The experiments at LUX and Xenon500 have all but ruled out particle form of dark matter. If DM exists in our galaxy and interacts with matter, we would have seen it by now. Also, there have been several studies recently confirming that the Higgs particle found in 2013 is the Standard Model Higgs, which pretty much shuts the door on SUSY (unless we redefine SUSY to be any particle that is just beyond the energy level of our accelerators - which seems to be the fashion). I can theorize that it's purple unicorns that makes objects in galaxies spin faster. Since the probability of a dark matter particle is quickly approaching the probability of purple unicorns, when is it appropriate on this Q&A to ask the question: What other models would explain our universe?

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    $\begingroup$ Uh...why did you delete your question and post it on meta? It isn't a question about Physics.SE (at least, the body is not, the title is, which leads to the question what the title has to do with the body). The issue people had with your question was not that you were asking about alternative models, but the combative tone in the title as well as the question - you don't need to discredit current models as "purple unicorns" to ask about others. $\endgroup$ – ACuriousMind Dec 7 '15 at 19:03
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    $\begingroup$ "The experiments at LUX and Xenon500 have all but ruled out particle form of dark matter." Is way too strong in two ways. First, neither experiment is up to their final expected scale yet; they are eating into the preferred space for WIMP masses, but they haven't excluded anything. Secondly, the theorists wave a couple of ranks of harder to nail down particle hypotheses waiting in the wings. All of those are things that are allowed by as-yet-not-ruled-out variations on the standard model. $\endgroup$ – dmckee --- ex-moderator kitten Dec 7 '15 at 21:47
  • $\begingroup$ They will never exclude every possibility for a particle. They have, however, reasonably excluded everything in the Standard Model. Now since dark matter is an imaginary particle to begin with, you can always conjure some more imaginary properties for it, like it doesn't actually interact weakly with baryonic matter. But the more imaginary qualities you give to dark matter, the more the theory becomes 'unprovable'. My point is we are very, very close to saying 'God did it' when it comes to dark matter theories. $\endgroup$ – user32023 Dec 7 '15 at 21:51
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    $\begingroup$ Look, a resurgence of interest in modified gravity theories is not unlikely if WIMPs don't pan out, but axions are not some arbitrary construct: they obey all the tested symmetries of standard model and solve an outstanding problem too. So why is everyone looking for WIMPs? Because axions are harder. It is easy to think that the theory community somehow "believes" in WIMPs as "the answer" if you only read the popular press, but if you go to particle-physics conferences you know that there is and has been strong interest in alternate approaches all along. $\endgroup$ – dmckee --- ex-moderator kitten Dec 7 '15 at 22:42
  • $\begingroup$ There's an old expression: Man makes gods by the dozens, but he can't make a single maggot. Why is it so much easier for you to invent ad hoc fields and particles with magic properties rather than just fix Newton's Second Law of Motion? $\endgroup$ – user32023 Dec 7 '15 at 23:09
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    $\begingroup$ If the fix is so easy, then write it down and send it to a journal. It's not like no one looked at modifying Newtonian mechanics. It wasn't terribly popular, but it was an active area of research with hundreds of papers published in good venues. But interest waned because of difficulties with covariance and then came the bullet cluster. $\endgroup$ – dmckee --- ex-moderator kitten Dec 7 '15 at 23:18
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    $\begingroup$ I did. The editor of the Astrophysics Journal told me if Copernicus himself walked into his office and drop Dē revolutionibus orbium coelestium on his desk, he wouldn't print it as it wasn't "mainstream" science. Why do you believe the journals are interested in solving this problem? $\endgroup$ – user32023 Dec 7 '15 at 23:24
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    $\begingroup$ Ethan T. Vishniac told me this himself in a correspondence. The exact term he used was 'highly theoretical', but the rest of the conversation is pretty much verbatim. $\endgroup$ – user32023 Dec 8 '15 at 13:29
  • $\begingroup$ @dmckee : re "if you go to particle-physics conferences you know that there is and has been strong interest in alternate approaches all along". But do those approaches only consist of dark matter in the guise of particles? $\endgroup$ – John Duffield Dec 8 '15 at 13:38
  • $\begingroup$ @KyleKanos - How can I argue with something I never said? MOND is a broken theory. There is absolutely nothing wrong with gravity. The problem is with Newton's Second Law of Motion. It's broken. Now go do your search again. Find me how many ApJ articles address fixing Newton's Second Law. I'll be here when you get back. $\endgroup$ – user32023 Dec 9 '15 at 2:14
  • $\begingroup$ You're right, misread your comments. And there's nothing wrong with Newton's 2nd law (in it's appropriate limit), so why bother trying to fix what isn't broken? $\endgroup$ – Kyle Kanos Dec 9 '15 at 2:24
  • $\begingroup$ I would guess from your comments that you never took college physics, so let me explain. The stars at the edges of spiral galaxies move too fast to be explained by Newton's 2nd Law. Also, using the Virial Theorem, which is based on Newton's Second Law, galaxies in clusters have too much kinetic energy for the potential. I can send you some excellent background material if you want to read up on the subject. $\endgroup$ – user32023 Dec 9 '15 at 10:54
  • $\begingroup$ Actually, I've got a BA, MS & PhD in Physics (the PhD in computational astrophysics, the latter you should be aware of since I just told you in a now-deleted comment), so you can drop the ad hominem. Call it what you want, but if you start with MOND thoughts (needing to modify Newton's laws), you're going to get a MOND theory (even if it's different from what other people have presented). $\endgroup$ – Kyle Kanos Dec 9 '15 at 18:30
  • $\begingroup$ Since there was never a falsifiable version of the Second Law proposed, MOND is a theory of modified gravity in spite of the label. I can prove to anyone with a high school education in physics and a basic understanding of statistics that Newton's Second Law is broken. $\endgroup$ – user32023 Dec 9 '15 at 19:06

