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I'm very skeptical of this because there is plenty of evidence that's explainable with dark matter but not with MOND. I believe one such thing is how the dark m
by eloff 6y ago
I'm very skeptical of this because there is plenty of evidence that's explainable with dark matter but not with MOND. I believe one such thing is how the dark matter can seem to separate from the host galaxy in a collision with another galaxy. Someone can correct me as I might not be remembering that quite right. That's tough to explain with just gravity alone.
I really like the idea of MOND as requiring an invisible unknown particle to explain galactic rotation always smacked of the old ether theories to me. But my understanding is it's nearly impossible for a mond theory to encompass all of the current evidence for dark matter without being totally contrived.
- dnautics 6y agothe difficulty of dark matter is that you can explain anything with dark matter. If the oxygen in my room moved to the upper left corner and suffocated me, there is a dark matter field that could account for that.
- andreareina 6y agoYou could explain anything with dark matter, but it's a lot harder to explain multiple things with the same dark matter. The different observations (galaxy rotation curves, the Bullet Cluster collision, CMB power spectrum) all put different constraints on what the explanation has to be, and a particle that feels gravity but not electromagnetism is consistent with all of that.
- dnautics 6y agoSorry, could you please define "same" here? That "same" dark matter could also explain the room evacuation. Should I stretch it further to include poltergeists?
- vadansky 6y agoI think the argument is that you can put exactly as much dark matter anywhere you want to explain whatever you want. Sure it has to be "a particle that feels gravity but not electromagnetism", but that just means it's invisible and the only effect it has is the exact effect you want. So it's basically being able to add whatever number you want to one side of an equation to get it to balance out.
- dnautics 6y agoalso "constraints" on DM values can be misleading. When you say "a DM particle cannot be more clumpy than X" given an average density of DM particles in the universe, you are saying "we must LOOSEN a constraint on the types of things that DM can explain", because for that particular parameter, there is an inverse relationship between constraint tightness and explanatory power.
- h2odragon 6y agoNicely expressed. Put that way, it becomes obvious the problem isn't that we have a class of unknowns gathered under the label "dark matter", its that the label comes to imply things unrelated to the real use. I take the "faith in human ignorance" view that there are many things extant that we have no inkling of. Its easy to mistake quantified and classified ignorance for knowledge, especially when it is all we have.
- magicalhippo 6y agoSure if your model is complicated enough you can add unicorns and Santa Claus. But those models aren't interesting, precisely because of that. The interesting models have as few parameters as possible, allowing them to be constrained in their behavior.
- jjk166 6y agoYes, and you can put exactly as much invisible air as you want to explain why branches move the way they do. If the branch is moving twice as much, isn't it so convenient to claim that there just happens to be twice as strong a wind?
- 8note 6y agoCan it? Thered have to be a lot of dark matter for that, and I would expect it to have clumped at the center of the earth, not be floating in your room
- dnautics 6y agoyou'd also expect two galaxy clusters to be travelling at each other at high speed with frequency less than one universes.