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Q & A: Galactic evidence for Dark Matter

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Most recent answer: 06/08/2014
Dear Department of Physics, Today I was watching the serie Cosmos: A Spacetime Odyssey. In the episode, the host Neil Tyson talked about gravity. More specifically he stated that once an object gets more gravitational pull, the quicker they move. He used mercury as an example. This planet has more gravitational pull from the sun than the earth, thus he moves quicker. He then went on about how this rule is apparently not working for galaxies. According to research that Tyson refers to, stars in the outer layers or edges of galaxies are moving quicker than they are supposed to. He then went on about how researchers explain this phenomenon with the mysterious "dark matter". I was just really curious about this and wondered: can't the movement of these stars at the edge be explained by gravitational pull from surrounding stars? So that it's not just the centre of a galaxy that causes a pull, but also each individual solar system? And if so, couldn't that partially explain why stars at the edge of the galaxy move quicker than if they were simply being pulled by the centre of the galaxy? I am no physics expert but just a really curious wanderer. Hope you could clear this out for me! Thanks in advance for the answer.
- Wouter (age 24)

Dear Wouter,  

Your question makes sense if the hypothesized dark matter has the same radial distribution as the ordinary matter in a galaxy.  However if the dark matter particles do not have electromagnetic or strong interactions and only interact weakly through the electro-weak force and gravity then their galactic radial distribution doesn't have to be the same as ordinary matter. Their gravitational interaction would, however, affect the radial-orbital velocity distibution of ordinary stars.  See: 

These particles are sometimes called WIMPs for Weakly Interacting Massive Particles and would interact only via the electroweak and gravitational force.  See .   Many experiments are underway worldwide searching for these particles.  See .

So far no stong evidence has shown up.   Stay tuned.


p.s. The sorts of pulls from nearby stars that you ask about would give motions in all different directions as the stars circled about their neighbors. The motions that suggest dark matter are more systematic, part of the overall rotation of the whole galaxy, not just little swirls here and there. Mike W.

(published on 06/08/2014)

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