There is evidence that there are low-frequency waves

Gravitational-wave signature of the Hellings-Downs curve from the merger of two supermassive black hole binaries

The timing of a single pulsar would not be reliable enough to detect gravitational waves. Instead, each collaboration monitors an array of dozens. There is a signature called the Hellings–Downs curve which predicts how the correlation between two pairs of pulsars varies in the presence of Gravity waves coming from all directions. NANOGrav was first to spot the signal1, and reported it to colleagues in 2020. But the team decided to wait for the other collaborations to see hints of the curve before publishing.

After seeing the group’s results, the member of the European team said, “Bloody hell, there could be something interesting here.”

Einstein had predicted the creation of space waves in 1916. On 14 September 2015, the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington State confirmed his prediction by detecting a burst of waves from the merger of two black holes. Physicists have captured waves from a lot of events.

If the latest signal is from the combined gravitational waves of thousands of pairs of supermassive black holes across the Universe, it would be the first direct evidence that such binaries exist and that some have orbits tight enough to produce measurable gravitational waves. Colpi says a major implication is that each of the pairs will ultimately merge — creating bursts similar to the ones seen by LIGO, but on a much larger scale. The Laser Interferometer Space Antenna (LISA) will detect the signals of some of the collision in the future, due to launch in the 20s.

“Seeing the Hellings–Downs curve actually appear for the first time in a real way — that was a beautiful moment,” says Chiara Mingarelli, a gravitational-wave astrophysicist at Yale University in New Haven, Connecticut, and a member of NANOGrav. I never tire of seeing it.

“We can use them basically as clocks,” says Andrew Zic, a radio astronomer at the Australia Telescope National Facility in Sydney and a lead author of the Parkes paper3. Slight changes in the arrival time of a pulsar’s signals can mean that the space between the star and Earth has been altered by the passage of a gravitational wave.

Three collaborations have amassed decades’ worth of pulsar data and are reporting similar results: the North American group NANOGrav; the European Pulsar Timing Array, with the contribution of astronomers in India; and the Parkes Pulsar Timing Array in Australia. A fourth collaboration, the Chinese Pulsar Timing Array, says it has found a signal with merely three years of data, owing to the exceptional sensitivity of the Five-hundred-meter Aperture Spherical Telescope (FAST), which opened in 2016 in the Guizhou region.

Keija Lee, a radio astronomer at Peking University in Beijing who led the FAST study, says he was not surprised by the result4. “My calculation for the gravitational-wave sensitivity of FAST observation was done back in 2009, when I was a PhD student.”

Ransom says that they are not using the d word for detection yet. “But we do think this is strong evidence.” Ransom and others say that each group has seen hints of an expected signature of the waves, but without a firm discovery. The researchers will pool their data to see if they can reach that threshold together.