UAH team determine local dark matter density using direct acceleration measurements
by Russ Nelson for UAH News
Huntsville AL (SPX) Feb 26, 2025
Dr. Sukanya Chakrabarti, the Pei-Ling Chan Endowed Chair in the College of Science at The University of Alabama in Huntsville (UAH), and her team have pioneered the use of gravitational acceleration measurements of binary pulsars to help illuminate just how much dark matter there is in the Milky Way galaxy and where it resides.
Their previous study promised that as the number of data points grows with the addition of many more binary pulsars, the galaxy’s gravitational field could be mapped with great precision, including clumps of galactic dark matter. Now Chakrabarti and her team, including first-author Dr. Tom Donlon, a UAH postdoctoral associate, and UAH physics undergraduate student Sophia Vanderwaal, have published a new study that for the first time details a way to advance this field by using solitary pulsars instead.
“When we first began this work in 2021 and did the follow-up publication last year, our sample was composed of pairs of millisecond pulsars – binary millisecond pulsars,” Chakrabarti explains. A binary millisecond pulsar is a pulsar with a short rotation period that orbits another star.
“However, most pulsars are not in pairs,” the researcher notes. “Most of them are solitary. In this new work, we show how to effectively double the number of pulsars we can use to constrain dark matter in the galaxy by rigorously using solitary pulsars to measure galactic accelerations.”
“Constraining dark matter” means limiting the possible properties and characteristics of dark matter by analyzing observational data, essentially narrowing down the range of potential explanations for what dark matter could be, based on how it interacts with other matter and affects the universe’s structure on different scales.
“Because it’s a larger sample, we now have a breakthrough,” Chakrabarti says. “We are able to measure the local dark matter density using direct acceleration measurements for the first time. And on average we find that there’s less than 1 kilogram of dark matter in a volume of that of the Earth. If you compare that to millions of kilograms of gold produced every year – you can see that pound-per-pound, dark matter is more valuable than gold.”
Dark matter is thought to comprise over 80% of all matter in the cosmos, but is invisible to conventional observation, because it seemingly does not interact with light or electromagnetic fields.
Charting galactic “wobble”
“In my earlier work, I used computer simulations to show that since the Milky Way interacts with dwarf galaxies, stars in the Milky Way feel a very different tug from gravity if they’re below the disk or above the disk,” Chakrabarti says.
“The Large Magellanic Cloud (LMC) – a biggish dwarf galaxy – orbits our own galaxy, and when it passes near the Milky Way, it can pull some of the mass in the galactic disk towards it – leading to a lopsided galaxy with more mass on one side, so it feels the gravity more strongly on one side.
“It’s almost like the galaxy is wobbling – kind of like the way a toddler walks, not entirely balanced yet. So this asymmetry or disproportionate effect in the pulsar accelerations that arises from the pull of the LMC is something that we were expecting to see. Here with the larger sample of pulsar accelerations, we are actually able to measure this effect for the first time.”
“The incredibly strong magnetic field of the pulsars will twist and coil on itself as the pulsar spins, which leads to a kind of friction, like rubbing your hands together,” adds Donlon. “Pulsars also emit particles at very high speeds, which beams away energy. These effects lead to the pulsar spinning more slowly as time goes on, and currently there isn’t any way to calculate how much this happens from existing theory.”
This phenomenon is called “magnetic braking,” the process by which a star loses angular momentum (rotational speed) due to its magnetic field capturing charged particles from its surface and flinging them outward as a stellar wind, effectively taking away some of the star’s spin with them. Modeling this process turned out to be key to moving forward.
“Because of this spindown, we were initially – in 2021 and in our follow-up 2024 paper – forced to use only pulsars in binary systems to calculate accelerations, because the orbits aren’t affected by magnetic braking,” Donlon says. “With our new technique, we are able to estimate the amount of magnetic braking with high accuracy, which allows us to also use individual pulsars to obtain accelerations.”
In astronomy, magnetic spindown rates refer to the rate at which a celestial object, particularly a rotating neutron star (like a pulsar), slows down its rotation due to the loss of rotational energy through magnetic dipole radiation.
Through mapping the acceleration field of the galaxy, it should eventually be possible to determine the distribution of dark matter in the Milky Way with fairly high accuracy, the new study reports.
“In essence, these new techniques now enable measurements of very small accelerations that arise from the pull of dark matter in the galaxy,” Chakrabarti says. “In the astronomy community, we have been able to measure the large accelerations produced by black holes around visible stars and stars near the galactic center for some time now. We can now move beyond the measurement of large accelerations to measurements of tiny accelerations at the level of about 10 cm/s/decade – 10 cm/s is the speed of a crawling baby.”
Supporting this initiative has been especially impactful for an undergraduate like Vanderwaal. She recently traveled to the 2025 American Physical Society CU*IP Conference for Women and Gender Minorities in Tuscaloosa, Ala., with Dr. Chakrabarti where Vanderwaal placed in the poster competition. For the past year the UAH student has been working as a part-time student researcher at UAH.
“I’m a physics undergraduate student with a love for learning. Working with Dr. Chakrabarti and her team on this project was my first research experience, and I learned so much from it!” Vanderwaal says.
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