![]() NANOGrav was first to spot the signal 1, and reported it to colleagues in 2020. As a result, they have found a signature called the Hellings–Downs curve, which predicts how, in the presence of gravitational waves coming from all possible directions, the correlation between pairs of pulsars varies as a function of their separation in the sky. Instead, each collaboration monitors an array of dozens. The timing of a single pulsar would not be reliable enough to detect gravitational waves. 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. “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 paper 3. Millisecond pulsars rotate the fastest, up to several hundred times per second. Each time a pulsar rotates on its axis, its radio beam travels in and out of the line of sight to Earth, resulting in a pulse with regular intervals. These are incredibly dense neutron stars that spew radio waves from their magnetic poles. Credit: Danielle Futselaar, MPIfRĪll the groups use massive radio telescopes to monitor ‘millisecond’ pulsars. “My calculation for the gravitational-wave sensitivity of FAST observation was done back in 2009, when I was a PhD student.”Īn artist’s impression of gravitational waves caused by supermassive black holes. Keija Lee, a radio astronomer at Peking University in Beijing who led the FAST study, says he was not surprised by the result 4. ![]() 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. 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. “It will put an army of astrophysicists to work.” Catching a wave “If this is confirmed, we’ll have 20 years of work studying this new background,” says Monica Colpi, who studies the theory of gravitational waves and black holes at the University of Milan–Bicocca in Italy. Researchers will now pool their data to see if they can reach that threshold together. “But we do think this is strong evidence.” Each group has seen hints of an expected signature of gravitational waves, but without the statistical certainty of a firm discovery, Ransom and others say. “We’re not using the ‘d’ word - for detection - yet,” says Ransom. “We can tell that the Earth is jiggling due to gravitational waves that are sweeping our Galaxy,” says Scott Ransom, an astrophysicist at the US National Radio Astronomy Observatory in Charlottesville, Virginia, and a senior member of NANOGrav, one of four collaborations that announced separate results on 29 June 1 – 4. By contrast, the ripples detected since 2015 using a technique called interferometry are just tens or hundreds of kilometres long. These waves are thousands of times stronger and longer than those found in 2015, with wavelengths of up to tens of light years. Whereas the original discovery spotted waves originating from the collision and merger of two star-sized black holes, the most likely source of the latest finding is the combined signal from many pairs of much larger black holes - millions or even billions of times the mass of the Sun - slowly orbiting each other in the hearts of distant galaxies. The approach tracks changes in the distances between Earth and beacon stars in its Galactic neighbourhood called pulsars, which reveal how the space in between is stretched and squeezed by the passage of gravitational waves. Gravitational waves are back, and they’re bigger than ever.Īfter the historic first detection of the space-time rattles in 2015 using ground-based detectors, researchers could have now rediscovered Albert Einstein’s waves with an entirely different technique. ![]() A view from the Five-hundred-meter Aperture Spherical Telescope in Guizhou, China, which monitored pulsars to detect gravitational waves. ![]()
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