An international collaboration of European astronomers, together with Indian and Japanese colleagues, have seen evidence for ultra-low-frequency gravitational waves, which is expected to originate from pairs of supermassive black holes found in the centres of merging galaxies. This is the result of more than 25 years of observations with the most sensitive radio telescopes in Europe and India, including the Westerbork Synthesis Radio Telescope (WSRT). In doing so, they have opened a new window on gravitational wave research. These gravitational waves contain information about the Universe’s best-kept secrets. The research has been published in a series of articles in the professional journal Astronomy & Astrophysics.
Published by the editorial team, 29 June 2023
The European Pulsar Timing Array (EPTA) is a collaboration of scientists from more than ten institutions across Europe and brings together astronomers and theoretical physicists in order to use observations of ultra-regular pulses from extinguished stars called ‘pulsars’ to construct a gravitational wave detector the size of our Milky Way. From the Netherlands, astronomers from ASTRON and Radboud University (RU) are involved. Other teams from around the world have independently obtained the same results, also announced today.
The astronomically-large gravitational wave detector, spanning from the Earth to twenty-five specifically chosen pulsars scattered across the Milky Way, gives the opportunity to investigate gravitational waves at lower frequencies (wavelengths of a few light years) than is possible with other detectors such as LIGO and Virgo (wavelengths of several kilometres). At these nanohertz frequencies, unique sources and phenomena can be observed.
Emma van der Wateren, PhD student at the Netherlands Institute for Radio Astronomy ASTRON and Radboud University, explains: ‘Pulsars are excellent natural clocks. We use the changes in the extreme regularity of the clocks’ ‘ticking’ to detect the subtle stretching and squeezing of space-time caused by gravitational waves.’ The gravitational waves for which there is now evidence are most likely a sum of signals from multiple sets of supermassive black holes orbiting each other very slowly.
The researchers see the results as the beginning of a new exploration of the universe. Astronomer Gemma Janssen (ASTRON, RU): ‘These ultra-low-frequency gravitational waves contain information about the Universe’s best-kept secrets. This is the only way we can study the population of supermassive black hole binaries that form when galaxies merge – with masses of millions to billions times the mass of our Sun.’
‘It has been quite an undertaking,’ continues Ben Stappers of the UK’s Jodrell Bank Centre for Astrophysics. ‘These results are based on decades of coordinated observations with the five largest European radio telescopes: the Effelsberg radio telescope in Germany, the Lovell telescope at Jodrell Bank Observatory in the United Kingdom (UK), the Nançay radio telescope in France, the Sardinia radio telescope in Italy and the Westerbork Synthesis Radio Telescope in the Netherlands.’
Once a month, the European telescopes performed exactly simultaneous observations in addition to their individual observations to achieve increased sensitivity. Observations from the EPTA were complemented with data from the Indian Pulsar Timing Array (InPTA), resulting in a uniquely sensitive dataset.
The Dutch contribution to the EPTA result is a dataset that was generated of 16 years of monthly observations with the WSRT. Cees Bassa, scientist at ASTRON, says: ‘The unique feature of the Westerbork dataset is that the signals were recorded also at lower frequencies in addition to the more standardly-used higher observing frequencies. This allowed us to remove the effects of space weather from the data, making the entire dataset more sensitive to gravitational wave signals.’
Besides observing pulsars, astronomers in the Netherlands also stood at the basis of the development of a new generation of pulsar instruments. The first of such instrument was installed at WSRT in 2005, and similar instruments are now used at all European radio telescopes for pulsar observations.
The announcement of the EPTAs results is coordinated with similar publications by other collaborations across the world, namely the Parkes-based Australian, Chinese, and North-American pulsar timing array (PTA) collaborations, abbreviated as the PPTA, CPTA and NANOGrav, respectively. ‘Since the independently obtained results are in agreement with each other, this gives us additional confidence that this emerging signal really comes from gravitational waves,’ Janssen said.
Scientists from most of the leading PTAs are combining their datasets for the International Pulsar Timing Array. The aim is to expand and merge the PTA datasets, eventually creating a combined dataset with a stronger overall signal of gravitational waves. This will then eventually lead to new insights into the evolution of supermassive black holes and the massive galaxies in which they are formed.
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