1. Subscribersonhouse
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    12 Jun '14 11:22
    http://phys.org/news/2013-07-andromeda-milky-billion-years.html#nRlv

    This might be the end of dark matter. Hello MOND.
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    12 Jun '14 19:321 edit
    I have just read the link and I am impressed. So now the evidence they uncovered appears to supports MOND while actually contradicting the dark matter theory! While I don't think this comes close to actually proving MOND, surely this means MOND should be considered at least as likely to be true ( or more likely? ) than dark matter theory and casts very serious scientific doubt on the credence of dark matter theory?
  3. Subscribersonhouse
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    12 Jun '14 19:37
    Originally posted by humy
    I have just read the link and I am impressed. So now the evidence they uncovered appears to supports MOND while actually contradicting the dark matter theory! While I don't think this comes close to actually proving MOND, surely this means MOND should be considered at least as likely to be true ( or more likely? ) than dark matter theory and casts very serious scientific doubt on the credence of dark matter theory?
    I wonder what other evidence supports dark matter? Should be a contraction there somewhere, datter? darter? damatter? You could say 'what's damatter'?🙂
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    12 Jun '14 20:31
    Originally posted by humy
    While I don't think this comes close to actually proving MOND, surely this means MOND should be considered at least as likely to be true ( or more likely? ) than dark matter theory and casts very serious scientific doubt on the credence of dark matter theory?
    No, there are many different sources of evidence for dark matter. They have to show that MOND works just as well as, or better, as an explanation for all the other known phenomena, or find alternative explanations.
    I think they have a long way to go before they overturn dark matter. If they do succeed however, they could be in for a Nobel prize or something similar.
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    12 Jun '14 21:334 edits
    there are many different sources of evidence for dark matter

    Can you give any examples of different sources of evidence for dark matter that couldn't be interpreted as evidence for MOND?
    I am not implying here that no such evidence exists because I am not an expert on that but I have just googled “evidence for dark matter” and so far couldn't find a single example of evidence for dark matter that couldn't have a reasonable alternative MOND interpretation.
    Can you give an example of any phenomenon that dark matter explains but MOND cannot?
    And, so far, I have looked at:

    http://en.wikipedia.org/wiki/Dark_matter

    and

    http://www.astro.cornell.edu/academics/courses/astro201/dm_evidence.htm

    I also looked at this which says, to date, no dark matter particles have ever been known to have been detected;

    http://www.insidescience.org/content/date-particle-supercollider-detects-no-evidence-dark-matter/1545

    -which makes me can't help get the impression that the evidence for dark matter is rather wobbly although I think they shouldn't stop trying to look for dark matter particles just in case they do exist. Of course, if one such dark matter particle were to be detected, that would change everything and throw MOND into the dustbin.
  6. Standard memberDeepThought
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    13 Jun '14 00:10
    What they may have evidence for is a previous collision. If there had been such a collision then there should have been a merger if there is as much dark matter as they say. That would rule out dark matter, at least in the quantities current models suggest are needed.

    That does not rule in MOND or it's relativistic version Tensor-Vector-Scalar Gravity (see the Wikipedia page on MOND for a partial discussion). These theories have their own problems such as accounting correctly for CMB anisotropies.

