Gravitational waves are ripples in the fabric of space, and were predicted by Albert Einstein as early as 1916. This was shortly after he formulated his famous theory of general relativity, which describes gravity as a deformation of the spacetime geometry.

According to Einstein’s theory, these waves are generated by rapidly moving massive bodies generated in a similar way that accelerated charges radiate electromagnetic waves. However, in most cases, gravitational waves are very weak and hard to detect, and it took scientists almost a century to develop detectors capable of registering them.

“In 2015, scientists used an experiment called LIGO to detect gravitational waves for the first time and showed Einstein was right. But so far, those methods have only been able to catch waves at high frequencies,” explained Chiara Mingarelli, an astrophysicist at Yale University and a member of NANOGrav experiment. “Those quick ‘chirps’ come from specific moments when relatively small black holes and dead stars crash into each other.”

The frequency of gravitational waves that LIGO and its European and Japanese counterparts can observe ranges from a few hertz to a few kilohertz, sufficient enough to detect waves emitted by relatively small black holes and neutron stars with masses only a few times greater than the mass of our Sun. However, there are much larger objects that exist in the Universe that can collide and emit gravitational waves.

“Galaxies across the Universe are constantly colliding and merging together,” explained Szabolcs Marka, an astrophysicist at Columbia University. “As this happens, scientists believe the enormous black holes at the centers of these galaxies also come together and get locked into a dance before they finally collapse into each other.”

These gargantuan black holes can be millions, even billions, of times larger than the objects whose mergers have been recorded by LIGO and other detectors, resulting in gravitational waves that are also billions of times larger.

A galaxy-sized detector

Wavelength and frequency are inversely proportional, therefore, a massive gravitational wavelength will have a tiny frequency, in the case of gravitational waves emitted by supermassive black holes have nanohertz frequencies. Detecting this low-frequency radiation with such huge wavelengths, often reaching many trillions of kilometers in length, is not possible with detectors like LIGO.

“We had to build a detector that was roughly the size of the galaxy,” said NANOGrav researcher Michael Lam of the SETI Institute in a press release. This “detector” consisted of 67 pulsars, which the NANOGrav collaboration observed over the course of 15 years using the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, and the Very Large Array in New Mexico.

These pulsars are rapidly rotating neutron stars with very strong magnetic fields, and which emit powerful radio waves from their poles. Astronomers can observe this radiation only when the stars’ poles point toward the Earth, giving their radiation a characteristic pulsing.

“[Any pulsar is] like a perfectly regular clock ticking away far out in space,” said NANOGrav member Sarah Vigeland, an astrophysicist at the University of Wisconsin-Milwaukee. “But as gravitational waves warp the fabric of spacetime, they actually change the distance between Earth and these pulsars, throwing off that steady beat.”

In their recent study published in The Astrophysical Journal Letters, the NANOgrav team compared the observed radio signals from the 67 pulsars and concluded that the entire cosmos is actually filled with gravitational waves in the nanohertz range, probably emitted by merging supermassive black holes.

“It’s really the first time that we have evidence of just this large-scale motion of everything in the Universe,” said Maura McLaughlin, co-director of NANOGrav.

This discovery will hopefully allow scientists to better understand the history of the entire Universe, as the rate of formation and the number of supermassive black holes strongly depends on how they interacted with each other, all other forms of matter, and energy at different stages of the evolution of the Universe.

“The background noise [we] found is ‘louder’ than some scientists expected,” Mingarelli said. “This could mean that there are more or bigger black hole mergers happening out in space than we thought — or point to other sources of gravitational waves that could challenge our understanding of the Universe.”

Improving accuracy

Despite the long duration of the experiment and the large number of pulsars observed by the researchers, the measurements are still not very accurate as the amount of spacetime deformation caused by very distant black hole mergers is very small and difficult to detect. To increase accuracy, the scientists plan to continue observing the pulsars, as well as combine their results with those of scientific teams from Australia, China, Europe and India, who have also observed pulsars with other telescopes.

“It could open new doors to ‘cosmic archaeology’ that can track the history of black holes and galaxies merging all around us,” Marka said.

An interesting feature of these findings is that the gravitational waves may have another origin; that is, they may not be radiated by merging supermassive black holes. One of the hypotheses is that they might have been generated during a period of rapid expansion in the Universe called cosmological inflation, which, according to one popular theory about the evolution of the Universe, should have taken place in the first moments of its existence.

In any case, wherever these gravitational waves come from, future study promises to significantly enrich our understanding of the laws of nature at the most fundamental level. “We’re starting to open up this new window on the Universe,” concluded Vigeland.

Reference: Gabriella Agazie et al, The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background, The Astrophysical Journal Letters (2023), DOI: 10.3847/2041-8213/acdac6

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