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Originally Posted by knicholson
2. You know MOND is still alive and well in some circles. Can you prove it wrong?
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It isn't necessary to prove one theory wrong to prefer another, all we have to do is pass judgement over which of the current possibilities makes the most useful predictions. I have never yet seen MOND make a significant prediction that worked, it is all retrofit. Dark matter makes predictions, and some have worked and others have not-- yet. So it is still pretty clearly the better of the two theories, which is the only question science asks. That doesn't mean it always will be.
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They simply said that Newton's law had to be modified to get speed matches for galaxies and thus avoid dark matter, but their mods had to be special for nearly every case. Does that prove it wrong? I think so.
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At least it proves it's not our best theory, and really not much of a theory at all so far.
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They then use 5 to estimate the mass of the Galaxy, assuming 5 as constant over the entire radius. This is where I think the problem lies. I believe the average M/L ratio is closer to 10 for most galaxies, and that the M/L varies a lot for different galaxies and over radius in a given galaxy. Because of the possible situation where there is very low average light, I think it should be a L/M ratio to be more realistic.
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One thing I will freely admit in all this is that I have no idea what confidence we can put in L/M determinations. There are two problems with it that are potentially severe, one is that the L comes more from high-mass stars while the M comes more from low-mass stars, so there's really two sources of potential error there not just one. The other is that as soon as you presume the dark matter has to be spherical, it takes even more of it to fit the rotation curves. So even a fairly small shortfall in M/L in the disk will lead to a relatively large need for dark matter. That has a way of magnifying the importance of errors anywhere in the analysis.
The bottom line for me is, if galactic rotation curves were the only thing that forces us to think about dark matter, I would see it as being on pretty shaky footing. But there is an even stronger need for it in galaxy clusters and in cosmology, so extending that to galaxy rotation curves does not seem like a stretch. Indeed, if galaxy rotation curves worked fine without dark matter, using what seemed like a "normal" M/L, it would be a rather puzzling state of affairs.