What’s the matter?
Windchime will use gravity to detect certain types of dark matter. Ultrasensitive sensors would be jostled by the gravitational forces of a dark matter “wind” passing by Earth, James R. Riordon reported in “Dark matter could be assisted by gravity” (SN: 9/10/22, p. 14).
Because dark matter is affected in gravity, reader David Goldberg asked if dark matter orbits the Milky Way’s center just like our solar system does. How would dark matter winds be detected if the two were to move together?
It’s possible that dark matter circles the Milky Way’s center at least a little, though it’s hard to say for sure because no one has been able to measure the elusive stuff yet, Riordon says. But to search for dark matter using the Windchime method, it doesn’t really matter whether the mysterious substance moves with the galaxy, he says. That’s because as the sun circles the Milky Way’s center, Earth is also orbiting the sun. Even if the sun happens to move with the same velocity as nearby dark matter, the direction of Earth’s velocity changes over the course of a year, Riordon says. So we should sense the pull of a dark matter wind that shifts with Earth’s seasons.
What’s more, as the planet spins on its axis, the direction of the surface’s movement relative to the galaxy changes throughout the day. “It’s a little like a fish swimming in the ocean,” Riordon says. “Even though the water in general moves with Earth, as a fish swims in various directions, the creature will experience a flow of water relative to its own motion.”
Reader Jack Ryan wondered why, despite its gravitational attraction, dark matter doesn’t form stars, planets and other celestial bodies.
“Because no one knows what dark matter is, we can’t say for certain that it can or cannot form globs or come in very massive particles,” Riordon says. “It’s something that researchers like those on the Windchime team are looking out for.”
To create asteroids, or planets from normal matter, it must experience force beyond gravity. Riordon says. If two rocks collide in space, electromagnetic force would stop them simply passing through each others. The gravity would then hold the rocks together. If the concentration of matter continues building up, an asteroid or planet could form.
Some scientists believe that dark matter can experience other forces. But, as far as we know gravity is the only one that affects it. “If a dark matter particle that only experiences gravity approaches a rock, a planet or another dark matter particle, it would glide right through because there is no force that can stop it,” Riordon says. “Gravity can pull dark matter into a halo, but on its own, it probably can’t stick dark matter together.”
Correction
A line dropped from our feature “Learn from islands” (SN: 9/24/22, p. 22). The full sentence should read at the end of Page 26: Saban native Dahlia hassell-Knijff earned a biology degree in Mississippi and then returned to Saban, where she oversees projects for the Dutch Caribbean Nature Alliance.
