A decades-long investigation has discovered space-time ripples that are light years broad using beacon stars called pulsars.
There are more gravitational waves than ever before.
Researchers have the opportunity to recover Albert Einstein’s waves in 2015 using a completely new method after the historic first discovery of the space-time vibrations was made using ground-based detectors. The method monitors variations in the separations between Earth and pulsars, which are beacon stars in our galaxy’s vicinity that show how space is compressed and expanded as a result of gravitational waves.
The most likely source of the most recent discovery 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 centres of distant galaxies, as opposed to the original discovery, which detected waves originating from the collision and merger of two star-sized black holes. These waves have wavelengths up to tens of light years and are thousands of times stronger and longer than those discovered in 2015. Contrarily, the waves found since 2015 by means of an approach known as interferometry are just tens or hundreds of miles long.
According to Scott Ransom, a senior member of the NANOGrav collaboration and an astrophysicist at the US National Radio Astronomy Observatory in Charlottesville, Virginia, “We can tell that the Earth is jiggling due to gravitational waves that are sweeping our Galaxy,” which is one of four collaborations that announced separate results on 29 June1-4.
We’re not yet employing the “d” term, which stands for detection, according to Ransom. But we do believe that this is solid proof. According to Ransom and others, each group has detected evidence of a gravitational wave signature but without the statistical assurance of a confirmed finding. In order to test if they can jointly attain that barrier, researchers will now pool their data.
“If this is confirmed, we’ll have 20 years of work studying this new background,” says Monica Colpi, a researcher at the University of Milan-Bicocca in Italy who specialises in the theory of gravitational waves and black holes. An army of astrophysicists will be put to work as a result.
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.”
An artist’s impression of gravitational waves caused by supermassive black holes.Credit: Danielle Futselaar, MPIfR
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.
The Green Bank Observatory in West Virginia is another radio telescope used in the pulsar-monitoring effort.Credit: Jay Young for Green Bank Observatory
“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.”
