Omnipresent Vibrations: The entire cosmos is filled with invisible waves several light-years long – the gravitational-wave background. Astronomers have now detected these long-wave space-time oscillations for the first time. Unlike the very brief, high-pitched signals from stellar black hole collisions, these gigantic oscillations resemble a sustained “hum”. The researchers detected them through long-term observations of pulsars, whose radio pulses are deflected by the gravitational waves.
As early as 1916, Albert Einstein predicted that the movement of massive cosmic objects causes space-time to oscillate. In 2015, astronomers were able to detect the gravitational waves from a merging pair of stellar black holes for the first time. Since then they have detected a whole “periodic table” of such events. Translated into acoustic terms, the short-term, high-frequency space-time oscillations of such collisions of neutron stars or black holes resemble a short whistling or chirping sound.
Sustained buzzing instead of short whistles
But astrophysicists have long suspected that beyond these very short, sporadic events there must also be longer-lasting, long-wavelength gravitational waves in the universe. A single oscillation of such a wave can span several light-years. Such extremely low-frequency space-time oscillations arise when, for example, duos or trios of supermassive black holes interact with one another. Because these giants weigh several million to billions of solar masses, they release enormous amounts of energy in the form of long-wave gravitational waves.
Other waves of this type could still be from the early days of the cosmos or even represent the echo of a Big Bounce – the collapse of a predecessor universe before the Big Bang. All of these vibrations combine to create an omnipresent background of vast but invisible gravitational waves — a ubiquitous “floor noise” or “hum” of moving spacetime. This vibrating background forms the gravitational counterpart to the cosmic background radiation, which fills the entire universe with a faint radio noise.
Pulsars as search helpers
The problem, however, is that to detect a gravitational wave with a wavelength of several light years, you need a detector the size of half a galaxy – and a lot of time. Therefore, astronomers have taken advantage of cosmic “helpers” to search for these giant vibrations. The collaboration “North American Nanohertz Observatory for Gravitational Waves” (NANOGrav) used the now destroyed Arecibo radio telescope, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico.
With these large radio telescopes, astronomers repeatedly targeted 67 millisecond pulsars in our galaxy for 15 years. Pulsars are neutron stars that spin rapidly on their own axis, emitting a focused beam of radio waves like a kind of cosmic beacon. Seen from Earth, these pulsars emit fast but very regular radio pulses – like a cosmic metronome.
Subtle changes in clock speed
This is where the long-wave gravitational waves come into play: when they stretch and compress the space-time between us and the pulsars, this also changes the travel time of the pulsar radio pulses – and creates tiny irregularities in their actually regular “ticking”. Over the course of several years, these deviations should trace the shape of the space-time oscillations.
“Pulsars are relatively faint radio sources, however, so it took us thousands of hours of observing time per year at some of the world’s largest telescopes to conduct this experiment,” says Maura McLaughlin of West Virginia University. In 2020, after a good twelve years of pulsar observations, the researchers actually began to see the first signs of the gravitational waves they were looking for. Now, after 15 years of data collection, the NANOGrav teams have confirmed this suspicion.
“Music of the Gravitational Wave Universe”
“Now we know it’s actually the music of the gravitational-wave universe,” says Xavier Siemens of Oregon State University. The clock fluctuations of the pulsars distributed across the Milky Way suggest that the entire cosmos is filled with the “buzz” of these long-wavelength space-time oscillations. “This is the first detection of the gravitational-wave background, and we have opened up a whole new observation window into the universe,” says Chiara Mingarelli of the Flatiron Institute in New York City.
The astronomers assume that a large part of the space-time oscillations they detect can be traced back to the interaction of orbiting supermassive black holes – as predicted by the theory. The frequency of the gravitational waves released in the process depends, among other things, on the mass of the black holes and their movement. “It’s like a chorus that all these black holes join in at different frequencies,” explains Mingarelli.
Louder than expected
Surprisingly, however: “The gravitational-wave background is about twice as loud as expected,” reports Mingarelli. “It moves at the upper limit of what vibration models only predict for such black holes.” The researchers therefore suspect that other processes could also contribute to this buzzing chorus of space-time vibrations. What these are, however, is still unclear. So far, the data has only shown the combined “buzz” of all potential sources.
The aim of the NANOGrav collaboration is now to find out more about the individual sources of the gravitational-wave background. The researchers are therefore planning to evaluate their data to determine which frequencies are represented in this hum and to look for clues as to where the individual oscillations are coming from. “We are only at the beginning here,” says Mingarelli.
Pooled data could reveal more
Helpful: The astronomers of the NANOGrav collaboration are not the only ones who have found evidence of the gravitational-wave background with the help of radio telescopes and pulsars. At the same time, teams in Europe, India, China and Australia reported on very similar observations in several specialist articles. As part of the “International Pulsar Timing Array Consortium”, the groups want to combine their data in the future in order to increase the resolution.
“Our combined data will be far more meaningful,” says Stephen Taylor of Vanderbilt University, leader of the NANOGrav collaboration. “We are excited to see what secrets they will reveal to us about our universe.” (The Astrophysical Journal Letters, 2023; doi: 10.3847/2041-8213/acdac6)
Source: North American Nanohertz Observatory for Gravitational Waves (NANOGrav), National Science Foundation, Simons Foundation