Milliseconds measure galactic gravity

Scientists measuring tiny changes in the arrival times of signals from millisecond pulsars have found a new way to measure both gravitational waves and the galaxy’s distribution of dark matter.

Pulsars are rapidly rotating neutron stars that, like cosmic lighthouses, beam out radio signals that sweep the heavens – beams that can be detected by radio telescopes each time they point in our direction. Millisecond pulsars are ones that do this every few thousandths of a second.

They produce an incredibly steady metronomic beat. “Their temporal stability rivals atomic clocks,” says Sukanya Chakrabarti, of Rochester Institute of Technology, in New York, US.

When changes do occur, they provide important clues to astrophysical processes that have nothing to do with the pulsars themselves.

Yesterday, at a virtual meeting of the American Astronomical Society (AAS), Joseph Simon of the University of Colorado examined more than a dozen years of data from two of the world’s most powerful radio telescopes – Green Bank in West Virginia, US, and Arecibo in Puerto Rico, and found small changes that appear to be due to gravitational waves passing through the Earth. 

These waves, says Simon, toss the entire Earth slightly toward one group of pulsars and away from others, causing their signals to arrive slightly sooner or later than expected. “The only thing we know that causes this pattern is a passing gravitational wave,” he says.

Not that the Earth is shifting enough for anyone but an astrophysicist to care about. The pulsar signals’ expected arrival times changed by only a few hundred nanoseconds – the equivalent of the Earth being pushed a dozen metres or so over the course of years.

Still, it’s indicative of the incredible precision with which pulsar signals can be measured. And it might also be a fitting obituary to Arecibo, which recently collapsed when crucial cables frayed and broke. “The science legacy of Arecibo is really, great, and we are incredibly saddened by its loss,” Simon says.

Chakrabarti’s finding is, if anything, even more impressive.

She reported at the AAS meeting that, rather than using ordinary pulsars, she used binary pulsars – ones that orbit each other in pairs, much like binary stars. 

These orbits mean that the arrival times of each pulsar’s signals shifts as first one and then the other is closer to Earth. Tiny changes to this rhythm reveal how the pulsars are responding to the galaxy’s gravitational field.

“That allows us to use them as galactic accelerometers,” Chakrabarti says, “much like the accelerometers in your iPhone.” 

It’s not a big effect. In a study of 14 binary pulsars, Chakrabarti found that they were changing velocity by only a few centimetres per second. “That’s roughly the speed of a crawling baby,” she says. “And not a very fast baby, at that.”

But given the extraordinary precision with which pulsar signals can be measured, it was enough for her team to measure the gravitational field of the galaxy over a sphere about 3200 light years in radius around the Earth. 

From that, she says, it was possible to map out the average density of matter in and around our part of the galaxy, and determine how much of that matter was invisible: dark matter, in other words.

It’s a finding that may help astrophysicists better understand the role dark matter plays in the galaxy as a whole, but it can also help physicists figure out how to detect it on Earth, by showing how much is in our vicinity. 

And it turns out that there isn’t a lot: only about 7 x 10^-25 grams per cubic centimetre. “If I look at the amount of dark matter within the Earth, it’s less than a kilogram,” Chakrabarti says.