After nearly five years of observations using the South African MeerKAT radio telescope, a research team led by the MeerKAT Pulsar Timing Array (MPTA) collaboration has provided further confirmation of the existence of a cosmic gravitational wave background at extremely low frequencies (1–10 nanohertz). This effort has produced the most detailed map to date of the distribution of these gravitational waves in the universe. The signal is believed to originate from a population of spiralling supermassive black hole binaries.
The results, which involved researchers from Italy’s National Institute for Astrophysics (INAF) and the University of Milano-Bicocca, are published today in three studies in the Monthly Notices of the Royal Astronomical Society (MNRAS).
The MeerKAT Pulsar Timing Array is an international experiment using the highly sensitive MeerKAT radio telescope, operated by the South African Radio Astronomy Observatory. MeerKAT observes dozens of pulsars every two weeks, measuring the arrival time of radio pulses with extraordinary precision—down to tens of nanoseconds.
"MPTA is the most powerful ultra-low-frequency gravitational wave detector in the southern hemisphere," explains Federico Abbate, INAF researcher in Cagliari and co-author of all three papers published today.
Pulsars, rapidly rotating neutron stars, act as natural clocks. Their regular radio pulses allow scientists to detect minute variations caused by passing gravitational waves. First theorised by Albert Einstein in the 1920s, gravitational waves are ripples in spacetime caused by some of the universe's most powerful events, such as the merger of binary black hole systems. These overlapping waves form a faint "cosmic hum" that holds vital clues to hidden processes shaping the structure of the universe.
The MPTA team has found strong evidence of signals associated with gravitational waves from supermassive black hole binaries slowly spiralling toward each other. The signals detected by MPTA are stronger than those observed in similar experiments using other instruments. However, further data and advanced analytical techniques are needed to confirm this hypothesis and pinpoint specific black hole binary systems.
"We are fortunate that nature has provided us with such precise clocks distributed throughout our galaxy," notes Kathrin Grunthal, researcher at the Max Planck Institute for Radio Astronomy and lead author of one of the published studies. "With MeerKAT, one of the most powerful radio telescopes in the world, we can monitor these objects with incredible precision and look for tiny changes in their behaviour caused by gravitational waves resonating across the universe."
"Studying the hum of gravitational waves allows us to tune into echoes of cosmic events that occurred over billions of years," adds Matthew Miles, researcher at OzGrav and Swinburne University of Technology and lead author of two of the MNRAS papers.
"Detecting gravitational waves at nanohertz frequencies will not only help us search for supermassive black hole binaries but also open a window into the early phases of the universe's formation and a variety of exotic physical processes," explains Golam Shaifullah, researcher at the University of Milano-Bicocca and contributor to the studies.
Eighteen months after similar findings were published by three other international experiments, including the European Pulsar Timing Array (EPTA)—involving INAF, the University of Milano-Bicocca, and the Gran Sasso Science Institute—the MPTA results offer new perspectives on the role of massive black holes in shaping the cosmos and the large-scale architecture they left behind.
"Understanding and modelling the background noise affecting pulsar signals—caused by the ionised gas between stars, the Earth, and the Sun—is key to confirming MPTA's results, as well as those from EPTA and other experiments," explains Caterina Tiburzi, INAF researcher in Cagliari and member of the EPTA collaboration. "The new low-frequency receivers of MeerKAT will be extraordinary tools for this purpose."
"Beyond the excitement of these new observational results," concludes Andrea Possenti, INAF researcher in Cagliari and MPTA collaborator since its founding in 2018, "this is a pivotal moment. It demonstrates how international collaboration in Pulsar Timing Array experiments, in which INAF has been involved for over 20 years, will finally open the door to gravitational wave astronomy at ultra-low frequencies."
Published Studies
The three papers published in Monthly Notices of the Royal Astronomical Society:
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The MeerKAT Pulsar Timing Array: Maps of the gravitational-wave sky with the 4.5 year data release" di K. Grunthal et al.
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“The MeerKAT Pulsar Timing Array: The 4.5-year data release and the noise and stochastic signals of the millisecond pulsar population” di Matthew T. Miles et al.
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“The MeerKAT Pulsar Timing Array: The first search for gravitational waves with the MeerKAT radio telescope” di Matthew T. Miles et al.
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