Pulsar Acceleration Used to Calculate Milky Way’s Dark Matter Density

Jan 13, 2021 | Daily Space, Dark Matter, Neutron Stars / Pulsars

IMAGE: The ripples in the Milky Way disk are shown, along with the tidal debris from the Sagittarius dwarf galaxy. The pulsars analyzed by Chakrabarti and collaborators to calculate galactic accelerations are shown in the inset. CREDIT: IAS; Dana Berry

Pulsars — these fast-spinning neutron stars — are essentially rotating tops that have beacons of light that are emitted toward an observer twice per rotation, like the double-sided lights in a lighthouse. Just like a top, pulsars slow down with time, but the deceleration is so slow that, for the most part, we can use the timing of the pulses as a metronome against which we can measure all kinds of phenomena. 

Back in 1990, the first exoplanets ever found were discovered orbiting a pulsar, and they were found because the slight orbital motions of the pulsar and the planet going round and round affected the timing of the pulses reaching Earth. I still remember exactly where I was when that discovery was made. I was a high school student working at a spare computer next to a laser printer at Haystack Observatory. It smelled like ozone, and my advisor was super excited to explain pulsar timing to me and why this discovery looked real. Ever since then, I’ve taken delight in all the ways people more creative than I have found to use pulsars to probe what is going on in our galaxy and the universe beyond.

The physics of our universe is weird. As a whole, the universe is expanding thanks to both the residual motion of the Big Bang and the accelerating force of dark energy. When I was little, and I first learned the universe was expanding, I was terrified we were expanding with it, and someday my neurons wouldn’t be able to talk to each other because of the universe’s expansion.

I was a super weird little kid.

But this kind of a “is the solar system expanding? Is the galaxy expanding?” concern is actually pretty common, and for the most part, astronomers brush this question off and say, “No, galaxies and solar systems are gravitationally held together. People and plants and other stuff are chemically held together. The push of the universe’s expansion can’t overcome these forces, so no, if you find yourself expanding, you can’t blame dark energy.”

Occasionally, we find situations where the infall of a galaxy into a cluster is balanced against the expansion of the universe, and we can’t quite tell what that galaxy’s fate may be. On smaller scales, seeing the effects of expansion just hadn’t really been possible.

At least not until a group of astronomers looked carefully at a suite of pulsars positioned around our galaxy. By looking carefully at their timing, researchers can tease out the tiny motions due to the inward acceleration of gravity as these systems orbit the Milky Way and the tiny weird acceleration associated with the expansion of our universe. Since they looked at pulsars at a variety of distances, they can map out the varied distribution of the material in our galaxy and the twisting accelerations that mass’ gravity can create.

This study was presented at the AAS meeting yesterday and was led by Sukanya Chakrabarti. A related paper is appearing in Astrophysical Journal Letters. One of the particularly beautiful things about these results is that it is equally sensitive to mass we can see, like stars and dust, and to the invisible distribution of dark matter, which we have struggled to map out in any detail within our own galaxy.

According to Scott Tremaine: For centuries astronomers have measured the positions and speeds of stars, but these provide only a snapshot of the complex dynamical behavior of the Milky Way galaxy. The accelerations measured by Chakrabarti and her collaborators are directly caused by the gravitational forces from the matter in the galaxy, both visible and dark, and thereby provide a new and promising window on the distribution and the composition of the matter in the galaxy and the universe.

This technique is new, and I am now looking forward to more pulsars allowing more detailed mapping of our universe to take place.

More Information

IAS press release 

“A measurement of the Galactic plane mass density from binary pulsar accelerations,” Sukanya Chakrabarti et al., to be published in Astrophysical Journal Letters (preprint on arxiv.org)


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