Based on what we know about gravitational waves, the Universe should be full of them. Every pair of colliding black holes or neutron stars, every collapsed supernova at the core – even the Big Bang itself – should have sent waves that sounded in space-time.
After all this time, these waves would be weak and hard to find, but they are all predicted to form a resonant “hum” that enters our universe, called the background of the gravitational wave. And maybe I just got the first clue.
You can think of the bottom of the gravitational wave as a sound left behind by massive events throughout the history of our universe – invaluable potential for 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 NANOGrav collaboration.
“However, because the gravitational wave signal we are looking for extends throughout our observations, we need to understand our noise carefully. This leaves us in a very interesting place, where we can strongly rule out some sources of noise. “But we still don’t know if the signal really comes from gravitational waves. We’ll need more data for that.”
However, the scientific community is delighted. More than 80 papers citing the research have appeared since the team’s prepress was posted on arXiv in September last year.
International teams worked hard, analyzing data to try to reject or confirm the team’s results. If the signal turns out to be real, it could open a whole new stage of gravitational wave astronomy – or reveal completely new astrophysical phenomena.
The signal comes from the observations of a type of dead star called a pulsar. These are neutron stars that are oriented in such a way as to flash beams of radio waves from their poles as they rotate at millisecond speeds comparable to a kitchen blender.
These flashes are incredibly accurate timed, which means that pulsars are probably the most useful stars in the Universe. Variations during them can be used for navigation, for probing the interstellar environment and for studying gravity. And since the discovery of gravitational waves, astronomers have used them to search for them.
This is due to the fact that gravitational waves deform space-time as they unfold, which theoretically should change – only very easily – the moment of the radio pulses given by the pulsars.
” [gravitational wave] the background stretches and shrinks the space between pulsation and earth, causing the pulsation signals to arrive a little later (stretch) or earlier (shrink) than would otherwise be the case if there were no gravitational waves, “astrophysicist Ryan Shannon Swinburne University of Technology and the OzGrav collaboration, which was not involved in the research, explained ScienceAlert.
A single pulsar with an irregular rhythm would not necessarily mean much. But if a whole bunch of pulsars were to display a correlated pattern of temporal variation, it could be evidence of the background of the gravitational wave.
Such a pulsar collection is known as the pulsar synchronization matrix, and this was observed by the NANOGrav team – 45 of the most stable millisecond pulsars in the Milky Way.
They did not detect well the signal that would confirm the background of the gravitational wave.
But they detected something – a “common noise” signal that, Shannon explained, varies from pulsar to pulsar, but displays similar characteristics each time. These deviations led to variations of several hundred nanoseconds over 13 years, Simon observes.
There are other things that could produce this signal. For example, a pulsar synchronization matrix must be analyzed from a non-accelerating frame of reference, which means that any data must be transposed to the center of the solar system, known as the center of gravity, rather than the Earth.
If the center of gravity is not calculated exactly – which is more complicated than it seems, because 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 calculated the center of gravity of the solar system at a distance of 100 meters (328 feet).
There is still a chance that this discrepancy will be the source of the signal they found and more work is needed to resolve this.
Because if the signal really comes from a hum of a resonant gravitational wave, it would be a huge deal, because the source of these background gravitational waves is probably supermassive black holes (SMBH).
Since gravitational waves show us phenomena that we cannot detect electromagnetically – such as black hole collisions – this could help solve puzzles such as the last parsec problem, which states that supermassive black holes may not be able to fuse and will helps us better understand galactic evolution and growth.
Furthermore, we can even detect gravitational waves produced immediately after the Big Bang, giving us a unique window to the early Universe.
There is, to be clear, a lot of science to be done before you get to this point.
“This is a possible first step toward detecting gravitational waves with nanohertz frequency,” Shannon said. “I would warn the public and scientists not to over-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 using pulsar synchronization matrices to detect gravitational waves. OzGrav is part of the Parkes Pulsar Timing Array, which will soon launch the analysis of its 14-year-old datasets. The European Pulsar synchronization system is also difficult to work with. The result of NANOGrav will only increase the excitement and anticipation that there is something to be found.
“It was incredibly interesting to see such a strong signal coming out of our data, but the most interesting things for me are the next steps,” Simon told ScienceAlert.
“Although we still have a long way to go to reach a definitive detection, this is only the first step. Beyond that, we have the opportunity to identify the source of GWB, and beyond that, we get to find out what the Universe can tell us. “
The team ‘s research was published in The Astrophysical Journal Letters.