In a series of papers published today in “Astronomy and Astrophysics”, scientists of the “European Pulsar Timing Array” (EPTA), in collaboration with Indian and Japanese colleagues of the “Indian Pulsar Timing Array” (InPTA), report on results from data collected over 25 years, which promise unprecedented discoveries in the study of the formation and evolution of our Universe and the galaxies that populate it.
«The results presented today by the EPTA collaboration are extraordinary for their scientific importance and for the future prospects for further consolidation of the results» comments Marco Tavani, president of INAF. « Italian astrophysics and INAF are world leaders in a large enterprise aimed at exploring the cosmos with gravitational waves, a line of research
that will see Italy as a protagonist in the coming years».
The EPTA is a collaboration of scientists from 11 institutions across Europe, including two in Italy (INAF with its headquarters in Cagliari and the University of Milan-Bicocca, with the collaboration of researchers from GSSI), and brings together astronomers and theoretical physicists, in order to use observations of ultra-regular pulses from neutron stars called ‘pulsars’ to build a Galaxy-sized gravitational wave detector. 
Pulsars behave like natural, high-precision clocks. With the repeated measurement of very small variations (less than a millionth of a second) in the arrival times of their pulses, it is possible to measure the minute expansions and compressions of space-time caused by the passage of gravitational waves from the distant universe.
This gigantic gravitational wave detector - spanning from the Earth to 25 chosen pulsars across the Galaxy, thousands of light years away from us -- makes it possible to probe gravitational wave frequencies that are much lower than those observed, starting in 2015, by gravitational wave interferometers, among which Virgo in Cascina (near Pisa) and LIGO in the United States. 
At INAF in Cagliari, the enthusiasm is palpable. «Thanks to the EPTA’s observations, we are opening a new window into the universe of ultra-long gravitational waves (corresponding to oscillation frequencies of a billionth of a hertz) which are associated with unique sources and phenomena», says Caterina Tiburzi, an INAF-Cagliari researcher. 
«These gravitational waves», explains her colleague Marta Burgay, «allow us to study some of the hitherto unsolved mysteries in the evolution of the universe, including, for example, the properties of the elusive cosmic population of binary systems formed by two supermassive black holes, with masses billions of times greater than that of the Sun». These black holes are found to orbit in the center of galaxies that are merging with each other, and during their orbit, Albert Einstein's theory of general relativity predicts that they emit ultra-long gravitational waves.
The instruments used to collect the data are the Effelsberg Radio Telescope in Germany, the Lovell Telescope of the Jodrell Bank Observatory in the United Kingdom, the Nancay Radio Telescope in France, the Westerbork Radio Synthesis Telescope in the Netherlands, and the Sardinia Radio Telescope (SRT) in Italy.

«These results», adds Delphine Perrodin, an astronomer at INAF-Cagliari, «are based on decades of painstaking and tireless observation
campaigns carried out using the five largestradio telescopes in Europe. Furthermore, once a month, the data from these telescopes are also
added together, further increasing the sensitivityof the experiment ». These observations were then further complemented by data provided by
the Giant Metrewave Radio Telescope in India, thereby making the dataset even more accurate. 

« It is a great satisfaction for all Italian astrophysics that SRT, the large radio telescope managed by INAF, is among the witnesses of the emergence of this slow breathing of space-time in the data », explains
Andrea Possenti, an INAF senior researcher at Cagliari and one of the founders of EPTA, together with the former president of INAF, Nichi D'Amico.  «This is another great scientific result», Possenti adds, «which confirms, on a worldwide level, the central role of Italy, and increasingly of Sardinia (with SRT and hopefully soon also the Einstein Telescope), in the study of gravitational waves for many decades to come ».
In parallel with the EPTA results, a series of independent publications were announced today by other collaborations around the world, based on the Australian, Chinese and North American PTA (pulsar timing array) experiments, known respectively as PPTA, CPTA and NANOGrav. The various results are consistent across all collaborations, further corroborating the presence of a gravitational wave signal in the data. However, the work does not end here, as the very nature of the observed signal requires that it manifest itself in a progressively clearer way. « I started my doctorate at the right time », recalls Francesco Iraci, a PhD student at the University of Cagliari who has been carrying out his research at the INAF of Cagliari for about a year, precisely in the
context of EPTA, « I can't wait to contribute to further refinement of the data ».
In fact, the gold standard, in physics, to claim the detection of a new phenomenon is for the result of the experiment to have a probability of occurring by chance of less than once in a million. The result reported by EPTA - as well as by the other international collaborations - is approaching this probability, but still does not yet meet this criterion: in fact, there is
still about a one in a thousand chance that random noise sources conspire to generate this signal. After having completed their analyzes independently, the researchers from the four collaborations – EPTA, InPTA, PPTA and NANOGrav – are now directly combining their data under the auspices of the International Pulsar Timing Array. The aim is to exploit measurements from an array of over 100 pulsars observed with thirteen radio telescopes around the world.
The increased quantity and quality of the data should therefore provide astronomers with irrefutable proof that a new era in the exploration of the Universe has begun.