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I So we've found the missing "dark matter"?

  1. Oct 14, 2016 #1
  2. jcsd
  3. Oct 14, 2016 #2

    phinds

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  4. Oct 14, 2016 #3
    I'll admit, my first thought on seeing the article was "well there's at least some of the missing matter". On reflection though, I guess the 'missing' matter is within galaxies rather than between or beyond them.
     
  5. Oct 14, 2016 #4

    phinds

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    Exactly. Galaxies, as far as is currently known/believed, formed BECAUSE dark matter clumped somewhat in the early universe and thus became the seeds of galaxies.

    I did not find the article convincing, but it SEEMS to be fact-based so maybe that's just me.
     
  6. Oct 14, 2016 #5
    who said dark matter could not be between galaxies? I thought that this was basically proposed as a fix for the expansion of the universe not matching models.
     
  7. Oct 14, 2016 #6

    phinds

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    I've not heard anyone say dark matter does not exist in intergalactic space any more than normal matter not existing in intergalactic space, but the density of dark matter (and normal matter as well) is believed to be MUCH higher around/inside galaxies.

    I was not aware that the expansion model needs any fixing, but I,m no expert on the subject.
     
  8. Oct 14, 2016 #7
    Dark matter was initially proposed to explain the rotational speed of stars within a galactic plane, but shortly after was applied to galactic clusters exhibiting similar properties. The version of the article I read did not go into enough detail to say whether the 'new' galaxies are inside clusters, which would count against dark matter, or not.
     
  9. Oct 14, 2016 #8
    So if we invent 10x more galaxies there will a lot less need to invent dark matter. That is what I meant about "finding" dark matter. I did not mean we had detected the undetectable, more like we found alternative frig factor.

    Hypothetical galaxies instead of hypothetical dark matter.
     
  10. Oct 14, 2016 #9

    phinds

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    yes.
    no, it would NOT count against dark matter, as far as I can see. You still need dark matter as the seeds of galaxies.
     
  11. Oct 14, 2016 #10

    phinds

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    I disagree. New galaxies have nothing to do with dark matter. How would new galaxies explain the Bullet Cluster?
     
  12. Oct 14, 2016 #11
    So if we keep the dark matter plus the 10x more unseen galaxies we'll need 10x more dark energy to accelerate it outwards.
     
  13. Oct 14, 2016 #12

    phinds

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    Don't know if that's true or not. Also, as I said in post #4 I'm still not convinced that this report of 10X is correct.
     
  14. Oct 14, 2016 #13

    Chalnoth

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    No. Not at all.

    This report, no matter how accurate it is, says nothing at all about the average density of matter/energy in the universe, which has already been measured to within a few percent through CMB observations.
     
  15. Oct 14, 2016 #14

    phinds

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    Now that I think about it, I realize that my having any doubt at all about that (needing more dark energy) not being the case was very misplaced and basically brain dead.

    We have an existing amount of dark energy and it is known to within a fair degree of accuracy that it is causing the existing expansion, so REGARDLESS of the amount of matter, we obviously have exactly the right amount of dark energy to cause the observed expansion for the existing amount of matter.
     
  16. Oct 14, 2016 #15
    i just read the article in space.com. It says in the last pararaph 10x more in the distant past. the first thought i had was. Is this 10x more matter also. It does not seem this is the case. only that there were more galaxies that merged into the same amount of mass that we know there is in the universe today.
    Need to reaed more thought.
     
    Last edited: Oct 14, 2016
  17. Oct 14, 2016 #16
    I'm not sure what they revealed... and how to interpret it.
    Sounds like in the early universe there were more smaller galaxies which then gradually merged to form the larger ones we see 'today'. That does not mean more matter - only its distribution changed over time.

    Edit: How this discovery does affect current matter/energy estimates is unclear to me.
     
  18. Oct 15, 2016 #17
    Thanks for all the replies. It seems rather unclear what they are proposing to have modelled but if it is just 10x the number but smaller bits, without changing the overall mass, it seems rather an empty claim that is being misrepresented because it seems like a dramatic change to existing ( alleged ) knowledge .

    There are also press articles claiming "up to 20x" which is starting to just look like clickbait. The term "up to" is usually a good indicator or sensationalism and not science.

    Isn't the expansion the reason for the hypothesised existence of DM, thus by definition it is the "right" amount?

    .... and so, if we change the amount of matter we believe is there, we will need to change the hypothesised amount of DE. No?

    I'm inclined to agree with more small bits because if they were saying 10x more mass it would be causing a lot of ( gravity ) waves in the community.
     
  19. Oct 16, 2016 #18

    Chalnoth

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    Yes, essentially. This is the norm with science reporting in the popular media.

    Not exactly. The expansion rate can be used to determine the total matter density (dark + normal), but isn't sensitive to dark matter alone. Dark energy is primarily detected through the rate of expansion of our universe over time.

    Dark matter is primarily detected through its effect on structure in the universe. Our most sensitive measurements of the overall dark matter density come from the CMB.

    It's not really possible. At this point, we have too many independent measurements of the expansion rate across a broad range of redshifts. The estimated dark matter density might shift by a few percent in one direction or the other, but not by any large amounts.
     
  20. Oct 16, 2016 #19
    Thanks for those explanations.

    But CMB is an incredibly noisy measurement and results derived therefrom seem to be crude fits to a sparse number of data points huge error bars.

    How do we conclude results to within 'a few percent' from that kind of data?
     
  21. Oct 16, 2016 #20

    phinds

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    That is exactly the opposite of what I have always understood to be the case. Do you have a reference for that statement?
     
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