Based on what we know about gravitational waves, the Universe should be full of them. Every colliding pair of black holes or neutron stars, every core-collapse supernova – even the Big Bang itself – should have sent ripples ringing across spacetime.
After all this time, these waves would be weak and hard to find, but they’re all predicted to make up a resonant ‘hum’ that permeates our Universe, referred to as the gravitational wave background. And we may have just caught the first hint of it.
You can think of the gravitational wave background as something like the ringing left behind by massive events throughout our Universe’s history – potentially invaluable to our understanding of the cosmos but incredibly difficult to detect.
“It is incredibly exciting to see such a strong signal emerge from the data,” said astrophysicist Joseph Simon of the University of Colorado Boulder and the NANOGrav collaboration.
“However, because the gravitational-wave signal we are searching for spans the entire duration of our observations, we need to carefully understand our noise. This leaves us in a very interesting place, where we can strongly rule out some known noise sources, but we cannot yet say whether the signal is indeed from gravitational waves. For that, we will need more data.”
Nevertheless, the scientific community is excited. More than 80 papers citing the research have appeared since the team’s preprint was posted to arXiv in September of last year.
International teams have been working hard, analysing data to try to refute or confirm the team’s results. If it turns out that the signal is real, it could open up a whole new stage of gravitational wave astronomy – or reveal to us entirely new astrophysical phenomena.
The signal comes from observations of a type of dead star called a pulsar. These are neutron stars that are oriented in such a way that they flash beams of radio waves from their poles as they rotate at millisecond speeds comparable to a kitchen blender.
These flashes are incredibly precisely timed, which means that pulsars are possibly the most useful stars in the Universe. Variations in their timing can be utilised for navigation, for probing the interstellar medium and studying gravity. And, since the discovery of gravitational waves, astronomers have been using them to look for those, too.
That’s because gravitational waves warp spacetime as they ripple through, which theoretically should change – just very slightly – the timing of the radio pulses given out by pulsars.
“The [gravitational wave] background stretches and shrinks space time between the pulsars and earth, causing the signals from the pulsars to arrive a bit later (stretch) or earlier (shrink) than would otherwise happen if there were no gravitational waves,” astrophysicist Ryan Shannon of Swinburne University of Technology and the OzGrav collaboration, who was not involved in the research, explained to ScienceAlert.
A single pulsar with an irregular beat would not necessarily mean much. But if a whole bunch of pulsars displayed a correlated pattern of timing variation, that could constitute evidence of the gravitational wave background.
Such a collection of pulsars is known as a pulsar timing array, and this is what the NANOGrav team has been observing – 45 of the most stable millisecond pulsars in the Milky Way.
They haven’t quite detected the signal that would confirm the gravitational wave background.
But they have detected something – a “common noise” signal that, Shannon explained, varies from pulsar to pulsar, but displays similar characteristics each time. These deviations resulted in variations of a few hundred nanoseconds over the 13-year course of the observing run, Simon noted.
There are other things that could produce this signal. For example, a pulsar timing array needs to be analysed from a frame of reference that isn’t accelerating, which means that any data needs to be transposed into the centre of the Solar System, known as the barycentre, rather than Earth.
If the barycentre isn’t calculated accurately – a trickier thing than it sounds, since it’s the centre of mass of all the moving objects in the Solar System - then you could get a false signal. Last year, the NANOGrav team announced that they’d calculated the Solar System barycentre to within 100 metres (328 feet).
There’s still a chance that this discrepancy could be the source of the signal they’ve found, and more work needs to be done to work this out.
Because if the signal really is from some resonant gravitational wave humming, it would be a huge deal, as the source of these background gravitational waves is likely supermassive black holes (SMBHs).
Since gravitational waves show us the phenomena we can’t detect electromagnetically – such as black hole collisions – this could help resolve such conundrums as the final parsec problem, which poses that supermassive black holes might not be able to merge, and help us better understand galactic evolution and growth.
Further down the road, we may even be able to detect the gravitational waves produced just after the Big Bang, giving us a unique window into the early Universe.
There is, to be clear, a lot of science to be done before we get to that point.
“This is a possible first step towards nanohertz frequency gravitational wave detection,” Shannon said. “I would caution the public and scientists to not overinterpret the results. Over the next year or two I think evidence will emerge as to the nature of the signal.”
Other teams, too, are working on using pulsar timing arrays to detect gravitational waves. OzGrav is part of the Parkes Pulsar Timing Array, which will be releasing analysis of its 14-year datasets soon. The European Pulsar Timing Array is also hard at work. NANOGrav’s result will only increase excitement and anticipation that there’s something there to find.
“It’s been incredibly exciting to see such a strong signal emerge from our data, but the most exciting things for me are the next steps,” Simon told ScienceAlert.
“While we still have further to go to get to a definitive detection, that is only the first step. Beyond that we have the opportunity to pinpoint the source of the GWB, and beyond that, we get to discover what they can tell us about the Universe.”
The team’s research has been published in The Astrophysical Journal Letters.