    I was never really very convinced by the dark matter hypothesis, since it requires some form of exotic matter (new particles that do what the theory needs it to). Also if there is all this dark matter around it should be in the solar system and gravitational interactions will cause it to be in the sun, so the sun should have a lot less hydrogen in it (by a factor of 5) and would have run out of hydrogen by now.
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    13 Jun '14 06:351 edit
    seems to me then that both dark matter theory and MOND have their own problems and it is currently difficult to rationally give favour to one over the other. I wonder if they could both be wrong!?
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    13 Jun '14 07:58
    I don't believe in dark matter before I see it.
  9. Cape Town
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    13 Jun '14 09:17
    Originally posted by humy
    Can you give any examples of different sources of evidence for dark matter that couldn't be interpreted as evidence for MOND?
    I don't know exactly what MOND does.
    However, the main bits of evidence for dark matter that I know are:
    1. The large scale structure of the universe. Formation of Galaxies and Galaxy clusters can be simulated to match observation by taking dark matter into account.
    2. The bullet cluster and similar: two galaxies that have passed through each other. The centers of mass as calculated from the gravitational lensing effect are not where the centers of mass of the visible matter is. If valid, I think this would contradict MOND.
    I believe we have many more examples of galaxy collision and simulations of the collision also require dark matter to explain them.
    3. Galaxy structure. I believe the different types of galaxy and their structures can be explained by taking dark matter into account. MOND may have a similar effect, but it would have to be shown that it results in the same structures.
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    13 Jun '14 09:19
    Originally posted by DeepThought
    Also if there is all this dark matter around it should be in the solar system and gravitational interactions will cause it to be in the sun, so the sun should have a lot less hydrogen in it (by a factor of 5) and would have run out of hydrogen by now.
    Please show your calculations or a reference to support your sun claim. Why haven't astronomers realized the sun is off by a factor of 5?
  11. Cape Town
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    13 Jun '14 09:29
    There is always wikipedia:
    http://en.wikipedia.org/wiki/Dark_matter#Observational_evidence
  12. Standard memberDeepThought
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    13 Jun '14 12:56
    Originally posted by twhitehead
    Please show your calculations or a reference to support your sun claim. Why haven't astronomers realized the sun is off by a factor of 5?
    Dark matter theories put the dark matter in a halo around the solar system. This is not what the rest of the matter in the solar system does. Most of the normal matter in the solar system is in the sun. So why isn't the dark matter? Either pretty well established stellar evolution theories are wrong or dark matter behaves differently in a way I've never seen an explanation for.
  13. Subscribersonhouse
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    13 Jun '14 13:45
    Originally posted by twhitehead
    Please show your calculations or a reference to support your sun claim. Why haven't astronomers realized the sun is off by a factor of 5?
    If dark matter is real, maybe the highest concentration of the stuff would be more towards the center of the galaxy. The fact our sun still has plenty of H2 would indicate there isn't much dark matter in our vicinity at least.
  14. Standard memberDeepThought
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    13 Jun '14 17:01
    Originally posted by sonhouse
    If dark matter is real, maybe the highest concentration of the stuff would be more towards the center of the galaxy. The fact our sun still has plenty of H2 would indicate there isn't much dark matter in our vicinity at least.
    I think that that is what is believed, even so they should have some explanation of why the Sun is still shining. It is at slightly under half it's expected life time as a main sequence star and so even if we are in a relatively sparse dark matter region a significant proportion of its mass should be taken up by dark matter.
  15. Cape Town
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    13 Jun '14 17:33
    Originally posted by DeepThought
    Dark matter theories put the dark matter in a halo around the solar system. This is not what the rest of the matter in the solar system does. Most of the normal matter in the solar system is in the sun. So why isn't the dark matter? Either pretty well established stellar evolution theories are wrong or dark matter behaves differently in a way I've never seen an explanation for.
    You have it all wrong. Dark matter theories put a dark matter halo around the galaxy - not the solar system.
    The reason why it doesn't all gather in the centre is because it is weakly interacting. In general, any particle in the galaxy orbits the centre. The only reasons for orbit decay (and thus migration to the centre) are:
    1. collisions with other particles.
    2. gravitational interactions which create some drag.
    1. results in the formation of a disk as is seen in the galaxy, the solar system and the rings of Saturn.
    However if there are no physical interactions other than gravity, then there is much less of a tendency to form a disc, so the dark matter halo is more spherical than the visible matter.
    The dark matter halo does gradually shrink but not as fast as the visible matter as the visible matter manages to radiate its potential energy much more efficiently due to the greater interaction that it undergoes.

    I must point out that neutrinos behave roughly like dark matter in that they too do not collect at the centre of the sun (although they are produced there in very large quantities). And neutrinos are very much in the realm of established physics.
    However the characteristics of dark matter suggest it consists of much more massive particles.
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