Based on what we know about gravitational waves, the Universe should be full of them. Every pair of black holes or colliding neutron stars, every collapsing supernova in the nucleus, even the Big Bang itself, should have sent ripples that sounded through space-time.
After all this time, these waves would be weak and hard to find, but it is predicted that they would all form a resonant “buzz” that permeates our Universe, called the gravitational wave background. And we may have just caught the first clue.
You can think of the background of gravitational waves as a sound left by massive events throughout the history of our Universe, potentially invaluable to our understanding of the cosmos, but incredibly difficult to detect.
“It’s 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, since the gravitational wave signal we are looking for covers the entire duration of our observations, we must carefully understand our noise. This leaves us in a very interesting place, where we can firmly rule out some known sources of noise. , but we still can’t say if the signal actually comes from gravitational waves. For that, we’ll need more data. “
Still, the scientific community is excited. More than 80 articles citing the research have appeared since the team’s prepress was published in arXiv in September last year.
International teams have been working hard, analyzing data to try to refute or confirm the team’s results. If it turns out that the signal is real, it could open a new stage in gravitational wave astronomy or reveal completely new astrophysical phenomena to us.
The signal comes from the observations of a type of dead star called a pulsar. These are neutron stars that orient themselves in such a way that they flash beams of radio waves from their poles as they rotate at millisecond speeds comparable to the kitchen mixer.
These flashes have an incredibly accurate timing, which means that pulsars are possibly the most useful stars in the Universe. Variations in time can be used for navigation, to probe the interstellar medium, and to study gravity. And, since the discovery of gravitational waves, astronomers have also used them to search for them.
This is because gravitational waves deform space-time as they grow, which theoretically should change (very slightly) the time of the radio pulses given by the pulsars.
“The file [gravitational wave] the background stretches and reduces the space-time between the pulsars and the earth, causing the pulsar signals to arrive a little later (stretch) or earlier (shrink) than would otherwise happen if there were no gravitational waves, “astrophysicist Ryan Shannon of Swinburne University said. OzGrav technology and collaboration, which did not participate in the research, told ScienceAlert.
A single pulsar with an irregular beat would not necessarily mean much. But if a whole bunch of pulsars showed a correlated pattern of time variation, this could be evidence of the bottom of the gravitational wave.
This collection of pulsars is known as a pulsar synchronization matrix, and this is what the NANOGrav team has been observing: 45 of the most stable millisecond pulsars in the Milky Way.
They have not fully detected the signal that would confirm the background of gravitational waves.
But they have detected something: a signal of “common noise” that, Shannon explained, varies from one pulsar to another, but has similar characteristics each time. These deviations resulted in variations of a few hundred nanoseconds during the 13-year observation race, Simon noted.
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, instead of the Earth.
If the center of gravity is not calculated accurately, which is more complicated than it seems, as it is the center of mass of all moving objects in the solar system, you could get a false signal. Last year, the NANOGrav team announced that it had calculated the barycenter of the solar system at less than 100 meters (328 feet).
There is still the possibility that this discrepancy is the source of the signal they have found and more work needs to be done to fix it.
Because if the signal actually comes from some resonant gravitational wave orange, it would be a big problem, since 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 final parsec problem, which suggests that supermassive black holes may not be able to merge and would help us better understand galactic evolution and growth.
Further down, we can even detect gravitational waves produced just after the Big Bang, giving us a unique window into the primitive universe.
To be clear, there is a lot of science to do before you get to this point.
“This is a possible first step toward detecting gravitational waves of nanohertz frequency,” Shannon said. “I would warn the public and scientists not to over-interpret the results. Over the next year or two I think evidence will emerge about the nature of the signal.”
Other teams are also working on the use of pulsating time matrices to detect gravitational waves. OzGrav is part of the Parkes Pulsar Timing Array, which will soon publish the analysis of its 14-year-old datasets. The European Pulsar Timing Array also works great. The result of NANOGrav will only increase the excitement and anticipation that there is something 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 more to come to a definitive detection, this is only the first step. Beyond that, we have the opportunity to identify the source of the GWB and, beyond that, find out what we they can say the Universe. “
The team’s research has been published in The Astrophysical Journal Letters.