Astronomers around the world announced Thursday that they have found the first evidence of a long-period gravitational wave that creates a “background hum” throughout the universe.
The breakthrough — achieved by hundreds of scientists using radio telescopes in North America, Europe, China, India and Australia after years of work — has been hailed as a major milestone that opens a new window on the universe.
First predicted by Albert Einstein more than a century ago, gravitational waves are ripples in the fabric of the universe that travel through everything at the speed of light almost unimpeded.
Their existence was not confirmed until 2015, when American and Italian observatories detected the first gravitational waves generated by the collision of two black holes.
These “high-frequency” waves are the result of a single violent event that sends a powerful, short, rippling blast toward Earth.
But for decades, scientists have been searching for low-frequency gravitational waves, which are thought to constantly roll through space like background noise.
Joining forces under the banner of the International Pulsar Timing Array Consortium, scientists working on gravitational-wave detectors on several continents revealed Thursday that they have finally found strong evidence of such background waves.
“We now know that the universe is flooded with gravitational waves,” Michael Keith of the European Pulsar Timing Array told AFP.
An artist’s rendering of a network of pulsars affected by gravitational ripples generated by a supermassive black hole in a distant galaxy. (Aurore Simonnet/NANOGrav collaboration/published via REUTERS)
As gravitational waves travel through space, they very subtly compress and stretch everything they pass through.
To find evidence of this compression and expansion at lower frequencies, astronomers examined pulsars, the dead cores of stars that have exploded into supernovae.
Some rotate hundreds of times per second, flashing beams of radio waves at very regular intervals, like cosmic beacons.
Keith said this means it can serve as a “very accurate watch”.
For the new research, radio telescopes around the world were targeting a total of 115 pulsars across the entire Milky Way galaxy.
The scientists then measured the incredibly small differences in the timing of the pulses, looking for telltale signs of gravitational waves.
French astrophysicist Antoine Petito said they were able to “detect changes of less than a millionth of a second over more than 20 years.”
Maura McLaughlin of the US Pulsar Search Collaboratory said she was “astonished” after seeing evidence of waves for the first time in 2020.
“It was truly a magical moment,” she said at a news conference.
Scientists said the early evidence was consistent with Einstein’s theory of relativity and science’s current understanding of the universe.
However, they stressed that they had not yet definitively “discovered” the waves, because they had not reached the five level of certainty in the gold standard. Five sigma indicates that there is a one in a million chance that something is a statistical fluke.
“We’re frustratingly short on target,” Keith said, adding that there is a 99% chance the evidence points to gravitational waves.
Each country or consortium group has published their research separately in a series of journals.
Once all the data is combined, the five sigma mark could be reached within a year or two, said Steve Taylor, head of the North American NANOGrav Gravitational-Wave Observatory.
The main theory is that the waves come from pairs of supermassive black holes at the centers of slowly merging galaxies.
Unlike those causing previously detected gravitational waves, these black holes are almost incredibly massive – sometimes billions of times more massive than the sun.
If confirmed, the waves would be “the sum of all binary systems of supermassive black holes orbiting each other in the cores of galaxies everywhere in the universe,” Daniel Reardon, a member of the Australian Parkes Pulsar Timing Array, told AFP.
Keith said the “background buzz of all these black holes” was “like sitting in a noisy restaurant and hearing all these people talking”.
Another theory is that gravitational waves could come from the rapid expansion that occurred in the second after the Big Bang, a period called cosmic inflation that scientists are keeping from view.
Keith said that galaxies between Earth and the Big Bang are likely to “drown” such waves.
But scientists said low-frequency gravitational waves could tell us more about this early expansion into the future and possibly shed light on the mystery of dark matter.
It could also help them better understand how black holes and galaxies form and evolve.
