The European Pulsar Timing Array (EPTA) is a scientific collaboration that brings together teams of astronomers from Europe's largest radio-telescopes, and coordinates groups of researchers specialising in data analysis and the modelling of gravitational wave signals. These teams include the astrophysics group of the "Giuseppe Occhialini" Department of Physics Milan-Bicocca University. The EPTA recently published, in the historical scientific journal Monthly Notices of the Royal Astronomical Society, a detailed analysis ("Common-red-signal analysis with 24-yr high-precision timing of the European Pulsar Timing Array: Inferences in the stochastic gravitational-wave background search"; DOI: 10.1093/mnras/stab2833) of a signal compatible with the much sought-after gravitational wave background from a cosmic population of supermassive black-hole binaries. Although it is not yet possible to confirm the gravitational origin of the signal, The findings of this research represents another significant step forward in efforts to detect gravitational waves frequencies in the order of one billionth of a Hertz for the very first time. The candidate signal emerged from an unprecedented detailed analysis conducted using two independent methodologies. The signal also has strong similarities to that identified in analyses by other teams.
Pulsars are very compact neutron stars that emit beams of radio waves collimated in the direction of their magnetic poles that are generally not aligned with the axis of rotation of the star, and therefore create a "lighthouse effect". If the beams cross our line of sight, from the earth we observe a very precise periodic pulse: a clock that ticks with each rotation of the pulsar. A pulsar timing array (PTA) is a set of very stable rotating pulsars, used as a huge galactic detector of gravitational waves. PTAs, which are sensitive to gravitational waves with frequencies of one billionth of a Hertz, are therefore observatories that are complementary to the current LIGO/Virgo/Kagra ground-based detectors, which are sensitive to frequencies of between 10 Hz and 1000 Hertz. Whereas the latter detect the sudden collisions of stellar-mass black holes and neutron stars, a PTA can observe waves emitted by binary systems of supermassive black holes spiraling around one another at the centre of galaxies. The superposition of a great many of these signals, originating from a cosmic population of massive binaries, creates what is known as a "stochastic background" of gravitational waves.
"We can measure small fluctuations in the times of arrival of pulsar radio signals on Earth that are caused by the distortion of spacetime due to the passage of a gravitational wave. Basically, these distortions present as a very low-frequency disturbance in the times of arrival of the observed pulses, a disturbance that is common to all the pulsars of a PTA", explains Dr. Golam Shaifullah, a researcher at the "Giuseppe Occhialini” Department of Physics and co-first author of the study. However, the amplitude of this disturbance is incredibly small (estimated to be in the order of tens of billionths of a second) and it can be confused or masked by many other physical effects that can create small instabilities in the rotation period of the pulsars comprising the PTA network. Multiple independent codes with different statistical techniques were therefore used to validate the results in order to mitigate alternative sources of noise and identify the gravitational signal. It is important to point out that two independent "end-to-end" procedures were used in the analysis to cross-check the results. Furthermore, three independent methods were used to account for possible systematic errors due to uncertainty in the knowledge of the masses and positions of the celestial bodies in the solar system, an uncertainty that can result in false positives in the detection of the stochastic gravitational wave signal.
The analyses performed by the EPTA with both procedures detected the presence of a clear candidate signal for a stochastic gravitational wave background, and its spectral properties (i.e. how the amplitude of the observed disturbance varies with frequency) remain within the theoretical expectations for this type of signal. Dr Nicolas Caballero, a researcher at the Kavli Institute for Astronomy and Astrophysics in Beijing and co-first author, explains: “The EPTA first found indications of this signal in data published in 2015; however, as the results had greater statistical uncertainties, they were discussed strictly only as upper limits. Our new data now clearly confirm the presence of this signal, making it a candidate for a stochastic gravitational wave background."
Einstein's General Relativity entails a very specific relationship between the spacetime distortions experienced by radio signals from pulsars located in different directions in the sky. Scientists call this spatial signal correlation or the Hellings-Downs curve. Its detection will uniquely identify the possible gravitational origin of the observed signal. "At the moment," explains Dr. Siyuan Chen, a researcher at LPC2E, CNRS in Orleans, France and study co-first author, “the statistical uncertainties in our measurements do not yet allow us to identify the presence of the spatial correlation expected for the signal due to gravitational waves. To obtain further confirmation we need to include data from more pulsars in the analysis; however, the current results are very encouraging."
The EPTA is a founding member of the International Pulsar Timing Array (IPTA). Since independent data analyses performed by the other IPTA partners (i.e., the NANOGrav and PPTA experiments) also indicated similar common signals, it became essential to apply multiple analysis algorithms to increase the confidence in a possible future detection of this stochastic background. The members of the IPTA are working together, drawing conclusions by comparing analyses of the different datasets in order to prepare in the best possible manner for the next steps ahead.
Alberto Sesana, associate professor at Milan-Bicocca University and member of the EPTA, concluded: "The detection of a gravitational wave background from a population of binary systems of supermassive black holes or other cosmic sources will provide unique information on the cosmological models of the Universe's evolution, consequently imposing strong constraints on the aggregation of galaxies as we see them today. We are therefore intensifying our efforts by adding data from new pulsars and stepping up the cross-checks on our analyses. There is no room for errors."