Finding this cosmic phenomenon could unlock mysteries of the ancient universe

Astronomers are on the verge of unlocking an entirely new way to observe the universe.

Since the first detection of gravitational waves in October 2015, scientists have been listening in to these cosmic hums caused by massive, violent events such as the merger between two black holes.

But scientists still can’t detect these waves at low frequencies that are often the result of even more massive objects colliding with one another 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 work was published Monday in the journal 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 travels towards us, it interacts with different elements in outer space, including dust, obscuring our view of the cosmos.

Gravitational waves are a way to listen to the universe rather than see it. These hums are caused by the accelerated masses of cosmic beings, which send out ripples through spacetime at the speed of light. Scientists can listen in on these echoes of the cosmos thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors and the Virgo detector.

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

Frank Ohme, leader of the Independent Max Planck Research Group for Gravitational Physics, explains that they oscillate faster or slower depending on what causes the gravitational waves.

“The effect is the same; it's got to stretch and squeeze space and time,” Ohme tells Inverse. “The low-frequency ones want to do it slower, so it takes a lot longer for things to squeeze and stretch than the high-frequency ones.”

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

Scientists also believe that low-frequency gravitational waves could come from events taking place shortly after the Big Bang, long before galaxies were formed.

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 paper, has been studying gravitational waves for several years.

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

The primary method used to detect low-frequency gravitational waves is through pulsars, compact, highly magnetized stars that rotate while emitting a regular pulse of radio waves. Scientists look for any fractional change to the timing of the pulsar’s beam that gravitational waves may cause.

“Nature has been kind enough to give us rapidly spinning millisecond pulsars, which are extremely good clocks — they rotate in a very, very stable way which makes them extremely good timekeeping instruments,” Moore says. “If a gravitational wave were to come across the Earth, you'd see the clocks speed up and slow down but in different ways.”

But while that may be the leading way to detect low-frequency gravitational waves, the authors behind the new study argue that it’s not enough since 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 may be hints of low-frequency gravitational waves by studying signals from distant stars, but those are yet to be confirmed.

“So what we were really trying to do in this paper is to see if there's any other probe, apart from pulsar timing, any other instrument, any other experiment, any way of trying to detect gravitational waves that could help, even a little bit,” Moore says.

One suggestion is combining 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 into Big Bang nucleosynthesis, a model of the early universe based on how many different atoms existed shortly after the Big Bang.

“So neither of those methods can detect gravitational waves yet, but they can place limits at different frequencies,” Moore says.

Although the paper does not come up with conclusive answers, it is a first step in conducting future studies on low-frequency gravitational waves.

Why do we study gravitational waves?

Since researchers first detected gravitational waves, these ripples through spacetime have opened up a new field for observing the universe.

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

“We just tap into the really massive black holes that we know exist in the universe, but we’re not exactly sure how many there are and how heavy they are,” Ohme says. “And because they are so heavy, the gravitational waves they create not only are of lower frequencies but also are super, super loud intrinsically.”

“So the heavier the black holes are, the larger spacetime distortion they create, and therefore we can look further out into the universe,” he adds.

But for low-frequency gravitational waves to be informative, scientists have 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 looking at a cosmological process happening, happening much closer to the Big Bang, much further back in time?” Moore says.

Moore predicts that scientists are on the verge of the first confirmed detection of low-frequency gravitational waves, which may help us peer further out into the universe or learn more about how supermassive black holes came to be in the first place.

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

Abstract: Gravitational waves at ultra-low frequencies (≲100 nHz) are key to understanding the assembly and evolution of astrophysical black hole binaries with masses ~106–109M⊙ at low redshifts1–3 . These gravitational waves also offer a unique window into a wide variety of cosmological processes4–11. Pulsar timing arrays12–14 are beginning to measure15 this stochastic signal at ~1–100 nHz and the combination of data from several arrays16–19 is expected to confirm a detection in the next few years20. The dominant physical processes generating gravitational radiation at nHz frequencies are still uncertain. Pulsar timing array observations alone are currently unable21 to distinguish a binary black hole astrophysical foreground22 from a cosmological background due to, say, a first-order phase transition at a temperature ~1–100 MeV in a weakly interacting dark sector8–11. This letter explores the extent to which incorporating integrated bounds on the ultra-low-frequency gravitational wave spectrum from any combination of cosmic microwave background23,24, big bang nucleosynethesis25,26 or astrometric27,28 observations can help to break this degeneracy