In 2015, scientists snagged the first detection of a gravitational wave, a ripple in the fabric of spacetime. The achievement marked the beginning of an entirely new field of astronomy and earned the 2017 Nobel Prize in Physics. Now, emerging research suggests that we may be on the cusp of yet another major milestone for gravitational wave astronomy: the detection of the so-called “gravitational wave background.”
The discovery of gravitational waves continues to be one of the most consequential breakthroughs in science because it allows researchers to examine cataclysmic events, such as the mergers of black holes, that could never be spotted with traditional light-based astronomy.
Detectors like Laser Interferometer Gravitational-Wave Observatory (LIGO), which captured the first gravitational wave over five years ago, are built to sense relatively loud, high-frequency waves. But scientists predict that there is also an ambient murmur of subtle, low-frequency ripples constantly flowing through everything in the universe, including Earth. Now, researchers think they’ve found a candidate signal after more than a decade of watching fast-spinning collapsed stars for the faintest sign of a discrepancy that might indicate a wave.
This background wave source, if discovered, would be an absolute goldmine of information about some of the most persistent mysteries of the universe. For instance, the stochastic waves could shed light on the enigmatic behavior of supermassive black holes, which can be billions of times more massive than the Sun and may produce wave events that last months or years. (By comparison, LIGO and similar detectors are focused on waves emitted by seconds-long interactions between star-scale objects.)
“These are the black holes that are at the centers of every massive galaxy that we know of,” said Joseph Simon, an astrophysicist at the University of Colorado Boulder, in a call.
“If we were to be able to detect this signal, we would actually be able to open a completely different window into the universe than what LIGO is able to probe,” he continued, “and we will be able to learn more about the way that these supermassive black holes grow and evolve through throughout kind of cosmic time with their host galaxies.”
To spot signs of this elusive background, Simon and his colleagues at the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) have used the universe itself as an observatory of sorts. The team has been observing pulsars, which are rapidly-rotating dead stars that emit clockwork blasts of light, for nearly 13 years.
NANOGrav has monitored dozens of pulsars all across our galaxy, the Milky Way, to spot hints of distortion as background waves pass through them. In this way, the project is essentially utilizing a natural, galaxy-sized observatory to hunt for tiny light fluctuations that might expose the gravitational wave background.
As a lead member of NANOGrav, Simon presented the latest findings from this search on Monday at the 237th meeting of the American Astronomical Society, which is being held virtually due to the coronavirus pandemic. The results were also published in The Astrophysical Journal Letters last month.
While the team has not yet confirmed an unambiguous detection of the gravitational wave background, its initial results reveal a signal that may point to this untapped source of cosmic data. To be totally sure that they are looking at background waves, the researchers are hoping to either expand their survey to encompass more pulsars, or spend up to five more years studying the 45 pulsars from their 12.5 year survey (or both).
With those observations in hand, “our simulations show that we should be able to robustly determine whether this signal is the gravitational-wave background,” said Maura McLaughlin, an astrophysicist at West Virginia University and a lead member of NANOGrav, in an email.
NANOGrav’s initial results are tantalizing, but the real fun is still to come—assuming the team is able to eventually conclude that the observed light distortions are caused by the gravitational wave background. These low-frequency waves may finally reveal whether, and how, supermassive black holes merge together when their host galaxies collide.
“There is a lot of evidence for hierarchical galaxy growth over cosmic time, whereby galaxies grow larger and more structured through mergers,” McLaughlin said. “However, there are many unanswered questions about this merger process. How many galaxies are the product of a merger? What are the roles of astrophysical processes such as stellar scattering and accretion in the merger process?”
“By measuring the amplitude and spectrum of the gravitational wave background,” she concluded, “we can place important constraints on these questions.”