Scientists from the European Pulsar Timing Array (EPTA) [1], based on the analysis of data accumulated over 25 years.
The experiment is based on the clockwork precision of pulsars [2], whose signals are received on Earth at ultra-regular intervals. 25 pulsars were selected as targets for large-scale, coordinated observations in the radio wave range (0.3 - 3 GHz). Perfectly characterized over two decades, they form a celestial grid and act as a detector of gravitational waves the size of a galaxy, as soon as a variation, however minute, is detected in their pulses.
![<multi>[fr]Vue d'artiste représentant le principe de la détection d'ondes gravitationnelles par une réseau de pulsar (Pulsar Timing Array - PTA).[en]Artist's rendering of the principle of gravitational wave detection by a pulsar array (Pulsar Timing Array - PTA).</multi>](IMG/jpg/image_pulsar1.jpg)
"Pulsars are remarkable natural clocks. By measuring the minute variations in the arrival times of their signals, it is possible to detect subtle dilations and contractions of space-time caused by the passage of gravitational waves from the distant Universe!", explains Lucas Guillemot, assistant astronomer at LPC2E (Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, CNES/CNRS/Université d’Orléans) (Orléans) and Université d’Orléans.
![<multi>[fr]Figure principale illustrant le résultat[en]Main figure illustrating the result</multi>](IMG/png/image_pulsar3.png)
Antoine Petiteau, researcher at CEA’s IRFU institute, adds: "This gigantic gravitational wave detector, which extends from Earth to 25 selected pulsars in the Galaxy, gives access to gravitational wave frequencies much smaller than those measured by the ground-based LIGO/Virgo detectors for the first time in 2015. By detecting changes of less than a millionth of a second over more than twenty years, we can detect waves that have a period of several months to several years, corresponding, for example, to the merging of supermassive black holes"
A new frequency window
These results mark a crucial milestone in the study of gravitational waves, opening up a new frequency window rich in astrophysical information.
Gravitational waves, observed at very low frequencies, hold the key to some of the Universe’s best-kept secrets, such as supermassive binary black holes. Indeed, theory suggests that this population of cosmic objects - whose mass is millions or even billions of times greater than that of the Sun, and which form during galaxy mergers - emit waves at these frequencies, in the order of a billionth of a hertz.
The salvo of highly convincing results published on June 29, 2023 marks the start of a new journey into the Universe, likely to lift the veil on some of these mysteries. "We are opening a new window on the Universe", says Gilles Theureau, astronomer at Paris Observatory - PSL. "For the first time, we can envisage probing the movements of pairs of supermassive black holes formed by successive galaxy mergers. Gigantic black holes, larger than our own Solar System, located in distant galaxies, and ready to collide. "
The results are based on a vast coordinated observing campaign using Europe’s five largest radio telescopes:
- the 100 m Effelsberg radio telescope in Germany,
- the Lovell telescope at Jodrell Bank Observatory in the UK,
- Nançay radio telescope in France,
- the Sardinia radio telescope in Italy
- and the Westerbork radio telescope in the Netherlands.
To complete this data set, observations from the GMRT radio telescope in India were included in the analysis.
![<multi>[fr]Le grand radiotélescope décimétrique [en]The large decimeter radio telescope</multi>](IMG/jpg/image_pulsar2.jpg)
"This has been a long-term project," says Ismaël Cognard, CNRS Research Director at LPC2E. "Coordinating the efforts of five European observatories and our Indian colleagues was no easy task. These results reward all our efforts and strengthen the case for cooperation at European level and beyond".
Results corroborated around the world
The announcement of EPTA’s results on June 29, 2023 is coordinated with similar publications from other collaborations around the world, namely the Australian (PPTA), Chinese (CPTA) and North American (NANOGrav) collaborations. The astronomers are convinced that these are signatures of gravitational waves, as their results are consistent and backed up by independent analyses, using data collected with different radio telescopes, observing other sets of pulsars around the world.
While the EPTA astronomers have identified a signal that meets the criteria for gravitational-wave identification, there are still aspects that need to be further investigated before a fully robust detection can be claimed. Astronomers point out that the absolute criterion for affirming the detection of a new phenomenon is that there should be less than a one-in-a-million chance of it occurring by chance, and that it is in fact a "false detection". The results reported by EPTA - as well as by the other international collaborations - do not yet quite meet this requirement, but there are high hopes of achieving this goal in the near future.
Scientists from the four collaborations EPTA, InPTA, PPTA and NANOGrav, joined by the CPTA group and the consortium formed around the South African MeerKAT radio telescope, are currently combining their most recent data, with the support of the International Pulsar Timing Array. To achieve the required level of detection, the aim is to increase the number of pulsars observed to a hundred, optimize the quality of data processing and exploit more powerful instrumentation.
These highly promising results augur unprecedented discoveries on the formation and evolution of our Universe and its galaxies.
French contribution
France’s contribution is based on intensive use of the Nançay Large Decimeter Radio Telescope (NRT), which has been almost entirely dedicated to pulsar studies since 2004, with 2,500 hours of telescope time per year devoted to the EPTA timing program. This represents almost 70% of the European collaboration’s data.
The project has also intensively monopolized the CNRS CC-IN2P3 computing center for the execution of the gravitational analysis (with nearly 2.9 million CPU computing hours since 2021).
The four research laboratories involved are LPC2E, ORN [3] and the CEA’s particle physics department, which houses all the know-how and expertise in statistical analysis and interpretation of the gravitational signal.
In particular, this project has given rise to seven doctoral theses and three post-doctoral contracts over the past 15 years.
References
This work is the subject of three articles published simultaneously:
- "The second data release from the European Pulsar Timing Array — I. The Dataset", Antoniadis et al. (EPTA Collaboration), Astronomy & Astrophysics, 29 juin 2023.
- "The second data release from the European Pulsar Timing Array — II. Customised Pulsar Noise Models for Spatially Correlated Gravitational Waves", Antoniadis et al. (EPTA Collaboration), Astronomy & Astrophysics, 29 juin 2023.
- "The second data release from the European Pulsar Timing Array — III. Search for gravitational wave signals", Antoniadis et al. (EPTA Collaboration), Astronomy & Astrophysics, 29 juin 2023.
[1] Created in 2006, EPTA brings together astronomers and theoretical physicists from eleven European institutions. In a series of articles published in the journal Astronomy and Astrophysics on June 29 2023, they report on results obtained in collaboration with their colleagues from the Indo-Japanese network InPTA (Indian Pulsar Timing Array) [[InPTA is another consortium bringing together scientists from several Indian and Japanese institutes
[2] Pulsars are the remnants of the explosion of massive stars, whose core has survived as a neutron star, a very compact object of 1 to 2 solar masses with a radius of around 13 km. The fastest pulsars rotate at a speed of 700 revolutions per second and emit a beam of radiation from their magnetic poles, much like a beam of light from a distant lighthouse
[3] ORN is an observatory platform of Observatoire de Paris - PSL and a support and research unit of CNRS and Université d’Orléans. The ORN is home to radio astronomers and experts in pulsar timing, the APC[[Astroparticle and Cosmology (APC, CNRS/Université Paris Cité/Observatoire de Paris - PSL/CEA/CNES)