In a series of articles published today in "Astronomy and Astrophysics," scientists from the European Pulsar Timing Array (EPTA), in collaboration with Indian and Japanese colleagues from the Indian Pulsar Timing Array (InPTA), report the results of data collected over 25 years that promise unprecedented discoveries in the study of the formation and evolution of our Universe and the galaxies that inhabit it.
The EPTA is a collaboration of scientists from eleven institutions across Europe, including two in Italy (INAF and the University of Milano-Bicocca), bringing together astronomers and theoretical physicists to use observations of the extremely regular pulses from "dead" stars called "pulsars" to construct a gravitational wave detector the size of our Galaxy. "Pulsars are excellent natural clocks, and we can use the incredible regularity of their signals to search for tiny changes in their ticking caused by subtle dilations and compressions of spacetime caused by gravitational waves from the distant Universe," explains Golam Shaifullah, researcher at the University of Milano-Bicocca in the research group 'B Massive,' led by Alberto Sesana, a full professor at the university, and funded by the European Research Council.
This gigantic gravitational wave detector, which extends from Earth towards 25 pulsars selected within our Milky Way and thousands of light-years away from us, makes it possible to probe gravitational waves at much lower frequencies than those already observed by terrestrial interferometers, including Virgo in Cascina (near Pisa) and LIGO in the USA. These frequencies hide some of the best-kept secrets of the Universe, including the elusive cosmic population of binary black holes with masses billions of times larger than that of the Sun. They are found orbiting at the centers of merging galaxies, and during this cosmic dance, Albert Einstein's theory of general relativity predicts that they emit ultra-long gravitational waves.
The results are based on decades of coordinated observation campaigns using the five largest radio telescopes in Europe. These instruments are the Effelsberg Radio Telescope in Germany, the Lovell Telescope at the Jodrell Bank Observatory in the United Kingdom, the Nancay Radio Telescope in France, the Sardinia Radio Telescope in Italy, and the Westerbork Radio Synthesis Telescope in the Netherlands. These observations have been supplemented by data provided by colleagues from InPTA, and the exercise of combining EPTA observations with those of the Giant Metrewave Radio Telescope in India has made the dataset even more sensitive.
The results of the EPTA are compared with a series of publications announced today in parallel by other collaborations worldwide, belonging to the Australian, Chinese, and North American Pulsar Timing Array (PTA) experiments, known respectively as PPTA, CPTA, and NANOGrav. The various results are consistent across all collaborations, further corroborating the presence of a signal due to gravitational waves in the data.
Explaining the importance of this result, Professor Alberto Sesana states, "The EPTA dataset is extraordinarily long and dense and has allowed us to expand the frequency window in which we can observe these waves, enabling a better understanding of the physics of merging galaxies and the supermassive black holes they host." The length of the dataset allows for the exploration of gravitational waves that oscillate incredibly slowly, providing insights into binary black hole systems with orbital periods of tens of years. On the other hand, the data cadence also allows for the study of waves that undergo many oscillations per month, granting access to black hole systems with much shorter orbital periods, on the order of a few days.
The results of the EPTA data analysis presented today are in line with the predictions of astrophysicists. However, Nataliya Porayko, a visiting researcher at the University of Milano-Bicocca, emphasizes that "a golden rule in physics to confirm the discovery of a new phenomenon is for the experimental result to have a probability of occurring randomly of less than one in a million." The result reported by EPTA, as well as by other international collaborations, does not yet meet this criterion. In fact, there is still approximately a one-in-a-thousand chance that random noise sources conspire to generate the signal. "But work is already underway," as explained by Aurelien Chalumeau, a fellow of the B Massive group. "Scientists from the four collaborations - EPTA, InPTA, PPTA, and NANOGrav - are combining their data under the coordination of the International Pulsar Timing Array." The goal is to expand the current datasets by leveraging measurements from over 100 pulsars observed with thirteen radio telescopes worldwide. The increased quantity and quality of data should allow astronomers to achieve the objective in the near future, providing irrefutable evidence that a new era in the exploration of the Universe has begun.