ThrowawayPhysicist1 t1_jdvq71y wrote
Reply to comment by icrushallevil in Black holes may be swallowing invisible matter that slows the movement of stars by trevor25
You’re missing a lot of the easy physics to this. Photons can (mostly-or maybe entirely) pass through dark matter for the same reason neutrinos can pass through earth. Dark matter must not couple strongly to the electromagnetic force. This is not terribly surprising. It just means dark matter must be (electrically) neutral. It also can’t couple strongly to the weak or strong force (otherwise it would be easy to observe). This again isn’t terribly shocking (a low coupling is easy to put in). Therefore, we know dark matter only interacts strongly through gravitation. This will mean it will probably never form a structure like a star but even if the densities got that high, it wouldn’t result in fusion since there no reason to believe dark matter is capable of forming hydrogen/helium analogs. There are some people who talk about a “dark QED sector” which would have dark photons and other things but so far nothing we’ve tested for dark matter has panned out and this is where the interesting physics lies.
MACHOs (massive objects like black holes) have been pretty conclusively ruled out by lensing studies. Which leaves us with particles (WIMPs and axions being the most discussed).
arcanum7123 t1_jdvzdt1 wrote
>This will mean it will probably never form a structure like a star but even if the densities got that high
Can you explain more about this? What's to stop it forming dark planets? (I understand they're being no stars under the assumption that it can't form element/element-like particles)
sight19 t1_jdx0z40 wrote
For structure to form, you need to compress gas in a small volume. As gas is compressed, it's temperature increases and so do the random motions of the theoreticized WIMPs. That means that at a certain point, the gas cloud stops collapsing (this is also called 'virialization'). The only way to collapse further is if the gas cloud would leak out temperature somehow, and that can only happen via radiation. And dark matter can't radiate, so it can't cool further
ThrowawayPhysicist1 t1_jdx8ewv wrote
This is a good explanation but there’s also just the simpler fact that if things can’t collide (and dark matter is mostly collisionless as famously seen in the bullet cluster) it’s hard to get it to “stick together”. So while gravitational force will get it form “dense halos” it’s fairly unlikely you’ll get something like planets or stars.
[deleted] t1_jdzvbk2 wrote
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ThrowawayPhysicist1 t1_je03l5d wrote
Because MOND doesn’t really explain the data. For one, it doesn’t really explain all the mass but for another, the bullet cluster is kinda a death sentence for MOND. Like good scientists, we can’t rule it out completely yet but dark matter explains all the discrepancies much better than MOND and so MOND is a fringe theory among physicists. In addition to the Bullet Cluster, MOND also poorly fits several other features which dark matter explains naturally, including much of cosmology. Less damningly, MOND requires a rather complex, random looking change to physics while dark matter is actually quite simple. We are pretty convinced there it is possible there are particles we haven’t seen yet so it’s not terribly surprising some of these could have astronomical effects. Also, I suspect you have a hard time grasping much of physics (QFT and GR for example) but QFT has been confirmed at the highest precision of any scientific theory ever. The fact laymen struggle with some physics isn’t a good reason to believe something else.
Bensemus t1_je12v44 wrote
No offence but that logic is terrible.
I don't get something so the only answer is it's wrong?
Not understanding something is expected. There is way too much for people to understand everything and in physics the leading edge is mind-numbingly complex.
However that doesn't mean it's wrong.
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