Commission D1 Gravitational Wave Astrophysics


Pulsar Timing Array Projects Announce New Evidence for Ultra-Low Frequency Gravitational Waves


For two decades, collaborations of astronomers around the world have been in the pursuit of the detection of ultra-low (nanohertz) frequency gravitational waves. On 29 June 2023, they announced the results of their most recent searches. The results showed the strongest evidence yet for these gravitational waves.

To find these waves, the groups have been using the world’s largest radio telescopes to monitor ultra-stable millisecond pulsars. These stars beam pulses that can be used as clocks, forming a Galactic-scale gravitational wave observatories. The passage of gravitational waves through the Milky Way causes miniscule irregularities in the times the pulses are detected. By monitoring a set of pulsars (a Pulsar Timing Array) it is possible to discern the imprint of gravitational waves, which induce smoking gun spatial correlations, from other signals.  

The results were announced by the Chinese Pulsar Timing Array, European Pulsar Timing Array, Indian Pulsar Timing Array, the North American Observatory for Gravitational Waves, and the Parkes Pulsar Timing Array. The findings show the first evidence for spatial correlations from gravitational waves. The most likely source of this background is from binary supermassive black holes embedded in distant galaxies, but other more exotic sources that have also been hypothesized.

The future for nanohertz-frequency gravitational wave astronomy is immense. The signal of the radiation will get stronger through further, continued pulsar timing observations. It will also become more pronounced when the data sets from the PTAs are combined as part of the International Pulsar Timing Array project, which also includes observations from the MeerKAT radio telescope.      

The community is excited to share the results, in particular at the IAU General Assembly in Cape Town in August 2024.

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