2023-06-29 00:11:36
The universe is filled with invisible, inaudible vibrations – a background of very long-wavelength gravitational waves. The existence of such waves in space-time, which are up to ten light years long, has been suspected for some time, but astronomers have now proven them for the first time. This was achieved with the help of millisecond pulsars, stellar remnants rotating quickly and extremely regularly around their axis. Research teams used several large radio telescopes to follow the “ticking” of these pulsars, looking for subtle changes in clock rates. These arise when the radio waves of the pulsars are influenced by the long-wavelength gravitational waves on their way to us. After 15 years of observation, astronomers might see the effect in their data. This confirms that there is this background hum of long gravitational waves.
As early as 1916, Albert Einstein predicted that the movement of massive objects in the cosmos might cause space-time to oscillate. However, it was only almost 100 years later, in 2015, that astronomers using the LIGO gravitational wave detectors in the USA were able to detect these gravitational waves. They detected the short-term, high-frequency space-time oscillations that are released when two stellar black holes merge – when translated acoustically, these signals resemble a short whistling sound or chirp. But physicists have long suspected that there must be other, longer-lasting and very long-wavelength gravitational waves in the cosmos. According to the theory, such extremely low-frequency space-time oscillations should arise, for example, when supermassive black holes from colliding galaxies interact with each other, other waves might still come from the early days of the cosmos.
Pulsars as gravitational wave detectors
Just as the cosmic background radiation fills the entire universe with a weak but detectable noise everywhere, this gravitational-wave background should also permeate the entire cosmos, according to the theory. The problem, however, is detecting these very long-wavelength gravitational waves. Because each individual oscillation can be several light-years long and space-time changes accordingly slowly and gradually, it cannot be detected with terrestrial measuring instruments. “To detect such gigantic gravitational waves, you need a detector of similar size and a lot of patience,” explains Maura McLaughlin of West Virginia University. Therefore, astronomers have joined forces in the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) to search for these giant vibrations with the help of cosmic “helpers”.
Pulsars are rapidly rotating neutron stars that regularly emit radio pulses. © NASA/ Goddard Space Flight Center
To do this, the 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 extremely quickly around their own axis, emitting a focused beam of radio waves like a kind of cosmic lighthouse. As a result, these pulsars emit fast but very regular radio pulses from Earth. This is where long-wavelength gravitational waves come into play: when they stretch and compress space-time between us and the pulsars, they also change the travel time of the radio pulses from the pulsars. Theoretically, this should show up in tiny irregularities in their actually regular “ticking”. Over the course of several years, the nature of these shifts should trace the shape of spacetime oscillations. “Pulsars are relatively faint radio sources, however, so it took us thousands of hours of observing time per year at some of the largest telescopes in the world to do this experiment,” says McLaughlin.
The music of the gravitational wave universe
In 2020, following a good twelve years of pulsar observations, scientists from the NANOGrav collaboration began to see the first signs of the subtle shifts they were looking for. But it was only this year, following 15 years of data collection, that this initial suspicion was confirmed: “Our early data already revealed that something might be heard,” says NANOGrav co-director Xavier Siemens from Oregon State University. “But now we know it’s actually the music of the gravitational-wave universe.” In contrast to the brief “chirps” of the high-frequency, short-term gravitational waves from the collision of stellar black holes or neutron stars, the research team compares the faint but extremely long-wavelength gravitational-wave background with a low hum. “This is the first detection of the gravitational-wave background, and we have opened a whole new observation window into the universe,” says NANOGrav researcher Chiara Mingarelli from the Flatiron Institute in New York City.
The astronomers assume that a large part of the space-time oscillations they have detected can be traced back to the interaction of orbiting supermassive black holes. Because these can weigh several million to billions of solar masses, they can release enormous amounts of energy as long-wavelength gravitational waves. Their frequency 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,” says Mingarelli. “However, the gravitational-wave background is regarding twice as loud as I would have expected. It’s on the upper edge of what models predict for vibrations only from such black holes.” That’s why the astrophysicists suspect that other processes may also be contributing to this buzzing chorus of space-time vibrations.
The next goal of the NANOGrav collaboration is therefore to find out more regarding the individual sources of the gravitational-wave background. So far, the team has only been able to detect the common “hum” of all potential sources. The researchers are now planning to evaluate their data to determine which frequencies are represented in this hum and look for clues as to where the individual oscillations are coming from. “We are only at the beginning here,” says Mingarelli. It will help that the NANOGrav collaboration 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 are also reporting very similar observations at the same time. As part of the International Pulsar Timing Array Consortium, the groups will combine their data to increase 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 regarding our universe.”
Quelle: NANOGrav Collaboration, The Astrophysical Journal Letters, two: 10.3847/2041-8213/acdac6
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