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Is Time Travel (going backwards) Possible?
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<blockquote data-quote="freyar" data-source="post: 6042783" data-attributes="member: 40227"><p>You're welcome. Sorry for the delay on this response, busy day.</p><p></p><p></p><p></p><p>The basic framework of DM models, specifically, "there exists a non-luminous, nearly pressureless matter with approximately 5x as much total mass as normal matter," has not been changed, no. The reason is that it hasn't needed to be changed. The error bars have gotten smaller regarding how much DM you need as the data has gotten better, but that's it. It has worked very well. There is a variation in the ratio of normal to dark matter in different galaxies, but you'd expect that the very complicated evolution of structure in the universe would lead to that, and the best computer simulations agree in broad terms. I should say that structure formation isn't completely understood in either DM or MOND theories.</p><p></p><p>As for the Bullet Cluster, the observations agree very well with what you expect for DM and not what you'd expect for MOND. In particular, you expect the DM to go through, like the galaxies (ie, to miss everything), while the gas piles up in the middle. From what I've read on the MOND description of it, you'd expect extra long-range attraction in MOND, so it's MOND where you'd expect the galaxies to be moving faster than they are.</p><p></p><p></p><p></p><p></p><p>MOND makes very good predictions for galaxy rotation curves but is not so good for other things. I want to stress the measurement of the CMB. The reason that is so important is that the universe was much simpler back when the CMB was formed (looking at the CMB is looking very far back in time). What that means is that, given a theory (either DM or MOND), you can make precise predictions with very few assumptions and compare them to precise experiments, and there aren't many confounding factors. So CMB measurements carry a lot of weight because we know what's going on. There isn't a complicated history to things, no astrophysics (like stars blowing up, etc). And this is where DM works very well and MOND does not. Well, last I saw, there was argument about how to do this calculation even. EDIT: Let me clarify this last statement based on re-reading what I'd remembered, which is worse for MOND than I'd recalled. Even a fairly strong proponent of MOND agreed that the standard theory taken as the relativistic version of MOND can't reproduce the CMB and furthermore that it's hard to imagine a modified theory of gravity that could if you didn't include dark matter. <a href="http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/" target="_blank">Link</a></p><p></p><p>I have to go but want to address the rest of this later.</p></blockquote><p></p>
[QUOTE="freyar, post: 6042783, member: 40227"] You're welcome. Sorry for the delay on this response, busy day. The basic framework of DM models, specifically, "there exists a non-luminous, nearly pressureless matter with approximately 5x as much total mass as normal matter," has not been changed, no. The reason is that it hasn't needed to be changed. The error bars have gotten smaller regarding how much DM you need as the data has gotten better, but that's it. It has worked very well. There is a variation in the ratio of normal to dark matter in different galaxies, but you'd expect that the very complicated evolution of structure in the universe would lead to that, and the best computer simulations agree in broad terms. I should say that structure formation isn't completely understood in either DM or MOND theories. As for the Bullet Cluster, the observations agree very well with what you expect for DM and not what you'd expect for MOND. In particular, you expect the DM to go through, like the galaxies (ie, to miss everything), while the gas piles up in the middle. From what I've read on the MOND description of it, you'd expect extra long-range attraction in MOND, so it's MOND where you'd expect the galaxies to be moving faster than they are. MOND makes very good predictions for galaxy rotation curves but is not so good for other things. I want to stress the measurement of the CMB. The reason that is so important is that the universe was much simpler back when the CMB was formed (looking at the CMB is looking very far back in time). What that means is that, given a theory (either DM or MOND), you can make precise predictions with very few assumptions and compare them to precise experiments, and there aren't many confounding factors. So CMB measurements carry a lot of weight because we know what's going on. There isn't a complicated history to things, no astrophysics (like stars blowing up, etc). And this is where DM works very well and MOND does not. Well, last I saw, there was argument about how to do this calculation even. EDIT: Let me clarify this last statement based on re-reading what I'd remembered, which is worse for MOND than I'd recalled. Even a fairly strong proponent of MOND agreed that the standard theory taken as the relativistic version of MOND can't reproduce the CMB and furthermore that it's hard to imagine a modified theory of gravity that could if you didn't include dark matter. [url=http://blogs.discovermagazine.com/cosmicvariance/2012/05/09/dark-matter-vs-modified-gravity-a-trialogue/]Link[/url] I have to go but want to address the rest of this later. [/QUOTE]
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