Scientists are on the trail of low-frequency gravitational waves

The gravitational waves can give us a unique insight into how large galaxies evolve.

A century after Albert Einstein predicted the existence of gravitational waves, they were actually detected for the first time a few years ago. And in the meantime, gravitational waves – nothing more than ripples in spacetime – have been observed much more often by the gravitational wave detectors LIGO and Virgo. Some were caused by colliding black holes. Others by colliding neutron stars. All gravitational waves detected to date have one thing in common: they have a relatively short wavelength.

Other gravitational waves
Low-frequency gravitational waves – or gravitational waves with a longer wavelength – have not yet been observed, but this may change soon. Astronomers are on their heels, so is the magazine The Astrophysical Journal Letters to read. In data from the recently collapsed Arecibo radio telescope, researchers from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav for short) namely traces of these low-frequency gravitational waves were found.

Possible first
If researchers are actually tracking low-frequency gravitational waves, that’s a first. Although LIGO and Virgo have already detected many gravitational waves, they have never been able to detect these low-frequency gravitational waves. “The biggest difference (between these gravitational waves and the gravitational waves previously detected by LIGO and Virgo) is simply the frequency,” explains researcher Joseph Simon when asked. “As with the electromagnetic spectrum, when it comes to the spectrum of gravitational waves, you need different instruments to study different frequencies. Just as X-ray and radio telescopes complement each other, LIGO and NANOGrav also complement each other. In addition, the sources that generate gravitational waves at these different frequencies are also different. The gravitational waves observed by LIGO come from black holes tens of times the mass of our sun, while the black holes we expect to produce low-frequency gravitational waves are millions or billions of times more massive than the sun. These supermassive black holes are located at the heart of massive galaxies and are created when two black holes orbit each other after galaxies merge. ”

Sea of ​​gravitational waves
In the billions of years that the universe has existed, there have been many supermassive black holes orbiting each other in a narrow orbit, producing long-wavelength gravitational waves. The result is, as it were, a sea filled with a cacophony of gravitational waves generated by these supermassive black holes since the beginning of the universe. That may sound like it is not that difficult to detect these low-frequency gravitational waves, but appearances can be deceiving. Because the gravitational waves have a long wavelength, it can take years before one is detected by a stationary detector. And that makes the search for these gravitational waves much more time consuming than that of LIGO and Virgo; two stationary detectors could theoretically detect thousands of gravitational waves per second, because the waves they chase have a much shorter wavelength.

Pulsars
And yet researchers now think they are tracking low-frequency gravitational waves. In their search for these gravitational waves, they have been guided by pulsars: rapidly rotating neutron stars that emit electromagnetic radiation. Seen from Earth, the pulsars seem to flash continuously; Although the radiation is constantly being released on both sides of the neutron star, because the star is rotating very quickly, we see it emitting electromagnetic radiation at short intervals. Since pulsars are very stable, so is their apparent blink; you can set the clock there. Unless – and now it comes – a gravitational wave cycles in between. When that happens, spacetime is very subtly stretched and compressed again and that has – again, a very subtle – impact on the time it takes for the electromagnetic radiation from the pulsar to reach us. “You have to imagine that the Earth is floating on an ocean of gravitational waves,” says Simon. While the earth floats around like this, it moves up and down on the waves in turn. “And what actually happens is that spacetime is alternately stretched and compressed. As a result, pulsars in one part of space arrive a little earlier than expected (because spacetime is compressed), while pulsars in another part of space arrive a little later than expected (because spacetime is stretched ). ”

Arecibo at Green Bank
In other words, gravitational waves can cause fluctuations in the timing of the otherwise stable pulsars. And that is exactly what researchers have now seen happen. In data from the radio telescopes Arecibo and Green Bank, they have detected fluctuations in the timing of 45 pulsars over a period of 12.5 years. These fluctuations seem to be attributable to low-frequency gravitational waves, the researchers say.

Battle for the arm
The scientists are clearly still cautious. And that is not for nothing. Much more data is needed to confirm that the fluctuations are the result of low-frequency gravitational waves. According to Einstein’s general theory of relativity, low-frequency gravitational waves would not affect the timing of each pulsar in the same way, but the effect should be slightly different depending on the positions the pulsars take in relation to each other. At the moment, the signal that researchers have found in the data is still too weak to determine that this is the case. “The signal in our data indicates for now that the pulsars all see the same bobbing motion of the Earth, but we don’t yet have a specific correlation pattern between the pulsars that tells us that this bobbing motion is also due to gravitational waves – and not. is the result of other sources of noise that we have not yet identified and isolated, ”said Simon. But that could change in the short term. “If the signal we see in our data is actually a first indication of such a sea filled with low-frequency gravitational waves, then we expect the distinctive correlation pattern confirming this to emerge in the coming years.”