We had mentioned that the universe might be humming and that an announcement would be made on Thursday. Here we are with that announcement.

It's official. There's something out there shaking the stars in a way that can no longer be attributed to coincidence.
Several teams around the world have independently found a signal that points to giant, long-wavelength gravitational waves travelling across the galaxy in the timing of blinking stars called pulsars. These gravitational waves have yet to be fully detected, but there's more than a 99 per cent chance that what we're looking at is something important.

Teams in Australia, the US, Europe, China and India are simultaneously publishing their results in a series of papers.
"Stephen Taylor, astrophysicist at Vanderbilt University and head of the US team NANOGrav, said at a press briefing: "For the past 15 years we have been on a mission to find a low-pitched gravitational wave hum that reverberates throughout the Universe, bathing our galaxy and measurably bending space-time.

"We are very happy to announce that our hard work has paid off and ... we have exciting evidence for this background of gravitational waves."
Gravitational wave astronomy is a relatively new field, following the detection of space-time fluctuations caused by two colliding black holes in 2015. Since then, our Earth-based gravitational wave detectors have, at the time of writing, detected nearly 100 confirmed gravitational wave events, all caused by the merger of compact stellar-mass objects (black holes and neutron stars).

Gravitational waves are caused by large events in the universe. Think of a collision between black holes as a stone thrown into a lake, and gravitational waves as ripples. The medium is space-time itself, and the waves travelling at the speed of light spread out in all directions, stretching and compressing space-time in a way that we can perceive. If you want to go deeper, we have explained the whole background in more detail here.
Now imagine how many black holes must be colliding in the universe. And how many other massive events must be generating these waves. Space-time must surely be humming with gravitational waves, but there's a problem. Earth is too small to detect them at the longer wavelengths on the nanohertz scale, which can stretch for light years, expected from larger events such as the merger of supermassive black holes at the centres of galaxies.

Fortunately, we live in a galaxy much bigger than Earth. And there is something in our galaxy that emits very precisely timed signals that can be affected by nanohertz gravitational waves: radio pulsars. These are neutron stars that spin extremely fast and blast radio light from their magnetic poles. As they spin, these beams pass by Earth like a cosmic lighthouse, and because the timing of these pulses is so precise, we can use them to detect the way space stretches and compresses as gravitational waves roll in.
A small glitch in timing is not enough. But if you have enough pulsars with correlated glitches over a long enough time period, you can compile evidence of a large gravitational wave. That's what different teams around the world did, analysing a total of 115 pulsars over 18 years for the Parkes Pulsar Timing Array in Australia.

"The pulsar timing array is a galactic-scale gravitational wave detector. We detected a common 'rumble' among the pulsars in our array - a signal at ultra-low frequencies."
"Together with our international colleagues, we also see a hint of the fingerprint that identifies this rumble as originating from gravitational waves."
In January 2021, NANOGrav published a paper detailing what they thought was the first hint of a gravitational wave background in pulsar timing data. In January 2022, the International Pulsar Timing Array did the same with its own pulsar set.

Now, after painstaking work to determine that the signal is not generated by pulsars or other noise in the data, the researchers have concluded that the signal is significant.
NANOGrav's signal has a 4 sigma, or 99.349 per cent confidence level among 67 pulsars. PPTA's signal has a lower confidence level because it studied fewer pulsars; its detections are based on only 30 stars, but over a longer period of time. The gold standard for a discovery is 5 sigma. So there is still a lot of work to be done.

"This is not yet a gravitational wave detection," says Reardon. "To confirm that this is a gravitational wave detection, this fingerprint will need to become clearer, for example by using more data. But this is still very exciting because the fingerprint was expected to slowly emerge in our data sets, as in the evidence observed by the collection of pulsar timing sequence collaborations."
Since there has not yet been a confirmed detection of gravitational waves, the researchers cannot say for sure what caused it. The most obvious answer is supermassive black holes. Supermassive black hole mergers should occur at a rate that fills the Universe with gravitational wave noise, like the noise of the sea.

This is not the only potential source of the gravitational wave background. Cosmic strings, phase shifts in the Universe, the rapid inflation of space following the Big Bang - all of these could produce low-frequency gravitational waves (the Big Bang could have produced them too, but the wavelength would be the size of the Universe - for which we certainly don't have a detector big enough).
What we are probably looking at now is the supermassive black hole background.
"We know that every massive galaxy has a supermassive black hole at its core. We also know that galaxies collide, and when they collide we expect supermassive black holes to collapse into the centre and start orbiting each other, emitting gravitational waves," explains Reardon.

"The further out in the Universe we look, the more of these supermassive black hole systems we can see. A very large population of supermassive black holes orbiting in the distant Universe creates a random ocean of gravitational waves that bathe the Earth and pulsars in our galaxy."
Pulsar timing sequence astronomy is a long game, but we are very close to the confirmed signal. Separate pulsar timing arrays around the world have now merged their datasets and are working under the IPTA collaboration to validate their findings. This confirmation will come within a year, perhaps two years at most.

This could usher in a bold new era in nanohertz gravitational wave astronomy. Researchers will be able to decompose the signal, study its properties and find the sources of the massive gravitational disturbances that explode in space. From there, we could even begin to study the properties of supermassive black holes in more detail than ever before.
"Reardon tells ScienceAlert: "These pulsar timing sequence studies are the first hints of nanohertz-frequency gravitational waves.

"It's incredible to imagine that the Universe is really a rumbling ocean stretching and compressing space. Supermassive black holes are cosmic giants at the heart of galaxies, feeding on gas and disrupting star formation. I'm excited for a future where our pulsar observations reveal a complex map of gravitational waves rippling from supermassive black hole pairs. We should see the background hum of the universe with precisely identified 'hot spots' of gravitational waves from supermassive black hole pairs in galaxies we can identify."

 

Source: https://www.sciencealert.com/