The PINGU (Pulsar Timing Array Inference of the Nanohertz Gravitational wave Universe) project, led by Alberto Sesana, Professor of Astrophysics at the Department of Physics of the University of Milano-Bicocca, has been awarded a €2.3 million ERC Advanced Grant by the European Union. The aim is to exploit, over the next five years, the scientific potential of the astronomical technique known as 'Pulsar Timing Array' (PTA) - based on the observation of extremely regular pulses (timing) from a group (array) of 'dead' stars called 'pulsars' - to understand the evolution of the Universe and of supermassive black holes, the largest black holes in the Universe.
Pulsars rotate at very high speeds - between 100 and 1,000 rotations per second - and emit two beams of radiation in antipodal directions. When one of the emitting beams hits the Earth, it is observed by radio telescopes in the form of very regular pulses. "These objects are excellent 'galactic clocks'," explains Alberto Sesana, "which allow us to measure very low-frequency gravitational waves. By comparing the 'ticks' of these 'clocks', i.e. the arrival times of the radiation beams, we can determine whether the space between us and the observed stars is expanding or contracting. We can then use the incredible regularity of the pulsar signals to look for tiny changes caused by the passage of gravitational waves from the distant Universe".
Recently, several international scientific collaborations, including the European PTA (EPTA) - in which the research team led by Alberto Sesana also participates -, NANOGrav, Parkes PTA (PPTA) and Chinese PTA (CPTA), have reported evidence of a signal in their observations that is compatible with a gravitational wave origin. This observation opens a completely new window on the universe,' continues the professor from the Department of Physics of Milano-Bicocca. PTAs are sensitive to waves of a few nanohertz (billionths of a hertz), i.e. at frequencies more than 10 orders of magnitude lower than those detected by the ground-based interferometers LIGO and Virgo. At such low frequencies, one would expect to observe waves from a cosmological population of supermassive black holes or a 'gravitational background' from the early universe, in practice the gravitational analogue of the cosmic background radiation. Although the observed signal is compatible with that produced by a cosmic population of supermassive black holes, its origin cannot yet be determined with certainty.
By the end of the decade, new PTA observations made by international scientific collaborations, together with those made by the MeerKAT radio telescope in South Africa, under the coordination of the International PTA (IPTA), will not only confirm the signal but also map its origin in the sky. "PINGU proposes to cross-correlate this 'gravitational map'," says Sesana, "with a 'synthetic map' of supermassive black hole binaries in the Universe, built by combining theoretical models for the evolution of galaxies and the black holes they host with detailed maps of galaxies and clusters from the most advanced observational campaigns. By cross-correlating these maps, PINGU will make it possible to identify the origin of this signal and, if the origin is astrophysical, to identify the brightest supermassive black hole binaries (in the gravitational sense) and the galaxies that host them, This will allow us to map the gravitational universe in the nanohertz range, giving us unique insights into the evolution of supermassive black holes and their role in galactic evolution, providing an important missing piece in our understanding of the formation and evolution of cosmic structures.
If, instead, the detected signal is incompatible with an astrophysical origin, but comes from the early universe, "it would be by far the closest signal to the Big Bang ever observed, allowing us to get closer than ever before to the origins of the universe," concludes the PINGU project leader.
A dozen PhD students and research fellows from Milano-Bicocca will work on Pingu under the supervision of Alberto Sesana.
Since 2014, the University of Milano-Bicocca has received funding for 18 ERC projects: 2 Advanced Grants, including that of "PINGU", 5 Starting Grants, 7 Consolidator Grants, 2 Proof of Concept Grants and 2 Synergy Grants. "The funding of the PINGU project", says Guido Cavaletti, Pro-Rector for Research at the University of Milan-Bicocca, "is an important result for our University, which confirms the ability of our researchers to carry out excellent and cutting-edge research in international scientific contexts. It is a success that clearly shows us that we are on the right track and we are therefore very confident, not only in the positive outcome of this specific project, but also in the possibility for other colleagues to obtain similar results in European calls, where we have shown that we can be very competitive'.