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SYFY WIRE Dark Matter

The force of dark matter is with you — and something could measure it

It’s not the dark side of the Force, just the invisible side.

By Elizabeth Rayne
Star Wars Darth Vader

It’s not the dark side of the Force, just the invisible side. There is matter we can’t see flying around the entire universe, and it exerts a force on everything. That means you.

Think more in terms of Yoda than Darth Vader. Whatever power he uses to levitate things is unseen, but it pushes them to move, which isn’t too far from what gravity does. Dark matter exerts an influence on everything in this galaxy — and others far, far away. That is the galactic force. To measure it, you would need to find out how much force is affecting a given object, which would be easier if there weren’t so many other objects with their own gravity in the way.

Astrophysicists Edward Belbruno and Jim Green, who coauthored a study recently published in Monthly Notices of the Royal Astronomical Society, are proposing a method of finding out how much dark matter is influencing something passing through space. It has a negligible effect in the inner solar system. However, the further out you get, the less interference there is from other objects, which would hypothetically make getting a dark matter measurement possible.

“You can detect it when all the gravitational forces of the planets, and the Sun, are so small that the force due to dark matter can be measured,” Belbruno told SYFY WIRE. “This can be done hundreds of astronomical units from the Sun.”

Our solar system has the same amount of dark matter everywhere. How detectable it is depends on what it is up against, which is why a spacecraft trying to register its force will have to get away from objects as gargantuan as the Sun or a planet. These are going to have so much gravitational pull that it will overwhelm the relatively weak force of dark matter in the inner solar system. It’s when you get to the outer edge that you have a better chance of measuring its effect with a probe, because the Imperial troops (our star and its planets) aren’t taking over.

Liz Galaxy Cluster NASA

Dark matter slightly changes the trajectory of all spacecraft, though especially those such as New Horizons that venture to the outer reaches of the solar system. This could be an advantage for a mission designed to measure the force being exerted on it by matter that is otherwise unobservable. The gravity of the galactic force dominates at extreme distances, but NASA won’t necessarily have to send a spacecraft out 40,000 times the distance between the Earth and the Sun (astronomical units or AU) to see results. Belbruno thinks just a hundred AU should do it.

“The galactic dark matter force is towards the center of the galaxy, no matter where you are in the galaxy, and an object will feel that force If the spacecraft is far from the Sun, then the gravity of the galactic force will dominate,” he said.

Spacecraft traversing the solar system feel both the solar gravitational force and the galactic force. Which one ends up being dominant depends on how close or far that spacecraft is from the Sun, and Belbruno and Green think their Interstellar Probe concept, involving a spacecraft fueled by radioisotope power, could fly out about 100 AU, where it would drop a reflective ball. This is getting weird but it’s going to make sense. The ball would only feel the galactic force while the spacecraft would feel both that and the thermal force of radioactive decay.

Already known is that the direction of the galactic force is toward the center of the galaxy, and the direction of the thermal force from plutonium decay would be coming directly from the spacecraft. Measuring how much dark matter is influencing the spacecraft and the ball would come down to aiming a laser beam between the ball, which is only affected by the galactic force, and the spacecraft, affected by both the thermal and galactic force. The laser beam would keep moving between the ball and the spacecraft to get a read on the distance between them.

"If there were no galactic dark matter force, this distance would not change,” Belbruno said. “The dark matter force would affect the ball and the spacecraft slightly differently since they are in different locations. This very slight difference could be measured with instruments.”

Now NASA just has to approve this concept to make it a reality, and the first actual measurement of dark matter made in space could be beamed back to Earth. May the Force of dark matter be with them.