For the first time, astronomers have heard the background hum of the universe

(ORDO NEWS) — 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 core-collapsing supernova — even the Big Bang itself — should have caused ripples that ring through spacetime.

Over time, these waves have weakened, and it will be difficult to find them, but they are all predicted as a resonant “rumble” that penetrates our Universe, called the background of gravitational waves. And perhaps we just got the first hint of that.

You can think of the background of gravitational waves as something like the ringing left after massive events throughout the history of our universe – potentially invaluable to our understanding of space, but incredibly difficult to detect.

“It’s incredibly exciting to see such a strong signal,” said astrophysicist Joseph Simon of the University of Colorado at Boulder and a staff member at NANOGrav.

“However, since the gravitational wave signal we are looking for covers the entire duration of our observations, we need to carefully exclude noise. This leaves us in a very interesting place where we can completely rule out some known sources of noise, but we cannot yet tell if the signal is actually coming from gravitational waves. For this we need more data. ”

Nevertheless, the scientific community is delighted. Since the team’s preprint was posted to arXiv last September, there have been over 80 articles citing this study.

International teams have worked hard analyzing the data to try to disprove or confirm the scientists’ findings. If the signal turns out to be real, it could open up an entirely new stage in gravitational wave astronomy – or open up completely new astrophysical phenomena for us.

The signal comes from observations of a dead star called a pulsar. These are neutron stars that are oriented in such a way that they emit radio waves from their poles, spinning at millisecond speeds.

These flares are incredibly accurate in timing, which means pulsars are arguably the most useful stars in the universe. Changes in their pulsations can be used for navigation, for exploring the interstellar medium and studying gravity. And since the discovery of gravitational waves, astronomers have used to search for them.

This is because gravitational waves warp spacetime as they pass through it, which in theory should change – quite a bit – the timing of the radio pulses emitted by pulsars.

“The [gravitational wave] background stretches and contracts in the spacetime between pulsars and the Earth, causing pulsar signals to arrive a little later (stretch) or earlier (collapse) than would have happened if there were no gravitational waves,” – astrophysicist Ryan Shannon from Swinburne University of Technology.

One pulsar with an irregular beat does not necessarily mean much. But if a whole group of pulsars showed a correlated pattern of time variation, this could indicate a background of gravitational waves.

This set of pulsars is known as the Pulsar Temporal Array, and this is what the NANOGrav team observed – 45 of the most stable millisecond pulsars in the Milky Way.

They didn’t quite find a signal to confirm the gravitational wave background.

But they did find something – a “normal noise” signal that Shannon explained varies from pulsar to pulsar, but shows the same characteristics every time. These deviations resulted in variations of several hundred nanoseconds over the 13-year observation period, Simon noted.

There are other things that can trigger this signal. For example, a pulsar synchronization array must be analyzed from a frame of reference that is not accelerating, which means that any data must be transferred to the center of the solar system, known as the barycenter, and not to Earth.

If the barycenter is inaccurately calculated – and this is more difficult than it sounds, since it is the center of mass of all moving objects in the solar system – then you may receive a false signal. Last year, the NANOGrav team announced that they had calculated the solar system’s barycenter to within 100 meters.

There is still a possibility that this discrepancy could be the source of the signal they detected, and more work needs to be done to correct it.

Because if the signal really comes from a buzzing resonant gravitational wave, that would be a huge problem, since supermassive black holes (SMBHs) are likely the source of these background gravitational waves.

Since gravitational waves show us phenomena that we cannot detect electromagnetically, such as black hole collisions, it could help solve mysteries like the latest parsec problem, which suggests supermassive black holes cannot merge, and help us better understand galactic evolution. and growth.

In the future, we may even be able to detect gravitational waves that arose immediately after the Big Bang, which will give us a unique window into the early universe.

To be clear, there is a lot of scientific work to be done before we get to this point.

“This is a possible first step towards detecting nanohertz gravitational waves,” Shannon said. “I would like to warn the public and scientists not to overestimate the results. I think that in the next year or two, there will be evidence of the nature of the signal. ”

Other teams are also working on using pulsar synchronization 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.

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