Jacinta Bowler
An international team of researchers has today released simultaneous results about hearing hints of gravitational wave ‘background’ from the merging of supermassive black holes.
Scientists believe this background gravitational waves could provide information all the way back to the very few moments after the Big Bang.
The radio astronomy project was 18 years in the making, and includes instrumental results from the Murriyang, CSIRO Parkes radio telescope in Australia.
“We started in 2004 with the Parkes Murriyang radio telescope observing a set of millisecond pulsars,” Dr George Hobbs, CSIRO astrophysicist and lead for the Parkes Pulsar Timing Array project told Cosmos.
“When we started, gravitational waves were predicted but never seen. More recently, some of the ground-based systems like LIGO have detected gravitational waves, but not the supermassive binary black holes and not the background, which is what we’re looking for.”
The international collaboration is made up of five European radio telescopes, the Green Bank and Arecibo radio telescopes in the US, a Chinese and Indian telescope, and the Parkes telescope. It’s all under the larger umbrella of the International Pulsar Timing Array and all of these telescopes submitted a number of papers.
“[This result is] joining the new era, that LIGO and others have started. We are opening up a whole new frequency range in gravitational wave astronomy,” says Hobbs.
Gravitational waves are ripples in the fabric of spacetime. As objects interact with each other, gravity is affected, but with massive objects like black holes, so is spacetime.
“If you start the masses moving around, then the curved spacetime starts moving around as well, and you get these kind of little wobbles, which are gravitational waves,” Adam Deller, an astrophysicist from Swinburne University of Technology told Cosmos last year.
“People often summarise it as ‘mass tells spacetime how to curve, and curved spacetime tells mass how to move’.”
So far, LIGO – or the Laser Interferometer Gravitational Wave Observatory – has been able to find ground-based evidence of merging black holes, colliding neutron stars and neutron star/black hole mergers.
But these are massive, short-term events. What the new research is looking at is gravitational wave ‘background’, which is much quieter and exists for much longer periods of time. A single rise and fall of one of the waves could take years or even decades to pass Earth.
This gravitational waves background could be as important to astronomy as the cosmic microwave background. That’s because most types of radiation can’t tell us about the Universe before about 400,000 years after the Big Bang. But scientists believe there’s potential in background gravitational waves to provide information all the way back to just the very few moments post-Big Bang.
“We’re now opening a new window into the Universe, provided by these ultra low-frequency gravitational waves,” first author of the Parkes papers and CSIRO researcher Dr Andrew Zic told Cosmos.
“We also found an interesting anomaly in our results, whereby the ‘loudness’ of the gravitational waves we’re measuring seems to be growing with time, and we’re not sure why!”
These large, ancient waves can’t be detected by LIGO, so instead the international team started listening to pulsars, which are sometimes referred to as ‘lighthouse stars’.
Pulsars, or pulsating radio sources, are highly magnetised neutron stars that rotate and release beams of radiation – similar to a lighthouse.
“It’s like an ocean of gravitational waves permeating through the whole of space,” says Hobbs.
“And we’re trying to use these ‘lighthouses in space’ – the pulsars – which bob up and down in these gravitational waves to try and detect those waves.”
These pulsars are helpful for astronomers because they are VERY consistent. Some pulse every few milliseconds, while others take whole seconds, but importantly, because that are so reliable it means that scientists can use them as points of reference.
At radio telescopes in the US, China, Europe and Australia, the researchers have found a result.
“We have all seen a signal – a wobble – in our data sets that the pulse arrival times aren’t coming exactly when we expect them to come,” says Hobbs.
“Sometimes they come in a bit early by the tens to hundreds of nanoseconds. Sometimes they’re arriving a little bit late.”
If the team is right, this could be coming from the gravitational wave background signal – basically the merging of two supermassive black holes.
But Hobbs points out that we’re not absolutely there yet. Currently the results are in 3 or 4 sigma. This in scientific terms is ‘evidence’ but not necessarily proof.
“I’m sure everyone else will be saying, ‘Oh, it’s great. Let’s get the champagne open’. I would be doing that if it was what we’d call a ‘10 sigma result’ – a much more confident result,” he says.
“It’s not that we don’t think it’s exciting, we’ve been trying to do this for 15 years. But we just want to double check everything and be a bit careful.”
The next stage of the research will involve all the different countries pooling their results, which should lead to a stronger sigma, and hopefully Hobbs being able to crack open that champagne.
You can see the papers from the Parkes team here, and here.