The two Ligo detectors in the United States and the Virgo detector in Italy measured a gravitational wave signal from the collision (technically called coalescence) between two black holes. The observation of the event was recorded by all three detectors with a precision never achieved before and is the first performed by Advanced Virgo. The gravitational waves were emitted during the final moments of the fusion of two black holes, with masses of about 31 and 25 times the mass of the Sun, about 1,8 billion light years away from us. The black hole thus produced has a mass about 53 times that of our Sun: this means that, in the coalescence phase, about 3 solar masses have been converted into energy in the form of gravitational waves. This extraordinary discovery is important for at least two reasons.
The first reason is the precision with which the observation was made: “listening” to the universe with three interferometers instead of two allows a 10 times better localization of the phenomenon. This means that when Ligo and Virgo observe a promising event, after a few minutes the scientists can alert other astronomers and point other telescopes and instruments towards the source of gravitational waves.
In the event GW170814, the one taken into consideration, the precision pointing information allowed 25 instruments, on the ground and in space, to make their observations. In this case, no electromagnetic counterparts have been identified, thus confirming the predictions for black holes. But if the gravitational waves caused by the explosion of a supernova were observed in the future, it will be possible to study the developments of the supernova. And the same in the case of clashes between neutron stars that cause gamma-ray bursts.
The polarization describes how space-time is distorted in the three different spatial directions of propagation of a gravitational wave. The first tests, based on the GW170814 event, test extreme cases: on the one hand, the polarizations allowed by general relativity, and on the other, the polarizations prohibited by Einstein’s theory. Data analysis shows that Einstein’s forecast is strongly favorited.
The second important aspect concerns the discovery that gravitational waves are also polarized. Virgo does not respond in the same way as Ligo detectors to the passage of gravitational waves, because it has a different location and orientation on the Earth: it is not parallel to the other 2 interferometers. This implies that one can test another prediction of general relativity, which concerns the polarization of gravitational waves.
Now that thanks to Virgo we are finally able to listen in detail to the “melody” of gravitational waves, we have a new and powerful tool to study what happens in the universe. This will allow us to study black holes and look further into the universe, because gravitational waves are not attenuated by the matter they encounter, unlike light. We can better understand what happens inside pulsars and supernovae. And what happened when, with the Big Bang, gravity itself was born.
OK, BUT WHAT ARE GRAVITATIONAL WAVES? AND HOW DO YOU DETECT THEM?
Gravitational waves are ripples in the fabric of space-time, which we can imagine as a giant rubber carpet deformed by the interaction of any object with mass. They are produced, for example, by the Sun (the orbits of the planets slides in the deformation of the space around it); we also generate them when we move, but in order for these to be traceable, the interaction of objects of considerable mass, such as black holes, is necessary.
They were planned a century ago by Albert Einstein, with his General Relativity. Einstein had re-established gravity: more than a force between distant objects, he considered it a geometric effect. The effect of a planet or of a star in the “fabric” of space-time, in this perspective, is comparable to that of a ball resting on a stretched sheet, which deforms its surface.
The following year, Einstein realized that the most violent cosmic events can cause a ripple that spreads like a wave on that sheet. When a portion of the universe is traversed by perturbation, space and time dilate and shrink in a manner that is characteristic of the way the wave is formed.
Since space-time is deformed everywhere, dilating and shrinking, it is very difficult to detect the gravitational waves produced. Given that we can take a yardstick to measure such deformations, even that would suffer deformation. However, there is a “yardstick” that does not undergo dilation: the speed of light. If the space between two points expands or shortens, the light takes more or less time to go from one point to another. And it is on this concept that laboratories like Ligo and Virgo work. In 4-mile long tunnels, laser beams are fired to measure infinitesimal differences in distance between the two ends of the tunnel. When a gravitational wave arrives there is an expansion of the space in one direction of the tunnel. By measuring the interference between the laser beams that are reflected from one end to the other, it is possible to measure very precisely whether the space between the ends has dilated or compressed.