Astronomers use spinning stars as cosmic lighthouses to help detect gravitational waves
Astronomers may be on the verge of detecting gravitational waves from interactions between pairs of enormous supermassive black holes in galaxies up to halfway across the universe.
These waves affect the Earth “like a cork bobbing up and down on the surface of water,” according to astronomer Ingrid Stairs, as space-time itself is distorted as they pass through us.
Gravitational waves were predicted by Einstein in his theory of general relativity, and first detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment in the US in 2016. LIGO detected the collision of two black holes more than a billion light years away, each about 30 times the mass of our sun.
LIGO, however, is not the only effort to detect gravitational waves. This week at the meeting of the American Astronomical Society, the NANOGrav team (the North American Nanohertz Observatory for Gravitational Waves) presented data from more than 12 years of observations trying to detect the reverberations produced by systems vastly larger than those LIGO targets.
Their result wasn’t quite confirmed as a detection of gravitational waves, but the data collected so far is pointing in the right direction.
“We can’t say that we have yet,” said team member Stairs, an astronomer from the University of British Columbia. “It’s an important first milestone along the way.”
The NANOGrav project is attempting todetect gravitational waves caused by the decaying orbits of binary pairs of black holes that are roughly a billion times the mass of our sun. Black holes that massive are thought to form at the heart of large galaxies. A binary pair of black holes of that size would be a result of two large galaxies colliding and merging.
“Those black holes will eventually sort of sink down to the centre of the new galaxy together and they will get into a binary orbit around each other,” Stairs told Quirks & Quarks host Bob McDonald.
As the black holes “sink down,” they orbit around each other closer and closer. NANOgrav researchers aim to detect pairs of black holes orbiting each other at distances of a few per cent of a light year, when they’re moving at about a tenth the speed of light.
“It’s an incredibly extreme system, but that is what can produce the gravitational waves we might be sensitive to,” said Stairs.
While it takes huge and powerful black holes to create these gravitational waves, the result is paradoxically tiny.
“They stretch and shrink space-time, as they move through space. And we’re talking about very, very small stretches and shrinking,” said Stairs.
“The stuff we’re looking for with NANOgrav is about one part in 10 to the 15 [one quadrillion], which if you compare that to the size of the Earth, would be a fluctuation of about the size of a few nanometres.”
Searching for pulsars
Not surprisingly, detecting such a tiny fluctuation is very difficult. The NANOgrav team does it by looking for miniscule variations in the extremely precise radio emissions from a special class of neutron star called a pulsar.
Neutron stars are the remnants of large stars that have undergone a supernova, and collapsed into super-dense stellar cinders that pack the mass of a star into an object only about 10 kilometres in diameter. Pulsars are neutron stars that spin at enormous speeds, and their magnetic fields generate beams of radio waves that blast out from their magnetic poles.
These spinning beams can be visible from Earth as repeating radio bursts with incredible clock-like accuracy. The bursts from a pulsar can be seen hundreds of times a second, with nanosecond precision. That’s the key to the NANOGrav project.
“When these things are spinning around, they act a bit like lighthouses with these beams of radio waves.” said Stairs.
If the Earth is bobbing up and down and back and forth on gravity waves, it will slightly change the precise timing of the arrival of pulsar bursts at radio telescopes on Earth.
“What we’re doing is looking at a number of the most stable rotating pulsars out there,” said Stairs. “And we’re monitoring them to look for any changes over time and just how fast they spin. So we’re looking for a very distinctive pattern on the sky for how these things change.”
That “distinctive pattern” is caused because there are a lot of black holes in the universe producing gravitational waves. In fact there may be up to a million of these exotic black hole binaries in the detection range of NANOgrav, all producing ripples in space-time that are arriving at Earth simultaneously.
The team has been monitoring dozens of pulsars for more than a decade to try to hone in on a pattern of pulsar disturbances that they can be confident represent the detection of gravitational waves.
It’s painstaking and precise work, said Stairs.
“We’re looking for changes in the pulse arrival times of a few tens of nanoseconds, so that’s pretty small — really small.”
Their progress in the search has recently been interrupted. Their pulsar measurements have relied on the Green Bank radio telescope in West Virginia and the Arecibo observatory in Puerto Rico. Unfortunately a catastrophic collapse of the instrument platform at the Arecibo observatory in December caused irreparable damage to the telescope.
But there will be a Canadian contribution to help fill that gap, as Stairs and her colleagues are working to bring observations from the CHIME telescope in Penticton, B.C., into the collaboration.
Stairs is excited by this project, as it brings two disparate cosmic phenomena together — pulsar astronomy and gravitational wave astronomy.
“It’s a really neat combination of a couple of very fundamentally different but both fundamentally important aspects of physics, so in that sense it’s very stimulating,” she said.