The universe is full of invisible and inaudible vibrations – a background of very long wavelength gravitational waves. Such waves were suspected to exist in space-time up to ten light-years across, but astronomers have now proven they exist for the first time. This was achieved with the help of millisecond pulsars, stellar remnants that rotate quickly and very regularly on their axis. The research teams used several large radio telescopes to track the “beats” of these pulsars, looking for subtle changes in their clock rates. These arise when the radio waves of pulsars are affected by long-wavelength gravitational waves on their way to us. After 15 years of observation, the astronomers were able to see the effect in their data. This confirms the existence of background resonance for long gravitational waves.
As early as 1916, Albert Einstein predicted that the motion of massive objects in the universe could cause space-time to oscillate. However, it was only nearly 100 years ago, in 2015, that astronomers using LIGO gravitational wave detectors in the USA were able to detect these gravitational waves. They detected the short-range, high-frequency oscillations in space and time that are given off when two stellar black holes merge – when translated sonically, these signals resemble a short whistling or chirping sound. But physicists have long suspected that there must be other, very, very long-duration gravitational waves in the universe. According to the theory, such extremely low-frequency oscillations in space and time must have arisen, for example, when supermassive black holes from colliding galaxies interact with each other, other waves could still have come from the early days of the universe.
Pulsars as gravitational wave detectors
Just as the cosmic background radiation fills the entire universe with faint but ubiquitous detectable noise, this gravitational wave background should also permeate the entire universe, according to the theory. But the problem is detecting very long wavelength gravitational waves. Because each individual oscillation can be several light-years in length and space and time change accordingly slowly and gradually, it cannot be detected with ground-based measuring instruments. “To detect such giant gravitational waves, you need a similarly sized detector and a lot of patience,” explains Maura McLaughlin of West Virginia University. So, astronomers at the North American Nanohertz Gravitational-Wave Observatory (NANOGrav) have joined forces to search for these giant vibrations with the help of cosmic “helpers.”
To do this, astronomers targeted 67-millisecond pulsars in our galaxy using several large radio observatories, including the Arecibo radio telescope in Puerto Rico, the Green Bank telescope in West Virginia, and the Very Large Array in New Mexico. Pulsars are neutron stars that rotate very quickly on their axis, emitting a focused beam of radio waves like a kind of cosmic beacon. As a result, these pulsars emit fast but very regular radio pulses from Earth. This is where long-wavelength gravitational waves come into play: as they stretch and compress space-time between us and pulsars, they also change the travel time of the radio pulses from the pulsars. In theory, this should show up in small irregularities in regular “chimes” in reality. Over the course of several years, the nature of these transitions should follow the shape of space-time oscillations. “Pulsars are relatively faint radio sources, so it took us thousands of hours of observation time annually in some of the largest telescopes in the world to perform this experiment,” says McLaughlin.
Gravitational wave music of the universe
In 2020, after twelve years of pulsar observations, scientists from the NANOGrav Collaboration began to see the first signs of the subtle shifts they had been looking for. But only this year, after 15 years of data collection, that initial suspicion was confirmed: “Our early data did reveal that something could be heard,” says Xavier Siemens, co-director of NANOGrav from Oregon State University. “But we now know that it’s actually the music of the gravitational wave universe.” In contrast to the brief “chirps” of short-range, high-frequency gravitational waves from colliding stellar black holes or neutron stars, the research team compared the faint but extremely long-wavelength background with the lower gravitational-wave background. “This is the first detection of the gravitational wave background, and it has opened up a whole new observational window on the universe,” says nanograph researcher Chiara Mingarelli of the Flatiron Institute in New York City.
Astronomers hypothesize that a large part of the space-time oscillations they detect can be traced back to the interaction of orbiting supermassive black holes. Since these can weigh several million to billions of solar masses, they can release huge amounts of energy as long-wavelength gravitational waves. Its frequency depends, among other things, on the mass and motion of the black holes. “It’s like a choir that all these black holes join at different frequencies,” Mingarelli says. “However, the gravitational-wave background is about twice as high as I would have expected. It is on the upper edge of what models predict for vibrations from black holes only.” That is why astrophysicists think that other processes may also contribute to this boisterous chorus of space-time vibrations. .
The next goal of the NANOGrav collaboration is to learn more about individual sources of gravitational-wave background. So far, the team has only been able to detect the common “buzz” of all potential sources. The researchers now plan to evaluate their data to determine which frequencies are represented in this tinnitus and look for clues about the source of the individual oscillations. “We’re only beginning here,” Mingarelli says. It will help that the NANOGrav collaborative teams are not the only ones who have used radio telescopes and pulsars to search for and find evidence of the gravitational wave background. Teams in Europe, India, China and Australia also reported very similar observations at the same time. As part of the International Pulsar Timing Consortium, the groups will pool their data to increase accuracy. “Our combined data will be much more meaningful,” says Stephen Taylor of Vanderbilt University, leader of the NANOGrav collaboration. “We are excited to know what secrets they will reveal to us about our universe.”
Source: NANOGrav Collaboration, The Astrophysical Journal Letters, doi: 10.3847/2041-8213/acdac6
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