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Dark Matter versus Dark Mass

  1. May 17, 2013 #1
    Why do we talk about dark matter rather than dark mass?

    Where does the necessity for matter stem from?
     
    Last edited: May 17, 2013
  2. jcsd
  3. May 17, 2013 #2
    Dark mass is a combination of the mass of dark energy and dark matter.
    mass is either due to energy or matter. Dark matter explains why galaxies rotate at the rate that they do. Why certain gravitational lenses exists.
    We do not fully understand DM. However there is plenty of indirect evidence that it is there.
     
  4. May 17, 2013 #3

    cepheid

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    I'm not sure I agree. I have never heard the term "Dark Mass" before and am unsure what the OP is talking about.
     
  5. May 17, 2013 #4
    Thx Mordred. Is there any 'specific' evidence apart from the gravitational effects? How much does the mass of DE contribute to DM? Could it be around 66% (own figure).
     
  6. May 17, 2013 #5

    cepheid

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    Look, just to be clear, 'dark mass' is not really an accepted term in the cosmological literature. It means nothing: it's something you just made up.

    DM is dark matter, and DE is dark energy, and these are two totally different things.

    These two things contribute a certain amount to the total "budget" of energy density (energy per unit volume) in the universe. If you want to know how much: dark energy contributes to about 73% of the total energy density of the universe, and dark matter contributes about 22%. So that means that 95% of the energy density in the universe is due to two constituents of the universe that we don't fully understand yet, and only 5% of the energy density of the universe is due to the mass-energy of ordinary matter that is composed of atoms (and that includes everything that we can see).
     
  7. May 17, 2013 #6
    When I talk about mass I think of energy. In GR, energy is not conserved when space expands (thinking of DE). Is such an energy/mass gain visible or invisible (thinking of DM)? Wouldn't such be invisible apart from gravitational effects because the expansion of the universe affects everything the same? Does more energy necessarily come with more matter? Or could existing matter gain a higher mass, like i.e. a photon gets blueshifted when we put more energy in it? How exactly does this energy/mass gain that I think about look like? I have tried something and the numbers came up with 82.9%dark mass to 17.1%visible mass. Most likely coincidences. I'm looking why it’s wrong, i.e. where does the need for extra matter come from? Because I fear I waste all my time on it and annoy others with it. Sorry. Just trying to find an answer.
     
    Last edited: May 17, 2013
  8. May 17, 2013 #7
    I think the latest figures from Planck are 68.3% DE, 26.8% DM, 4.9% visible. And 4.9/26.8 = 18.3 visible to 81.7% invisible? Why do they say it's roughly 84%?

    I'm using the word "mass" (which is like energy) just to be careful because of the lack of evidence for matter particles other than for the gravitational effect which the "unknown" causes. Just wondering if there is a bulletproof reason that it cannot be any other way than "matter" because of course the word must have been chosen with good reason.
     
    Last edited: May 17, 2013
  9. May 17, 2013 #8

    cepheid

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    Sure, if you want to use the latest Planck values instead of the WMAP ones, then go right ahead. :grumpy: I was just trying to illustrate the *basic idea* to you. I am not interested in quibbling over specific values.

    Well who the hell are "THEY?" Could you please provide citations to the claims you are referring to so that we can see in what context they were made?

    EDIT: taking the WMAP numbers from the Planck website (22.7 / (4.5 + 22.7)) = 0.83455882352, which is pretty close to 84%. Maybe "THEY" just happened to be using some version of the WMAP numbers as well. :tongue: All of these numbers have an uncertainty associated with them. You shouldn't get all bent out of shape about an apparent "inconsistency" at this level.

    EDIT 2: I see that you also did your calculation wrong. It makes sense to speak of the ratio of invisible matter to the TOTAL amount of matter: (26.8 / (26.8 + 4.9)) = 0.845425.

    This is saying that some 84% out of *all* the matter in the universe is invisible matter. The ratio that you took isn't really meaningful. I didn't catch your mistake before.

    Yes, it's true that the only objects known to physics that have mass are elementary particles in the standard model of particle physics. And yes, we have good reason to believe that dark matter must consist of some things outside the standard model. However, we do have good reason to think (i.e. it's a leading theory) that those "things" that make up dark matter are also particles, just ones that are not known to the standard model. Some sort of particle, a WIMP, could have a lot of the necessary properties that we know dark matter must have.

    Even if dark matter isn't a particle in the traditional sense, but something more exotic than that, I really don't think that the term "matter" has a definition more specific than "stuff that has mass." The missing mass required to hold galaxies and clusters of galaxies together, and to make the large scale structure evolve in the way that it has, can be explained as some unknown substance i.e. "stuff that has mass." So calling it "Dark Matter" seems perfectly reasonable to me. These are just words, the physics is what is fundamental. I'm not going to sit here and quibble over what names should be used to refer to things.
     
    Last edited: May 17, 2013
  10. May 17, 2013 #9
    There is a mass energy relation. Dark energy does contribute to the total mass energy of the universe.

    http://en.m.wikipedia.org/wiki/Dark_energy
    However your correct in that its never referred to as dark mass. However if you want to tally the total mass if the universe. Dark energy density is also included in the calculations. So dark mass could simply mean mass energy of dark energy+dark matter. Thats how I took as the meaning of the OP. Yes energy density or mass energy are preferred terms. However as energy contributes to the total mass of the universe. Dark mass as a total of dark energy and dark matter isn't necessarily incorrect.

    http://arxiv.org/abs/1004.1035

    however I should note that dark energy is considered vacuum energy so converting from energy density to mass. affects the critical density differently from say dark matter or baryonic matter.
    I fully agree dark mass is a poor term for the latter reason.
     
    Last edited: May 17, 2013
  11. May 17, 2013 #10

    cepheid

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    Sorry, not sure if your remark is addressed to me. If it is: I am aware of this.

    Yeah. I included it in my tally above. The 73%.
     
  12. May 17, 2013 #11
    Thx to both of you !
     
  13. May 17, 2013 #12

    cepheid

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    Can you elaborate? The critical density depends only on H0 and G, to the best of my knowledge.
     
  14. May 17, 2013 #13
    Yeah I was about to edit that to state it contributes to the actual density which is compared to the critical density to define the universes curvature.
     
  15. May 17, 2013 #14
  16. May 18, 2013 #15
    Thank you very much Yenchin. Good stuff. From my own research into the topic I consider extra mass as almost certain. MOND is a phenomenological description. What I'm questioning is the what and 'why' DM is. I'm interested in observational evidence mapping its location - to see if it definitely must be some separate matter.
    Because I have tried an idea, which calculated there should be ~5 parts invisible mass to every part visible mass out there or ~83% to 17%. This match with observation is probably just coincidence because that idea demands them 'not' to be locationally separate.
    I can't give up on it just yet, because I expected the numbers to be many orders out for the case my idea was wrong. I just wished I would know. But 'who' knows? I think I would be happier if my results would have been way out, telling me my idea was definitely wrong, rather than being a perfect match, suggesting there could be something to it. Because now I'm stuck, and I don't feel good.
     
  17. May 18, 2013 #16
    ... 'not' to be locationally separate ... and apply to every visible matter accumulation equally. Is that complete rubbish?
     
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