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How to track down these cryptic signals

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Astronomers are about to unlock a whole new way to observe the universe.

Since the first detection of gravitational waves in October 2015, scientists have listened to these cosmic buzzes caused by massive and violent events like the merger of two black holes.

But scientists still cannot detect these low-frequency waves which are often the result of even more massive objects colliding with each other or events that took place shortly after the Big Bang.

A team of researchers from the University of Birmingham suggests combining different methods to detect ultra-low-frequency gravitational waves that hold the mystery of ancient black holes and the early universe.

Their job was published Monday in the newspaper Nature astronomy.

What are low frequency gravitational waves?

Astronomers have mainly relied on electromagnetic radiation, or light, to study objects in space. But as light moves towards us, it interacts with different elements of space, including dust, obscuring our view of the cosmos.

Gravitational waves are a way of listening to the universe rather than seeing it. These buzzing sounds are caused by the accelerated masses cosmic beings, which send ripples through space-time at the speed of light. Scientists can listen to these echoes from the cosmos using LIGO (Laser Interferometer Gravitational-Wave Observatory) detectors and the Virgo detector.

But most of the gravitational waves detected so far have higher frequencies in the millihertz. Meanwhile, low frequency gravitational waves, which are at nanohertz frequencies, are much more difficult to detect.

Frank Ohme, head of the independent Max Planck research group for gravitational physics, explains that they oscillate faster or slower depending on the cause of the gravitational waves.

“The effect is the same; it has to stretch and compress space and time, ”says Ohme Reverse. “TLow frequencies want to do it slower, so it takes a lot longer for things to tighten and stretch than high frequency ones.

While high frequency gravitational waves are caused by ordinary stars or smaller black holes between 20 and 30 solar masses, low frequency waves are caused by the fusion of supermassive black holes, which can be in the millions or billions. times the mass of the Sun.

Scientists also believe that low-frequency gravitational waves could originate from events occurring shortly after the Big Bang, long before galaxies formed.

Part of the Virgo gravitational wave detector.CLAUDIO GIOVANNINI / AFP / Getty Images

How to detect low frequency gravitational waves

Christopher Moore, a researcher at the Institute for Gravitational Wave Astronomy & School of Physics and Astronomy at the University of Birmingham and lead author of the article, has been studying gravitational waves for several years.

“I’ve been interested in gravitational waves for a long time,” says Moore Reverse. “But for most of my time, low frequency waves have been a niche interest with a lot less attention than high frequency waves, but I think that’s really starting to change.”

The primary method used to detect low-frequency gravitational waves is through the use of pulsars, compact, highly magnetized stars that spin while emitting a regular pulse of radio waves. Scientists are looking for any fractional changes in the timing of the pulsar’s beam that gravitational waves can cause.

“Nature has been kind enough to give us fast spinning millisecond pulsars, which are very good clocks. “If a gravitational wave passed through the Earth, you would see the clocks speeding up and slowing down, but in different ways.”

But while this may be the primary way to detect low-frequency gravitational waves, the authors of the new study argue that it’s not enough because it doesn’t specify the cause behind the waves.

Instead, they suggest combining different methods to determine the source of low-frequency gravitational waves.

In January, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) detected what could be hints of low-frequency gravitational waves by studying signals from distant stars, but these have yet to be confirmed. .

“So what we were really trying to do in this article is see if there is another probe, besides pulsar synchronization, another instrument, another experiment, a way to try to detect gravitational waves which might help even a little. “says Moore.

One suggestion is to combine the pulsar data with observations made by the European Space Agency’s Gaia mission, which has the ambitious task of creating a three-dimensional map of the Milky Way.

The authors also suggest looking at Big Bang nucleosynthesis, a model of the early universe based on the number of different atoms that existed shortly after the Big Bang.

“So none of these methods can detect gravitational waves yet, but they can set limits at different frequencies,” says Moore.

Although the article does not provide conclusive answers, it is a first step in conducting future studies on low-frequency gravitational waves.

Black hole mergers produce gravitational waves. We have detected “small” ones – but the bigger ones are more elusive. MARK GARLICK / SCIENTIFIC PHOTO LIBRARY / Scientific Photo Library / Getty Images

Why do we study gravitational waves?

Since researchers first detected gravitational waves, these ripples in space-time have opened up a new field of observation of the universe.

And now, as scientists are on the verge of unlocking low-frequency gravitational waves, it’s an exciting time to listen to the cosmos.

“We’re just tapping into the really massive black holes that we know exist in the universe, but we don’t know exactly how many there are and how heavy they are,” Ohme said. “And because they are so heavy, the gravitational waves they create are not only of lower frequencies, but are also inherently very, very strong.”

“So the heavier the black holes, the more space-time distortion they create, so we can look deeper into the universe,” he adds.

But for low-frequency gravitational waves to be informative, scientists need to know their source.

“And that’s really the point, are we looking at an astrophysical signal coming from black holes in the local universe, or are we watching a cosmological process happening, happening much closer to the Big Bang, much further in time? ? ” Moore said.

Moore predicts that scientists are on the verge of making the first confirmed detection of low-frequency gravitational waves, which could help us deepen the universe or learn more about the formation of supermassive black holes.

“It’s a whole new way of doing astronomy,” says Moore. “That’s one of the things that makes him really exciting.”

Abstract: Ultra-low frequency gravitational waves (≲100 nHz) are essential for understanding the assembly and evolution of astrophysical binaries of black holes with masses ~ 106-109M⊙ at low redshifts1–3. These gravitational waves also provide a unique window into a wide variety of cosmological processes4-11. The Pulsar12-14 timing matrices are starting to measure15 this stochastic signal at ~ 1-100 nHz and the combination of data from multiple matrices16-19 should confirm detection in the next 20 years. The dominant physical processes generating gravitational radiation at nHz frequencies are still uncertain. Pulsar synchronization network observations alone are currently unable21 to distinguish a binary black hole22 astrophysical foreground from a cosmological background due, for example, to a first order phase transition at a temperature. of about 1 to 100 MeV in a dark sector with weak interaction8-11. This letter explores the extent to which the incorporation of integrated limits on the spectrum of ultra-low frequency gravitational waves from any combination of cosmic diffuse background23,24, big bang nucleosynthesis25,26 or astrometric observations27, 28 can help break this degeneration

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