Based on what we know about gravitational waves, the Universe must be full of them. Every colliding pair of black holes or neutron stars, every nuclear-crashing supernova – even the big bang itself – must have rippled across space-time.
After all this time, these waves would be weak and difficult to find, but it is predicted that they will form a resonant ‘hum’ that permeates our universe, called the gravitational wave background. And we may have just gotten the first hint of it.
You can view the gravitational wave background as something like the sound left behind by massive events in the history of our universe – which is 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 understand our noise thoroughly. This leaves us in a very interesting place, where we can strongly exclude known noise sources, but we can not yet say whether the signal is indeed coming from gravitational waves. We will need more data for that. “
Nevertheless, the scientific community is excited. More than 80 papers citing the research have appeared since the team’s pre-print was posted on arXiv in September last year.
International teams worked hard to analyze data to refute or confirm the team’s results. If the signal turns out to be real, it could open up a new phase of gravity wave astronomy – or reveal completely new astrophysical phenomena to us.
The signal comes from observations of a kind of dead star called a pulsar. These are neutron stars that are so oriented that they radiate radio waves from their poles as they rotate at millisecond velocities comparable to a kitchen mixer.
These flashes are incredibly precisely fixed, meaning that pulsars are possibly the most useful stars in the universe. Variations in their timing can be used for navigation, to investigate the interstellar medium, and to study gravity. And since the discovery of gravitational waves, astronomers have been using them to search for them.
This is because gravitational waves distort space-time as they orbit, which theoretically has to change the timing of the radio pulses given by pulses.
“The [gravitational wave] background stretches and shrinks the space-time between the pulsars and earth, causing the signals from the pulsars to arrive a little later (stretch) or earlier (shrink) than would otherwise occur if there were no gravitational waves, “said 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 size will not necessarily mean much. But if a whole bunch of pulses show a correlated pattern of timing, it could be proof of the background of gravity.
Such a collection of pulsars is known as a pulsar timing, and this is what the NANOGrav team observed – 45 of the most stable millisecond pulsars in the Milky Way.
They have not yet fully detected the signal confirming the gravity wave background.
But they did detect something – a “common noise” signal that, according to Shannon, varies from pulsar to pulsar, but each time exhibits similar characteristics. These deviations led to variations of several hundred nanoseconds during the 13-year course of the observation run, Simon noted.
There are other things that can produce this signal. For example, a pulsating time series must be analyzed from a frame of reference that does not accelerate, which means that any data in the center of the solar system, known as the barycentres, must be transmitted instead of the earth.
If the barycentres are not calculated accurately – a more troublesome thing than it sounds, because it is the center 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 would calculate the Solar System Barycentres within 100 meters (328 feet).
There is still a chance that this discrepancy may be the source of the signal they found, and that more work needs to be done to work it out.
Because if the signal is really coming from a resonant gravitational wave, it will be a big deal, since the source of these gravitational waves is probably supermassive black holes (SMBHs).
Because gravitational waves show us phenomena that we cannot detect electromagnetically – such as collisions in black holes – it can help solve the final parsec problem, which means that supermassive black holes may not be able to merge, and it helps us to better understand galactic evolution and growth.
Furthermore, we can even detect the gravitational waves produced after the big bang, which gives us a unique window into the early universe.
There is, to be clear, a lot of science to do before we get to that point.
“This is a possible first step toward detecting nanohertz frequency gravitational waves,” Shannon said. “I want to warn the public and scientists not to interpret the results. Over the next year or two, I think there will be evidence about the nature of the signal.”
Other teams are also working on using pulsar timing devices to detect gravitational waves. OzGrav is part of the Parkes Pulsar Timing Array, which will soon release the analysis of its 14-year-old datasets. The European Pulsar Timing Array is also working hard. The result of NANOGrav only increases the excitement and anticipation 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 things for me are the next steps,” Simon told ScienceAlert.
“While we have to go even further to arrive at a definitive trace, this is only the first step. In addition, we have the opportunity to determine the source of the GWB, and then we can discover what they can tell us about the universe. . “
The team’s research was published in The astrophysical journal letters.