An artist’s interpretation shows an array of pulsars being affected by gravitational ripples produced by a supermassive black hole binary in a distant galaxy.NANOGrav/Sonoma State University/Aurore Simonnet
Somewhere, far away, the universe is humming, and for the first time, scientists are picking up the tune.
But that hum is not audible; rather, it consists of unimaginably long waves of gravitational energy that alternately stretch and squeeze space as they propagate in all directions. Their suspected source: hundreds of thousands of supermassive black holes swinging around each other like vigorous couples shaking the floor at a cosmic barn dance.
Such is the world of low frequency gravitational wave astronomy – a search for undulations so vast that, even thought they are coming at us at the speed of light, any single wave might take a decade or more to crest as it passes by Earth. The change is so subtle that scientists must employ special techniques just to demonstrate that the waves are there at all.
“These truly are among the lowest notes in the cosmic symphony. It’s an amazing feat to have found evidence for signals like this,” said Ingrid Stairs, an astronomer at the University of British Columbia and member of the NANOGrav collaboration, a North America-wide effort to search for the elusive low frequency waves.
The project’s latest measurements were published Thursday in the Astrophysical Journal Letters, in co-ordination with teams in Europe, India, Australia and China that have been independently looking for the same signal. All the findings are consistent with the existence of low frequency gravitational waves, though NANOGrav team members said the data are close but not yet at the 3.5 million-to-one-certainty level considered the gold standard for reporting new discoveries in physics.
“We’re not saying the word ‘detection,’ ” Dr. Stairs said.
If their interpretation is correct, researchers have used gravity to open up a new window into the unseen depths of the universe and shed light on the formation and evolution of the heaviest objects we know – or it could mean the discovery of something entirely new and unexpected.
Distant black hole caught in the act of annihilating a star
Long predicted by Einstein’s theory of general relativity, gravitational waves are disturbances in space that are produced whenever massive objects move very quickly. Their existence was first confirmed in 2015 by the Laser Interferometer Gravitational-Wave Observatory. The U.S.-based facility uses laser light reflecting back and forth between mirrors that are four kilometres apart to pick up slight vibrations that occur when gravitational waves are traversing through the experiment.
Since acquiring the sensitivity to detect the waves, LIGO has recorded many signals that come from colliding black holes a few dozen times the sun’s mass. Those signals appear as short-lived chirps in the data that momentarily jiggle the detector before fading away.
This week, scientists are reporting something quite different: not high-pitched chirps but a deep and continuous drone that permeates all of space. Such a drone would be expected to arise not from one collision but from the collective motion of many of the largest black holes in the universe – each one carrying the mass of millions of suns. Black holes of such extreme mass are known to form at the centres of distant galaxies. And while they may form separately, two such black holes can find themselves bound together in a tight orbit after their host galaxies merge.
Neither LIGO nor any other detector on Earth is large enough to sense the gravitational waves emanating from such a massive duo. However, by checking Earth’s position relative to other objects in space, astronomers have shown that our planet is acting very much like a cork bobbing around in slow motion exactly as would be expected from the low frequency waves.
“This is really compelling evidence for a background of gravitational waves,” said Steve Taylor, an astronomer at Vanderbilt University in Tennessee and current chair of NANOGrav during a briefing on the find. The collaboration harnessed researchers and facilities across North America to search for the effect.
To conduct its search, the team used radio telescopes in multiple locations to carefully monitor pulsars – compact, rotating objects scattered around our Milky Way galaxy that are left behind when stars exhaust their fuel and explode as supernovas. Some pulsars can spin as much as 1000 times per second, which makes then ideal natural timers because their rotations are so precise and consistent. As Earth is buffeted by gravitational waves, the planet’s back and forth motion can be spotted by comparing pulsars in different directions and checking for slight discrepancies in timing. The hitch is that it takes years of measurements to see the gradual change caused by passing low frequency gravitational waves.
Achieving that result reliably “has taken a small army of people to do everything right,” Dr. Stairs said.
Other teams used the same pulsar-timing approach to arrive at comparable results. And while the existence of low frequency gravitational waves has long been suspected, the details include a few puzzles. For example, if the background hum is produced solely by close pairs of supermassive black holes circling each other, then the pairs are more common and somewhat more massive on average than standard theories predict.
This had some researchers this week pointing to even more exotic possibilities for explaining the cause of the gravitational waves, including cosmic strings: hypothetical defects in spacetime that some theories predict could have formed during the Big Bang.
Luis Lehner, a researcher at the Perimeter Institute for Theoretical Physics in Waterloo, Ont., who was not part of the collaboration, said that theorists may have difficulty explaining how pairs of supermassive can get close enough to each other often enough to match what observers are now finding in their data.
“They take too long to merge, but we’re not seeing that,” he said. “Somehow they get together … it’s nature reminding us that it’s always going to be smarter than we are.”