LARGE SCIENTIFIC PROJECT
Gravitational-Wave Astronomy the theoretical predictions of gravitational-wave emission. This provided the first confirmation of the existence of gravitational waves, as predicted by Einstein’ s theory [ 7 ].
The birth of large interferometers
In the 1980s, large-scale projects were proposed: two detectors in the United States( to enable coincidence detection), which would become LIGO, and a third near Pisa( Italy), Virgo, the result of a Franco-Italian collaboration that has since grown into a broader European effort. Meanwhile, other experimental prototypes and key technological developments emerged: vibration isolation systems to isolate the test masses from the seismic noise and create an almost“ free fall” condition in the frequency region of interest, improvements in lasers, mirror coatings and a deeper understanding of fundamental noise sources. Particular attention was devoted to quantum noise( including squeezing, following the early works of Carlton Caves in the 1980s [ 8 ] and Brownian thermal noise, studied by Peter Saulson [ 9 ] and others). France played a crucial role in several of these technological developments, especially in laser stabilization, mirror coatings, optical metrology, and interferometric simulations, contributions that remain essential today.
The commissioning and the“ single machine”
At the beginning of the new millennium, LIGO and Virgo were“ switched on”, and the long and arduous commissioning phase began: the period between the end of detector integration and the start of scientific data-taking. The goal was to bring the detector into its operational regime and begin the systematic identification and mitigation of noise sources. Although theoretical models had driven the design of these instruments, their originality- and the extreme displacement sensitivity required, of the order of 10-
18 meters- meant that several noise sources, their coupling mechanisms, and their mitigation strategy had to be studied experimentally and in situ. Furthermore, their target sensitivity lies in a frequency range around 100 Hz, a region relatively new for precision metrology, where a dense " forest " of noise sources,
Figure 3. LIGO Hanford interferometer. Source: https:// www. ligo. caltech. edu / image / ligo 20150731 er including environmental, technical, and control-related noise, makes detection even more challenging. The first-generation versions of LIGO and Virgo did not succeed in detecting gravitational waves. Around 2011, both detectors were upgraded to their second-generation configurations: Advanced Virgo and Advanced LIGO [ 10,11 ]. This meant replacing mirrors, lasers, suspending optical benches and improving vibration isolation. Meanwhile, an important political development had occurred: LIGO and Virgo had signed a data-sharing and joint-publication agreement. Since 2007, they have effectively operated as a single global observatory, a“ single machine”, capable of triangulating and localizing sources. In fact, unlike telescopes, LIGO and Virgo are nondirectional: they observe the entire sky, continuously. In order to pinpoint a source, one must compare the arrival times of the gravitational-wave signal at least of three detectors. Beyond being a great scientific and technical adventure, the detection of gravitational waves has also proven to be a visionary exercise in international cooperation.
GW150914: The Birth of Gravitational-Wave Astronomy
In 2015, the first observing run of LIGO began. Virgo, due to a two-year historical delay from the time of its funding, was not yet ready. Just two days after the start of the data taking, LIGO observed a signal, lasting a fraction of a second, in coincidence between its two detectors. The event was interpreted as the merger of two black holes, each of approximately 30 solar masses, located at a distance of about 1.3 billion light-years. The signal was unmistakable, although for reasons of caution, it took the scientific community five months to finalize the analysis and announce the discovery.
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