. This allows new tests of Einstein's general theory of relativity, which describes the gravitational force. General relativity predicts that as a gravitational wave propagates, space is stretched in one direction while contracting in another, rather than, for example, deforming in just one direction or in all directions at once, as predicted by alternative theories. Analysis of the observed signals shows that this is indeed the case. It will be followed by other, more extensive tests of general relativity.
Pending fresh observations in autumn 2018, the first data from the three-detector network are still being analyzed and should soon be the subject of further announcements.