A series of articles published or being published in the journals “Astronomy and Astrophysics” and “The Astrophysical Journal Letters” report various aspects of the detection of very low-frequency gravitational waves. Researchers from the European Pulsar Timing Array (EPTA), the Indian Pulsar Timing Array (InPta), the Parkes Pulsar Timing Array (PPTA), the Chinese Pulsar Timing Array (CPTA), and the North American Nanohertz Observatory for Gravitational Waves (NanoGrav) analyzed data collected over the course of more than 25 years using groups of pulsars to obtain a kind of detector of gravitational waves at the galactic level. This was possible by exploiting the extreme regularity of the signals emitted by pulsars to detect variations of less than a millionth of a second and their correlations to identify gravitational waves. This technique expands the gravitational-wave astronomy opened up by the LIGO and Virgo detectors since the announcement of the first detection in February 2016.
Pulsars are neutron stars that emit electromagnetic pulses with extremely precise regularity. For this reason, they are used to carry out various cosmological researches, and one of these concerns gravitational waves with a frequency too low to be detected by instruments such as the ones that opened a new branch of astronomy by detecting mergers between objects such as black holes and neutron stars.
The various collaborations that carried out this type of research used various radio telescopes to conduct extremely precise measurements of the signals emitted by groups of pulsars kept under observation for long periods. This made it possible to detect even extremely small variations in the intervals between these signals and to create maps of correlations between these variations to detect the effects of gravitational waves at extremely-low frequencies.
Independently obtained results from collaborations in different countries or consortia of countries are consistent with each other and that confirms that those results are correct. This is just the beginning of a new phase in gravitational-wave astronomy that will lead to new coordinated observations within the International Pulsar Timing Array (IPTA) consortium to use thirteen radio telescopes to monitor over one hundred pulsars.
One of the goals of the next phase of studies is to find the sources of this type of gravitational waves. One possibility is that they are generated by pairs of supermassive black holes that start orbiting each other during a galaxy merger. There are also other hypotheses connected to other cosmological events in the early universe, up to the possible period of cosmic inflation.
Higher-quality results that could be obtained in the next phase of the study coordinated within the IPTA consortium could also offer new revelations about the early universe. For now, there is a new confirmation of the predictions of the general theory of relativity regarding gravitational waves. What’s currently a kind of background noise in the universe will be analyzed and more data will be collected, so we can expect many new announcements in the coming years.
