Pulsars have helped reveal ripples in space-time throughout the universe
NANOGrav
According to astronomers who have detected a hum of gravitational waves in the background, the fabric of the universe is constantly rippling. These waves could be produced by supermassive black holes merging in the Universe, but they could also have more exotic origins, such as leftover ripples in space-time created shortly after the Big Bang. By pinpointing their true nature, we can learn more about how supermassive black holes grow and affect their host galaxies, or even how the universe evolved in its early moments.
To find this mysterious hum, astronomers tracked rapidly rotating neutron stars called pulsars that emit light with extreme regularity. By looking at different pulsars in the Milky Way, astronomers can effectively use them as a galaxy-sized gravitational wave detector, called a pulsar timing array.
Although individual gravitational waves, which are ripples in space-time caused by collisions of massive objects, have been observed regularly since the first detection in 2015, the object of this search is different. Those earlier gravitational waves all have localized origins, rising and falling hundreds of times per second, but the newly discovered signal is more like a backdrop of gravitational waves that would permeate the entire universe at much lower frequencies, similar to the concept of the cosmic microwave. background, which is radiation left behind by the big bang and can be seen all over the universe today.
In 2021, there was first evidence that the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), a US-based collaboration that began in 2007 and uses a pulsar timing array, had detected this gravitational wave background using radio telescopes.
By measuring the light signals from pulsars as they arrive at Earth and checking for small time fluctuations that may have been caused by ripples in space-time, astronomers thought they had found signs of a common process that affects the timing of all pulsars in the same way. influenced. . At the time, however, they lacked a telltale signature, predicted by Albert Einstein’s general theory of relativity, that would confirm this buzz on a cosmic scale.
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NANOGrav used the Green Bank Telescope in West Virginia
NANOGrav
Now, after a total of 15 years of observations, the NANOGrav team has seen this signature in the signal for the first time, across a range of different gravitational wave frequencies. “It’s gone from a tantalizing hint to something that’s very strong evidence for the background of gravitational waves,” says team member James McKee at the University of Hull, UK.
This isn’t past the statistical threshold that would require scientists to call it a definitive detection of the gravitational wave background, but astronomers feel free to call it very strong evidence, at a 3-sigma level of statistical significance, meaning the probability of such a signal . emergence in the absence of the gravitational wave background are about 1 in 1000.
Three other pulsar timing array (PTA) collaborations, comprising Europe and India (EPTA), China (CPTA) and Australia (PPTA), also released their results today. The CPTA claims to have found the gravitational wave background with an even higher confidence level than NANOGrav, but for only one frequency, while both EPTA and PPTA see hints of it at a slightly weaker statistical level.
“They’re starting to see this very characteristic correlation signal in their data as well,” says NANOGrav team member Megan DeCesar at George Mason University in Virginia. “We kind of see it all, which is very exciting because that suggests it’s probably real.”
Huge scale
But confirming these signals and gaining more confidence in them isn’t easy, he says Aris Karastergiou at the University of Oxford. “It’s on a massive scale, with incredibly difficult data to work with.”
The gravitational wave background is minuscule — the strength of the signal that astronomers must extract compared to the noise picked up at the same time is equivalent to one part in a quadrillion, while the gravitational waves themselves span a light year — more than 9 trillion kilometers — across one wavelength. Therefore, pulsars, which are suitably spaced and are some of the most sensitive clocks in the universe, are key to this quest. If a constant background of gravitational waves distorts all of space-time, then it should also affect all of the pulsars’ light pulses in the same way, but measuring this isn’t easy because of the many other factors that can affect the timing of the pulsar. the signals from each pulsar in the array.
“We have to be able to account for all of them and that takes a long time,” says McKee. “It takes many years of observations, it takes a lot of understanding of the noise properties of spin anomalies, the interstellar medium, things like that.”
