Gravitational waves produce a background hum across the whole universe

According to astronomers who have discovered gravitational wave swells in the background, the structure of the universe is always rippling. These waves could be produced by supermassive black holes merging throughout the universe, but they could also have more exotic origins, such as leftover ripples in spacetime produced shortly after the Big Bang. Finding out what they are may also tell us how supermassive black holes grow and affect their host galaxies, or how the universe evolved in its first moments. .

To find this mysterious hum, astronomers have tracked fast-spinning neutron stars called pulsars that emit light in a highly regular fashion. Observing various pulsars across the Milky Way galaxy allows astronomers to effectively use them as galaxy-sized gravitational wave detectors called pulsar timing arrays.

Individual gravitational waves, ripples in space-time produced by colliding massive objects, have been observed regularly since their first detection in 2015, but this time the search is different. All gravitational waves to date have local origins, rising and falling hundreds of times per second, but the newly discovered signals resemble background gravitational waves that permeate the entire universe at much lower frequencies. and is conceptually similar to cosmic microwaves. The background is the radiation left by the Big Bang that is seen throughout the Universe today.

In 2021, the North American Nanohertz Gravitational Wave Observatory (NANOGrav), a US-based collaboration that began in 2007 and uses pulsar timing arrays, will be the first to detect this gravitational wave background using radio telescopes. I had a hint.

Astronomers measure the light signals from pulsars as they arrive on Earth and see small time variations that may have been caused by ripples in spacetime, affecting the timing of all pulsars alike. I thought I had discovered signs of a common process. . At the time, however, the telltale signs predicted by Albert Einstein’s general theory of relativity were lacking to support this cosmic hum.

Now, after a total of 15 years of observation, the NANOGrav team is the first to see this signature in the signal across a range of different gravitational wave frequencies. “It provided very strong evidence for the gravitational wave background from an interesting hint,” said a team member. James Mackie at the University of Hull, UK.

While this does not exceed the statistical threshold required for scientists to call it an unambiguous detection of the gravitational wave background, astronomers call it 3 sigma of statistical significance, meaning the probability of such a signal. I have no hesitation in calling it very strong evidence at the level. The probability of it occurring in the absence of a gravitational background is about 1 in 1000.

Three other Pulsar Timing Array (PTA) collaborations comprising Europe and India (EPTA), China (CPTA) and Australia (PPTA) also announced results today. CPTA claims to have discovered the gravitational wave background with even higher confidence than NANOGrav, but only at one frequency, and both EPTA and PPTA see signs of it at a slightly weaker statistical level. there is

“They are also starting to see this very characteristic correlation signal in the data,” says a member of the NANOGrav team. Megan DeCeaser at George Mason University in Virginia. “We’re all kind of watching it. It probably suggests it’s real, so we’re very excited.”

huge scale

But confirming these signals and gaining further confidence in them is not easy, he says. Alice Kalastagiu at Oxford University. “It’s very large and very difficult to work with.”

The background of gravitational waves is very small. The strength of the signal that astronomers need to extract compared to the noise they pick up at the same time is equivalent to a factor of 10 trillion, while the gravitational waves themselves stretch about a light-year. Over 9 trillion kilometers – over 1 wavelength. So pulsars, well-spaced and one of the most sensitive clocks in the universe, are key to this search. If a constant background of gravitational waves were distorting all spacetime, it should affect all pulsar light pulses in the same way, but there are many other factors that can affect pulsar timing. This is not easy to measure due to Signal from each pulser in the array.

“We have to be able to explain them all, and that takes a long time,” McKee says. “This would require years of observations and a lot of time to understand the noise properties such as spin irregularities and the interstellar medium.”

Only now is the Pulsar Timing Array team confident enough in the data to identify the characteristic patterns in the signal predicted by general relativity. As astronomers track pairs of pulsars in the sky, the difference in timing of light from them should become nearly dissimilar as the angle between them increases. This is because light from pulsars that appear close to the sky follow similar paths to Earth, i.e., through the gravitational wave background, while light from pulsars that appear more distant follow different paths. It’s from

Thanks to a peculiarity of general relativity, this relationship is actually reversed for very distant pulsars, making the timing differences more similar when comparing pulsars on opposite sides of the sky. This perfect pattern can be illustrated using a graph called the Herlings Down curve. It’s this pattern that NANOGrav was missing in 2021.

“They couldn’t specifically characterize it and say yes, this is gravitational waves,” he says. Carlo Contaldi at Imperial College London. “But now that they’ve measured this Herlingsdown curve, it’s just the killer ball.”

conflicting description

So, assuming the signal remains even as astronomers collect more data, what causes the gravitational wave background?

The main explanation involves twin merging supermassive black holes (SMBHs), giant black holes at the centers of many galaxies with millions of times the mass of the Sun. When these bodies are locked into orbit around each other as so-called binaries, their extreme masses should bend spacetime in the same frequency range that pulsar timing arrays are supposed to measure the gravitational wave background. Because these events occur throughout the universe, both in time and space, the waves they produce should intertwine to produce a unique cosmic noise.

“It’s inevitable that they will happen.” [pairs of] Supermassive black holes will eventually converge to form a binary star,” team members say. Laura Breca at the University of Florida. “It’s just a question of the timescales that really get close enough to produce gravitational waves that NANOGrav and other pulsar timing arrays can observe.”

This explanation makes the most sense, but when Blecha and colleagues modeled the gravitational wave background caused by the merger of supermassive black holes throughout the universe, they found a slightly different signal from that of NANOGrav. bottom. This suggests that these cosmic behemoths are either larger or larger. It’s more common in space than previously thought. If true, this could change our understanding of both galaxy formation and how the universe is structured on a large scale.

One way to support the supermassive black hole explanation would be to see an increase in the strength of the gravitational wave background signal in certain parts of the sky. This may have been caused by nearby coalescence. Australia’s PPTA finds hints of that in its analysis, but it’s too early to tell.

They say that the NANOGrav signal has enough uncertainty to open the door to alternative explanations. Nelson Christensen at Carleton University, Minnesota. “In the next few days, we will receive hundreds of papers from theorists introducing other models.”

One possibility is that the background wave originated from a phase change defect in the very early universe. The idea is that it leaves a mark in space-time, like the cracks that form when water freezes to ice. Another is that the background actually consists of the long-theorized primordial gravitational waves produced by the rapidly expanding universe just after the Big Bang during a period known as cosmic inflation. .

nothing is excluded

But at this point, the data aren’t accurate enough to rule out either scenario, he said. Pedro Ferreira at Oxford University. “The problem with this topic is that it can indeed be different types of new physics, but we can’t really distinguish between them.”

We need more data to solve it. Recently built telescopes such as FAST in China and MeerKAT in South Africa, as well as the world’s largest telescope Square Kilometer Array under construction in Australia and South Africa, will allow pulsars to be measured more frequently and with greater precision. . Finding new, more regular pulsars will also help, McKee said.

Combining all the different PTA datasets in a global collaboration also allows for more detailed analysis. There are some pulsars that can only be seen with Australian telescopes, and vice versa with European telescopes. A combined analysis of all the results is already underway and should be published in the next few years, Desizer said.

“This is the golden age of gravitational waves,” says Christensen. “Within about eight years, not only did we detect gravitational waves on the ground, but we were able to detect gravitational waves at a completely different frequency and in a completely different way, which is very exciting.”