The first detection of gravitational waves by the LIGO interferometer ushered astrophysics into a new era of observation. The study of these space-time oscillations has led to a better understanding of certain characteristics of black holes and how these cosmic objects behave during a merger.
On September 14, 2015, just days after LIGO was first activated, with greatly improved sensitivity, a gravitational wave passed through the Earth. Like the billions of similar waves that have traversed the Earth throughout its history, this one was generated by a collision and merger of two massive, ultra-distant objects from outside the Milky Way.
More than a billion light-years away, two black holes merged and the signal — traveling at the speed of light in a vacuum — finally reached Earth. Both LIGO detectors saw their arms expand and contract by a subatomic distance, but that was enough for the laser to move and produce a telling change in an interference pattern. For the first time, gravitational waves were detected. Three years later, a total of 11 were listed, 10 of which were from black holes.
There were two runs LIGO data:a first from September 12, 2015 to January 19, 2016 and a second, with slightly improved sensitivity, from November 30, 2016 to August 25, 2017. This last analysis was, halfway through, joined by the VIRGO detector in Italy , which not only added a third detector, but also greatly improved the ability to pinpoint the location of these gravitational waves.
LIGO is currently discontinued as it undergoes upgrades which will make it even more responsive. Indeed, the instrument is preparing for a new data observation campaign which will begin in the spring of 2019. On November 30, the LIGO scientific collaboration published the results of its improved analysis, sensitive to the final stages of object fusion. between about 1 and 100 solar masses.
Of the 11 detections made so far, 10 of them represent black hole mergers, and only GW170817 represents a neutron star merger. The merger of these neutron stars was the nearest event, 130-140 million light-years away. The most massive fusion ever seen — GW170729 — is coming from a location that, with the expansion of the Universe, is now 9 billion light-years away.
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These two detections are also the lightest and heaviest gravitational wave mergers ever detected:GW170817 from the collision of neutron stars of 1.46 by 1.27 solar masses, and GW170729 from the merger of black holes of 50.6 and 34.3 solar masses.
1. Mergers involving the most massive black holes are the easiest to detect
One of the advantages of studying gravitational waves is that it is easier to observe them when they are emitted from afar, than light sources. Stars appear fainter in proportion to their distance squared. While gravitational waves are weaker in proportion to their distance:the merger of black holes 10 times further away produces 10% of the signal.
As a result, it is possible to detect very massive objects over very large distances. However, no black hole mergers have involved objects larger than ~50 solar masses. Masses of 20 to 50 solar masses are common but, beyond that, no observation has yet been able to reveal any.
2. Adding a third detector increases the detection rate and location accuracy of gravitational waves
During his first run , LIGO ran for 4 months, then for 9 months on its second run . However, almost more than half of the total detections were made in the last month of observation, after the addition of Virgo. In 2017, gravitational waves were detected:
3. When black holes merge, they release more energy than all the stars in the observable Universe combined
The Sun is the reference star in stellar physics. It produces a total power of 4×10 26 W, the equivalent of transforming 4 million tons of matter into pure energy, every second. With approximately 10 23 stars in the observable Universe, the total power produced by all stars shining in the sky is greater than 10 49 W at any time.
But for a few milliseconds, at the crucial moment of the merger, each of the 10 detections showed energy greater than that of all the stars in the observable Universe combined.
4. During the merger, about 5% of the total mass of the two black holes is transformed into pure energy
The space-time oscillations detected by interferometers derive their energy from the conversion of the mass of black holes by the relation E =mc². According to the detections made and the analysis of the amplitude of gravitational waves, at the time of merger, black holes lose 5% of their total mass in the form of gravitational energy. These events are the most powerful known since the Big Bang, producing more energy than neutron star mergers, gamma-ray bursts and supernovas.
5. Numerous and frequent mergers involving low-mass black holes are yet to be detected
The most massive black hole mergers produce the largest amplitude signals, and are therefore the easiest to detect. But given the way volume and distance are related, twice the distance means eight times the volume. As LIGO becomes more sensitive, it is easier to spot large objects at greater distances than nearby low-mass objects.
Although observations have shown that very many stellar black holes with masses between 7 and 20 solar masses exist in the Universe, it is much easier for LIGO and Virgo to detect mergers of very massive black holes at great distances. With the improved sensitivity of interferometers and the existence of binary black holes of very different masses, scientists hope to be able to detect more such mergers in the future.
