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The Most Convincing Proof Yet for Low-Frequency Gravitational Waves

The strongest proof yet for low-frequency gravitational waves has been discovered by astronomers using information gathered by CSIRO’s Parkes radio telescope, Murriyang.


Artist's impression of a pulsar timing array. Image Credit: OzGrav/Swinburne, Carl Knox

The Parkes Pulsar Timing Array collaboration has been keeping an eye on a set of pulsars—rapidly spinning stars that pulse like lighthouses—for almost 20 years.

To support Einstein’s general theory of relativity and advance the comprehension of the universe, they are searching for nanosecond pulse delays caused by gravitational waves.

The team has advanced its efforts to use the study of pulsars to detect gravitational waves by gathering and analyzing this substantial data set.

Their most recent findings were published on June 29th, 2023, in the Astrophysical Journal Letters and Publications of the Astronomical Society of Australia.

Albert Einstein hypothesized that space-time is a four-dimensional fabric in 1916 and that events like star explosions and the merger of black holes cause ripples, or gravitational waves, in this fabric.

In 2015, over a century after the discovery of gravitational waves, scientists working with the LIGO and Virgo collaborations achieved the first direct observation of gravitational waves produced by the collision of two stellar-mass black holes.

The Parkes Pulsar Timing Array collaboration is looking for gravitational waves produced by binary supermassive black holes at the centers of galaxies as opposed to these gravitational waves, which oscillate many times per second. These gravitational waves oscillate over several years.

According to Dr. Daniel Reardon, a researcher at OzGrav and the Swinburne University of Technology who oversaw the searches, these gravitational waves would alter the apparent rotation frequency of fast-spinning pulsars as they move across the galaxy and wash over the Earth.

We can detect gravitational waves by searching for pulses that arrive earlier or later than we expect. Previous studies have shown an intriguing signal in pulsar timing array observations, but its origin was unknown.

Dr Daniel Reardon, Researcher, Swinburne University of Technology

Dr. Reardon added, “Our latest research has found a similar signal among the set of pulsars we have been studying, and we now see a hint of the fingerprint that identifies this signal as gravitational waves. Unlike stellar-mass bursts of gravitational waves, supermassive black holes take years or decades to complete their orbits, and so their signature takes a decade or more to emerge.

Studying pulsars has been a top priority for astronomers searching for this gravitational-wave signal.

There is a similar signal in the data from other collaborations in China (CPTA), Europe (EPTA), India (InPTA), and North America (NANOGrav), and their findings have also just been published in several academic papers.

While it is encouraging that all the major partnerships are detecting traces of the waves, CSIRO astronomer Dr. Andrew Zic, who co-led the analysis, noted that the ultimate test will occur in the near future, when all of the data are integrated into a worldwide dataset.

This signal could still be caused by things like variations in a pulsar’s rotation over a long period of time, or may simply be a statistical fluke. Our Parkes radio telescope, Murriyang, has an advanced receiver and an excellent view of the best pulsars in the southern sky, which are essential for this work. The next step is to combine pulsar data sets collected by telescopes in both the northern and southern hemispheres to improve the sensitivity of our observations.

Dr Andrew Zic, Study Co-Lead and Astronomer, Commonwealth Scientific and Industrial Research Organisation

The Parkes Pulsar Timing Array collaboration in Australia is one of many groups working together through the International Pulsar Timing Array consortium to aggregate their data to further characterize the signal and establish its origin.

Dr. Zic added, “The next stage of our research will combine the full power of the global array and rule out any anomalies.

Using pulsars to validate the observation of low-frequency gravitational waves would broaden this growing field of study, which will be further investigated by future devices such as the SKA telescopes currently under construction in Australia and South Africa.

The Parkes Pulsar Timing Array project is a collaborative effort by astronomers from several institutions to observe pulsars using the CSIRO's Parkes Radio Telescope, Murriyang.

The Parkes radio telescope near Murriyang is part of the Australia Telescope National Facility, which is sponsored by the Australian Government and managed by CSIRO, Australia’s national science agency. The Wiradjuri People are the traditional owners of the Parkes Observatory site, which is acknowledged by the researchers.

The Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav) provided funding for this study.

OzGrav is a collaboration between several Australian universities, including the University of Queensland, The Australian National University, The University of Sydney, Monash University, The University of Adelaide, The University of Western Australia, The University of Melbourne, and CSIRO. It is headquartered at Swinburne University of Technology.

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