This isn't the Physics Forum, it's Physics Stack Exchange.

Anyway, you've got two things going on in this question. First, you are presenting evidence to support the claim that dark matter is ruled out.

The experiments at LUX and Xenon500...
...is quickly approaching the probability of purple unicorns

None of that is relevant to this meta post.

The second thing you have going on is

...when is it appropriate on this forum to ask the question: What other models would explain our universe?

In the sense that you probably mean it, never. This site is not the place for a comprehensive listing of models that explain anything.

What would be reasonable, I think, is to ask what alternatives to a certain model exist within mainstream physics. In other words, you could ask what competing explanations are being seriously considered and actively researched by the community of scientists in the field. Of course, this requires you to (1) know what you're talking about well enough, and (2) do enough prior research, to establish that there are competing models within mainstream physics. If you ask about alternatives to, say, quantum field theory, it's not going to go over very well. (Actually I'm not sure that's true; it's not my area, but perhaps there are competing models that are actively being researched. But it's about as well established a theory as I can think of at the moment.)

  • $\begingroup$ I agree that a comprehensive list would be a zoo. A lot of those 'theories' are just regurgitate Star Trek dialog. However, 'mainstream' as you use the term is just a euphemism for 'court of public approval'. Geocentrism was very popular in it's time. ΛCDM and MOND both have serious shortcomings. I can make a case that LSB galaxies disprove ΛCDM and that galaxy clusters disprove MOND. So why isn't there room on this forum for something more scientific? What would be wrong with opening up the discussion for 'disprovable' theories? $\endgroup$ – user32023 Dec 7 '15 at 19:37
  • $\begingroup$ I think there are issues in that the people who advocate one particular theory will try to persuade the media & public that theirs is "the only game in town". We then read things like "according to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle". But it simply isn't true. So to then say that some competing theory is non-mainstream and should not be discussed is unwelcome. That sort of thing belongs in a medieval theocracy, not in 21st century science. NB: see this. $\endgroup$ – John Duffield Dec 7 '15 at 21:04
  • $\begingroup$ I guess the case that I want to make is the moderators of this forum have decided to throw out the baby with the bathwater. Yes, there are a lot of cranks out there. But this is Science, bitches! We have a way of dealing with cranks: it's called the Scientific Method. If someone has a case to make, let them post some disprovable theory or formula. It's our responsibility as critical thinkers to evaluate the ideas on their merits, not on the number of 'likes'. $\endgroup$ – user32023 Dec 7 '15 at 21:28
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    $\begingroup$ @DonaldRoyAirey The problem with opening something up for discussion is that this is not a site for discussion. If you bring up an idea that is currently not accepted, you have to convince people of its merits. That requires discussion, and Physics Stack Exchange is not for that. $\endgroup$ – HDE 226868 Dec 8 '15 at 16:56

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