On september 14, 2015, at 9:50 GMT, the two detectors of Laser Interferometer Gravitational-Wave Observatory (LIGO) based in Livingston, Louisiana and in Hanford, Washington, simultanuesly (but within 7 msec) catched a transient signal produced by a gravitational wave (GW).
An oscillating waveform less than 1 sec duration and frequency increasing from 35 to 250 Hz, whose shape matched that predicted by general relativity theory for the inspiraling coalescence and subsequent collapse of two black holes into a single one.
The confidence in the observation is practically absolute: Given the experimental parameters in effect, a false alarm could occur once in 203.000 years. The event source, located at 410 Megaparsec (Mpc), corresponds to a red shift Z=0,09. Before collapse, the masses of approaching black holes amounted to 36 and 29 solar masses, respectively, whereas the single black hole produced in the collapse had solar mass of 32. Thus, the excess of 3 solar masses was converted into gravitational radiation.
At which distance was the event detected?
At a distance hard even to conceive. The black holes giving rise to the GW were observed at 1 billion and 337 millions light-years far from us, namely about 8,500 times farther than Large Magellanic Cloud.
The experiment demonstrated the existence of binary systems of black holes and is the first direct detection of GW and the first observation of a merger of two black holes.
The observation was described in an article “Observation of Gravitational Waves from a Binary Black Hole Merger” published on Physical Review Letters on February 11, 2016, i.e. just one century after the publication of Einstein’s article that predicted, inside the General Relativity theory, the existence of GW, whose this recent result is the first and undisputable experimental evidence. In such symbolic date one of the most important discoveries of our century was unveiled to the planetary audience, and the article above will likely be among the most cited in next future by the pertinent scientific communities. It was authored by over 1000 scientist from all over the world, belonging to several international scientific cooperations, standing among these LIGO and VIRGO (the latter a French-Italian initiative) engaged in an extraordinary joint effort. In this magnificent venture some research groups of Salerno University, especially in engineering and physics, are participating, Some are among the authors of above mentioned article: Fausto Acernese, Paolo Addesso, Fabrizio Barone and Rocco Romano; others, as Francesco Chiadini, Roberto Conte, Maurizio Longo, Stefano Marano, Vincenzo Matta, Fabio Postiglione, are authors of several previous publications, witnessing a research journey now thirty years’ worth. The studies of said groups from Salerno University focused, among other subjects, on signal processing and on the enhancement of mirror suspensions so us to mitigate the external perturbations. The direct experience and sense of cooperation in an international experiment was exciting, as put forth by Roberto Conte (former Research Associate with del DIEM, now in Leonardo-Finmeccanica): “In the control room of interferometers, researchers from all over the world switch up for their turns. We stay hours to look at the displays projected in real-time on the big wall of the lab: in the center, the largest, the queen, the spectral density. We stay hours to observe it breathing about that noise floor (10-23Hz-1) with a dream to unveil the secrets of the universe”.
The event of September 14 marks the beginning of the age of gravitational astronomy: the new science which allows us to look ever deeper into the space-time, tracking back from the observed GW to the sources and remote events whereby it originated.
Indeed, besides coalescing binary stars, there are prospects of other sources of GW, either transient, as supernovae explosions and formation of galaxies , or continuous, as pulsar or cosmic gravitational background radiation, residual of Big Bang. In its “full-sky” version, the detection requires the simultaneous observation at all possible angles of celestial sphere. In this challenge, alongside the further enhancement of instruments, the capacity to process an increasing available amount of data is also crucial. Scientists from Salerno University are currently studying inside LIGO Scientific Collaboration (LSC) novel signal processing techniques based on sparse representations in time-frequency as well as based on the theory of stochastic resonance, particularly promising for the detection of periodic gravitational signals. The aim is to achieve ever sharper images of the GW and thus to refine the classification of the source and the analysis of the dynamics of gravitational phenomena
Have we thus added another important dowel toward the comprehension of the fascinating mystery of the birth of the universe?
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