It is only now that pulsar timing array teams are confident enough in their data to recognize the distinctive pattern within the signal predicted by general relativity. As astronomers track pairs of pulsars across the sky, the timing differences in the light from them should broadly become less similar as the angle between them increases. This is because light from pulsars appearing close in the sky will have taken a similar route to Earth, meaning it will take a similar path through the background of the gravitational wave, while light from those appearing further apart will take different paths. to take.
Thanks to a quirk of general relativity, this relationship actually reverses for pulsars that are very far apart, with the timing differences more similar when you compare pulsars on opposite sides of the sky. This entire pattern can be described using a graph called the Slope-Downs curve, and it is this pattern that NANOGrav missed in 2021.
“They couldn’t characterize it specifically and say, yes, it’s gravitational waves,” says Carlo Contaldi at Imperial College London. “But now that they’ve measured this Slope-Downs curve, that’s really just a smoking gun.”
Competitive Explanation
So, assuming the signal persists as astronomers collect more data, what’s causing the gravitational wave background?
The main explanation involves pairs of merging supermassive black holes (SMBH), the giant black holes at the center of many galaxies with masses millions of times greater than that of the sun. Once these objects are locked into orbit around each other, as so-called binary stars, their extreme masses should bend space-time in the same frequency range that the pulsar timing arrays seem to be measuring for the gravitational wave background. Since these events happen throughout the universe, both in time and space, the waves they produce should unite to create a distinctive hum that permeates the cosmos.
“It is inevitable that those [pairs of] supermassive black holes will eventually coalesce to form binary stars,” says team member Laura Blecha at the University of Florida. “It’s just a matter of the time scale at which they would get close enough to produce these gravitational waves that NANOGrav and other pulsar timing arrays could observe.”
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The Effelsberg radio telescope in Germany is part of the EPTACopyright:
Tacking/MPIfR
While this explanation makes the most sense, when Blecha and her colleagues modeled a gravitational wave background caused by the merger of supermassive black holes in the universe, they found a slightly different signal than NANOGrav’s, suggesting that these cosmic behemoths are either more massive or larger. common in the universe than previously thought. If true, it could change our understanding of both how galaxies form and how the universe is structured on a large scale.
One way to support the explanation of the supermassive black hole would be to see a gravitational wave background signal increasing in strength in a specific part of the sky, possibly caused by a nearby merger. Australia’s PPTA sees hints of this in its analysis, but it’s too early to say.
There is enough uncertainty in the NANOGrav signal that the door is open to alternative explanations, says Nelson Christensen at Carleton College in Minnesota. “We will receive hundreds of papers from theorists in the coming days in which they will present other models.”
One possibility is that the background waves come from defects in the very early universe when it changed phase. The idea is that this has left an imprint in space-time, like the cracks that form when water freezes into ice. Another is that the background is actually long-theorized primordial gravitational waves produced by the rapidly expanding universe shortly after the Big Bang during a period known as cosmic inflation.
Nothing excluded
However, the data is currently not accurate enough to rule out one scenario or the other, he says Pedro Ferreira at the University of Oxford. “The problem with this topic is, yes, it could be any number of new physics types, but you can’t really differentiate between them.”
To solve that, we need more data. Recently built telescopes such as FAST in China and MeerKAT in South Africa, as well as the Square Kilometer Array, the world’s largest telescope under construction in Australia and South Africa, allow us to see the pulsars more frequently and with much greater precision. to measure. Discovering new and more regular pulsars will also help, McKee says.
Combining the datasets of all the different PTAs in a global collaboration also enables a more detailed analysis. There are some pulsars that only the Australian telescopes can see, and vice versa for the European ones. An analysis combining all the results is already underway, DeCesar says, and should be released in the coming years.
“This is a golden age for gravitational waves,” Christensen says. “In about eight years, not only have we detected gravitational waves on the ground, but now we’ve detected them using a completely different method at a completely different frequency — this is just super exciting.”
Subjects:
- cosmology/
- gravity waves