Readers discuss the search for dark matter and 25 years of Mars rovers

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Cover of September 10, 2022 issue of Science News

What’s the matter?

The proposed Windchime experiment plans to use only gravity to detect some 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 “Gravity could aid dark matter search” (SN: 9/10/22, p. 14).

Since dark matter is affected by gravity, reader David Goldberg asked if dark matter orbits the Milky Way’s center just like our solar system does. If the two move together, how would there be a dark matter wind to detect?

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.”

For normal matter to create asteroids or planets, it must experience some force beyond gravity, Riordon says. For instance, if two rocks collide in space, electromagnetic forces would prevent them from simply passing through each other. This would then allow gravity to hold the two rocks together. And if the concentration of matter continues to build up, then an asteroid or a planet could eventually form.

Some physicists hope that dark matter experiences other forces, but gravity is currently the only one known to affect 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 “Island lessons” (SN: 9/24/22, p. 22). At the end of Page 26, the full sentence should have read: Saban native Dahlia Hassell-Knijff got a degree in biology in Mississippi, then returned to the island, where she oversees projects at the regional Dutch Caribbean Nature Alliance.

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