Astronomers find first direct evidence of gravitational wave background

It is much louder than previously believed possible and its discovery will alter our understanding of the universe.
Chris Young
An artist's impression of gravitational waves coming from a pair of black holes.
An artist's impression of gravitational waves coming from a pair of black holes.

Aurore Simonnet / NANOGrav Collaboration 

Scientists have heard the "chorus" of gravitational waves emanating throughout the universe for the very first time, and it's louder than they expected, a press statement reveals.

The new discovery was made by scientists using the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).

To reach their findings, they closely observed stars called pulsars that essentially act as cosmic metronomes, allowing scientists to measure a great deal of space phenomena.

Crucially, the new findings constitute the first direct evidence for the gravitational wave background, which only existed in theories until now.

"This is just the beginning"

The gravitational wave background has been theorized since the first detection of a gravitational wave confirming Albert Einstein's theory that they are generated when two massive objects, such as black holes or neutron stars, collide.

Now, the discovery of direct evidence for the gravitational wave background provides a wealth of new insight into the frequency of these types of mergers, as well as a whole host of other cosmological questions.

The newly-detected gravitational waves, or ripples in space-time, are the most powerful ever measured. According to a new paper detailing the findings, they carry roughly a million times the energy of a one-off burst of gravitational waves from a typical black hole or neutron star merger.

In their paper, published in The Astrophysical Journal Letters, the researchers explain that the gravitational waves were likely produced by pairs of supermassive black holes spiraling toward massive collision events.

"It's like a choir, with all these supermassive black hole pairs chiming in at different frequencies," NANOGrav scientist and Yale University professor Chiara Mingarelli, explained in the statement. "This is the first-ever evidence for the gravitational wave background. We've opened a new window of observation on the universe."

"The gravitational wave background is about twice as loud as what I expected," Mingarelli continued. "It's really at the upper end of what our models can create from just supermassive black holes."

The volume may be the result of experimental limitations, though it could also be down to louder and more abundant supermassive black holes than the ones accounted for in the researchers' models. Alternatively, something else that we do not yet understand is generating gravitational waves.

"What's next is everything," Mingarelli said. "This is just the beginning."

How scientists measured the gravitational wave background

The scientists reached their findings using a completely new method. Unlike the Earth-based LIGO and Virgo gravitational wave observatories, which detect high-frequency waves, NANOGrav was designed to measure ultra-low-frequency waves. Some of these waves are so low frequency that a wavelength could be tens of light-years long.

An experiment on Earth wouldn't be able to detect such enormous waves, as these could take decades to pass through a detection facility. Instead, the NANOGrav team closely observed pulsars for many years, which are ultra-dense remnants of large stars that died and went supernova.

Pulsars shoot out beams of radio waves so regularly that they're sometimes referred to as cosmic clocks. For their experiment, the researchers measured the way in which these regular pulses of radio waves were warped by gravitational waves over the course of 15 years.

Astronomers find first direct evidence of gravitational wave background
The Very Large Array in New Mexico.

In total, they analyzed 67 pulsars using the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia, and the Very Large Array in New Mexico.

"Pulsars are actually very faint radio sources, so we require thousands of hours a year on the world’s largest telescopes to carry out this experiment," Maura McLaughlin of West Virginia University, co-director of the NANOGrav Physics Frontiers Center explained.

The next step for the NANOGrav scientists will be to pinpoint and analyze sources of the cosmic background noise. By doing so, they may reveal mechanisms of the universe that were previously unknown to humanity.

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