Gravitational waves are back, and they’re bigger than ever.
After 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. 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.
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. These waves are thousands of times stronger and longer than those found in 2015, with wavelengths of up to tens of light years. By contrast, the ripples detected since 2015 using a technique called interferometry are just tens or hundreds of kilometres long.
“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 June1–4.
“We’re not using the ‘d’ word — for detection — yet,” says Ransom. “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. Researchers will now pool their data to see if they can reach that threshold together.
“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. “It will put an army of astrophysicists to work.”
Catching a 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.”
All the groups use massive radio telescopes to monitor ‘millisecond’ pulsars. These are incredibly dense neutron stars that spew radio waves from their magnetic poles. 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. Millisecond pulsars rotate the fastest, up to several hundred times per second.
“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.
The timing of a single pulsar would not be reliable enough to detect gravitational waves. Instead, each collaboration monitors an array of dozens. 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. 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.
“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’m never tiring of seeing it.”
Alberto Vecchio, an astrophysicist at the University of Birmingham, UK, and a member of the European team, says his first reaction when he saw his group’s results2 was, “Bloody hell, there could be something interesting here.”
The long game
Einstein first predicted gravitational 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 since captured gravitational waves from dozens of such 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 signals of some of these collisions will be detected in space by the Laser Interferometer Space Antenna (LISA), a European Space Agency mission due to launch in the 2030s.
Researchers hope that they will eventually go beyond the Hellings–Downs curve and see signals of individual supermassive-black-hole binaries close enough to our Galaxy — and therefore loud enough, in gravitational-wave terms — to stand out of the background signal. “To see an isolated source, it has to be really strong,” says Vecchio.
But for now, other origins of these waves cannot be ruled out, including possible residual gravitational noise from the Big Bang.
“It’s been a long and patient game,” says Zic. “Now we’re really starting to open the window into this ultra-low-frequency gravitational-wave spectrum.”
doi: https://doi.org/10.1038/d41586-023-02167-7
