Dark matter, widely known as the universe’s most mysterious stuff, is rarer on Earth than gold — and that’s despite the fact that dark matter outweighs “ordinary matter” by a staggering ratio of five to one.
The finding came courtesy of scientists who propose a novel way to map dark matter using the “wobble” of the Milky Way. That wobble is due to the influence of the Milky Way’s satellite galaxies, like the Large Magellanic Cloud (LMC), and rapidly rotating neutron stars, or “pulsars.” Fascinatingly, pulsars act like “cosmic lighthouses” in the cosmos, sweeping beams of light across vast distances.
The team’s previous work has in fact used these extreme stars, when orbited by stellar companions in systems called “binary pulsars,” as dark matter probes. The scientists’ new research, however, further suggests solitary pulsars could be used in such an investigation, too.
“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,” Sukanya Chakrabarti of the University of Alabama in Huntsville (UAH) said in a statement. “However, most pulsars are not in pairs. 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.”
By “constraining dark matter,” Chakrabarti means limiting the possible properties and characteristics of dark matter.
As more neutron star data is collected, the gravitational acceleration measurement of binary pulsars and their single counterparts could shine a light on the gravitational field of the Milky Way and, thus, the shape and distribution of dark matter in our galaxy.
“Because it’s a larger sample, we now have a breakthrough,” Chakrabarti said. “We are able to measure the local dark matter density using direct acceleration measurements for the first time.”
The team found that there is less than 2.2 pounds (1 kilogram) of dark matter in a volume equivalent to that of the entire Earth.
“If you compare that to millions of kilograms of gold produced every year — you can see that pound-for-pound, dark matter is more valuable than gold!” Chakrabarti said.
Dark matter glue and wobbly galaxies
Dark matter, which makes up about 85% of the total matter in the universe, has been sort of a problematic phenomenon for scientists because it doesn’t interact with light — or, if it does, that interaction is too weak to be detected with current technology.
That tells researchers that dark matter can’t be made of atoms like everyday matter is, because the particles that comprise atoms — electrons, protons, and neutrons — do interact with light.
The only way we can know whether dark matter exists at all is via its interaction with gravity and the influence this interaction has on light and everyday matter. In fact, this influence is crucial.
If galaxies weren’t packed with invisible dark matter, the gravitational influence of their “everyday matter” — stars, planets, dust clouds, and so on — would not be sufficient to prevent them from flying apart as they spin.
Galactic dark matter content is thought to be heavily concentrated in the centers of galaxies, but it’s believed the substance also extends out to form a spherical shell that extends far beyond the limits of a galaxy’s visible matter.
That explains how dark matter can be less common in an average sphere around the size of Earth than gold is here on our planet — but still vastly outnumber atoms of all types. Space is vast, and dark matter is way more spread out across the universe than gold or other elements are.
Chakrabarti explained that in her earlier work, she used computer simulations to show that, as the Milky Way interacts with its satellite galaxies, the stars in our galaxy feel a very different tug from gravity depending on whether the stars are located above or below what is known as the “galactic disk.”
The LMC is one of the Milky Way’s larger dwarf galactic satellites, for instance. As it orbits our own galaxy and passes near to the Milky Way, it can pull some of the mass in the Milky Way’s galactic disk towards it, leading to a lopsided galaxy with more mass on one side. As a result, Chakrabarti said that gravity is felt more strongly on one side of the Milky Way.
“It’s almost like the galaxy is wobbling — kind of like the way a toddler walks, not entirely balanced yet,” she continued. “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.”
Cosmic lighthouses
Pulsars, like all neutron stars, are born when stars at least eight times as massive as the sun run out of their fuel supply needed for nuclear fusion and can no longer support themselves against the inward push of their own gravity.
As the cores of these stars crush down, the stars’ outer layers, and most of their masses, are blown away in tremendous core-collapse supernovas.
This leaves a stellar remnant with between one and two times the mass of the sun condensed into a width of around 12 miles (20 kilometers). This means neutron stars are composed of the densest matter in the known universe.
If a teaspoon of neutron matter were scooped up and brought to Earth, for context, it would weigh 10 million tons. That is equivalent to stacking 85,000 blue whales onto a teaspoon.
The rapid reduction in size of the massive stars’ cores has another consequence: it speeds the neutron star up to rates of rotation that can reach 700 turns per second. Think of this as being like the cosmic equivalent of an ice skater drawing in their arms to increase their rate of spin.
Luckily for scientists, this rapid spin and its precise frequency make pulsars excellent timing mechanisms.
Pulsars and other young neutron stars are also notable for possessing some of the strongest magnetic fields in the known universe.
“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,” team member and UAH postdoctoral associate Tom Donlon said in the statement. “Pulsars also emit particles at very high speeds, which beams away energy. These effects [known as magnetic braking] lead to the pulsar spinning more slowly as time goes on.”
The magnetic field of a pulsar captures ejected particles and then flings them away as the neutron star rotates. These particles, dispersing as “stellar wind,” also carry away angular momentum, slowing the spin of the pulsar, or causing it to spin down. The spin-down process is key in the team’s research.
“Because of this spin down, we were initially forced to use only pulsars in binary systems to calculate accelerations because the orbits aren’t affected by magnetic braking,” Donlon said. “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.”
Using this technique and the excess data it provides, researchers should be able to better determine how dark matter is distributed through our galaxy as more data is gathered.
“In essence, these new techniques now enable measurements of very small accelerations that arise from the pull of dark matter in the galaxy,” Chakrabarti said. “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 centimeters per second each decade, 10 centimeters per second is the speed of a crawling baby.”
The team’s research is available as a pre-peer-reviewed paper on the paper repository site arXiv.
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