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 supernova with a collapse of the nucleus – even the Big Bang itself – should send waves to ring in space-time.
After all this time, these waves would be weak and difficult to find, but they are all expected to create a resonant “buzz” that penetrates our universe, called the gravitational background of the wave. And maybe we just caught the first hint of that.
You can think of the background of the gravitational wave as something like the ringing left after massive events throughout the history of our universe ̵
“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 NANOGrav.
“However, since the gravitational signal we are looking for covers the entire duration of our observations, we need to understand our noise carefully. This leaves us in a very interesting place where we can categorically exclude some known sources of noise, but we still can’t say whether the signal is really from gravitational waves. We will need more data for that. “
However, the scientific community is excited. More than 80 articles citing the study surfaced after the team’s prepress was published in arXiv last September.
International teams work hard, analyzing data to try to disprove or confirm the team’s results. If the signal turns out to be real, it can open a whole new stage in the astronomy of gravitational waves – or reveal completely 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 emit radio waves from their poles as they rotate at a speed of milliseconds comparable to a kitchen mixer.
These lightning flashes have an incredibly accurate time, which means that pulsars are probably the most useful stars in the universe. Variations in time can be used for navigation, probing the interstellar medium, and studying gravity. And since the discovery of gravitational waves, astronomers have used them to look for them.
This is because gravitational waves distort space-time as they pulsate, which should theoretically change – quite slightly – the time of the radio pulses given by the pulsars.
“The [gravitational wave] the background stretches and shrinks the space time between the pulsars and the earth, causing the pulsar signals to arrive a little later (stretching) or earlier (shrinking) than it would otherwise be if there were no gravitational waves, “said astrophysicist Ryan Shannon of the University. Swinburne in Technology and Collaboration of OzGrav, which is not involved in the study, explained to ScienceAlert.
A pulsar with an irregular rhythm does not necessarily mean much. But if a whole bunch of pulsars show a correlated pattern of time change, this may be evidence of the background of the gravitational wave.
Such a collection of pulsars is known as a time array of pulsars and this is observed by the NANOGrav team – 45 of the most stable millisecond pulsars in the Milky Way.
They have not fully detected the signal that would confirm the gravitational background of the wave.
But they found something – a signal of “general noise”, which, Shannon explained, varies from pulsar to pulsar, but shows similar characteristics each time. These deviations led to variations of several hundred nanoseconds over the 13-year course of observation, Simon noted.
There are other things that could produce this signal. For example, the array of time pulses must be analyzed by a reference frame that does not accelerate, which means that all data must be transposed at the center of the solar system, known as the barycenter, and not at Earth.
If the barycenter is not calculated exactly – something more complicated than it sounds, as it is the center of mass of all moving objects in the solar system – then you can get a false signal. Last year, the NANOGrav team announced that it had calculated the barycenter of the solar system to within 100 meters (328 feet).
There is still a chance that this discrepancy is the source of the signal they have found, and more needs to be done to resolve this.
Because if the signal really is from some resonant buzzing of a resonant gravitational wave, that would be a huge deal, since the source of these background gravitational waves is probably supermassive black holes (SMBH).
As gravitational waves show us phenomena that we cannot detect electromagnetically – such as black hole collisions – this can help solve such mysteries as the last parsec problem, which puts that supermassive black holes may 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 immediately after the Big Bang, giving us a unique window into the early universe.
To be clear, a lot of science needs to be done before we get to this point.
“This is a possible first step toward detecting a gravitational wave with a nanohertz frequency,” Shannon said. “I would warn the public and scientists not to interpret the results. In the next year or two, I think there will be evidence of the nature of the signal.”
Other teams are also working on the use of pulsar synchronization grids to detect gravitational waves. OzGrav is part of the Parkes Pulsar sync array, which will soon release an analysis of its 14-year data arrays. The European Pulsar Timing Array is also working hard. The result of NANOGrav will only increase the excitement and expectation that there is something to be found there.
“It was incredibly exciting to see such a strong signal coming out of our data, but the most exciting thing for me is the next steps,” Simon told ScienceAlert.
“Although we have yet to reach a final discovery, this is only the first step. Beyond that, we have the opportunity to point to the source of the GWB, and besides, we can find out what the universe can tell us.”
The team’s research was published in Astrophysical Journal Letters.