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 collapsing supernova – even the Big Bang itself – should have been ringing ripples through spacetime.
After all this time, these waves would be faint and difficult to find, but they are all predicted to form a resonant ‘hum’ pervading our universe, also known as the gravitational wave background. And maybe we just got the first hint of it.
You can think of the background of the gravitational waves as something like the sound left behind by massive events in the history of our universe – possibly invaluable to our understanding of the cosmos, but incredibly difficult to detect.
“It’s incredibly exciting to see such a strong signal coming out of the data,” said astrophysicist Joseph Simon of the University of Colorado Boulder and the NANOGrav collaboration.
“However, because the gravitational wave signal we are looking 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 can’t say yet whether signal does indeed come from gravitational waves, so we need more data for that. ‘
Nevertheless, the scientific community is excited. More than 80 articles have come out citing the research since the team’s preprint was posted on arXiv last September.
International teams have worked hard to analyze data to try to disprove or confirm the team’s results. If the signal turns out to be real, it could open up a whole new phase of gravitational wave astronomy – or reveal entirely new astrophysical phenomena to us.
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 a speed of milliseconds that is comparable to a kitchen blender.
These flashes are timed with incredible precision, which means that pulsars are possibly the most useful stars in the Universe. Variations in their timing can be used for navigation, to explore the interstellar medium and to study gravity. And since the discovery of gravitational waves, astronomers have been using them to find them too.
That’s because gravitational waves distort spacetime as they ripple through it, which theoretically should change – just a tiny bit – the timing of the radio pulses delivered by pulsars.
“The [gravitational wave] background stretches and shrinks the spacetime between the pulsars and Earth, causing the pulsars’ signals to arrive a little later (stretch) or earlier (shrink) than they would otherwise if there were no gravitational waves, ” says astrophysicist Ryan Shannon of de Swinburne University of Technology and the OzGrav collaboration, which were not involved in the study, explained to ScienceAlert.
A single pulsar with an irregular beat doesn’t necessarily mean much. But if a whole series of pulsars showed a correlated pattern of timing variation, that could be 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 observed – 45 of the most stable millisecond pulsars in the Milky Way.
They haven’t fully detected the signal that would confirm the background of the gravitational waves.
But they detected something – a “common noise” signal that, Shannon explained, varies from pulsar to pulsar, but exhibits similar characteristics each time. These deviations resulted in variations of a few hundred nanoseconds over the 13-year course of the observation run, Simon noted.
There are other things that can produce this signal. For example, a pulsar timing array must be analyzed from a frame of reference that is not accelerating, which means that all data must be transposed to the center of the solar system, known as the center of gravity, rather than to Earth.
If the center of gravity is not calculated accurately – a bit trickier than it sounds, given that it is the center of gravity of all moving objects in the solar system – you could be getting a false signal. Last year, the NANOGrav team announced that they had calculated the solar system’s center of gravity to within 100 meters (328 feet).
There’s still a chance that this discrepancy could be the source of the signal they found, and more needs to be done to fix this.
Because if the signal is really from some resonant gravitational wave that’s buzzing, it would be a big deal since 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 – like black hole collisions – this could help solve riddles like the latest parsec problem, which states that supermassive black holes may not be able to merge, and help us better understand galactic evolution and growth.
Ahead, we may even be able to detect the gravitational waves produced just after the Big Bang, giving us a unique look at the early Universe.
To be clear, there’s still a lot of science to be done before we get that far.
“This is a possible first step towards detecting gravitational waves at the nanohertz frequency,” said Shannon. “I would caution the public and scientists not to overinterpret the results. I think evidence will emerge in the next two years about the nature of the signal.”
Other teams are also working on using pulsar timing arrays to detect gravitational waves. OzGrav is part of the Parkes Pulsar Timing Array, which will soon release an analysis of its 14-year-old datasets. The European Pulsar Timing Array is also hard at work. The result of NANOGrav will only increase the excitement and the expectation that there is something to be found.
“It was incredibly exciting to see such a strong signal coming from our data, but the most exciting thing for me is the next steps,” Simon told ScienceAlert.
“While we have to go even further to get to a definitive detection, that’s just the first step. In addition, we have the ability to trace the source of the GWB, and then we can discover what they can tell us about the Universe. “
The team’s research is published in The Astrophysical Journal Letters.